Why You're Not Getting The MPG You Expect
Hybrids Fair Worst
Consumer Reports v.70, i.10 1oct2005
For years, automakers have been criticized for producing vehicles that get so-so gas mileage. But as gas prices climb and consumers seek more miles per gallon, it turns out that fuel economy is much worse than it appears--50 percent less on some models, a new Consumer Reports analysis reveals.
Drivers who track their own fuel economy have long known that their results seldom match the gas mileage claimed by the Environmental Protection Agency on new-car stickers. Our study, based on years of real-world road tests over thousands of miles, quantifies the problem across a wide swath of makes and models.
We compared the claimed EPA fuel economy with the mileage per gallon we measured for 303 cars and trucks for model-years 2000 to 2006. Our selection represents a good cross-section of mainstream, high-volume vehicles. We looked at city highway, and overall mpg.
Highlights of our study:
- Shortfalls in mpg occurred in 90 percent of vehicles we tested and included most makes and models.
- The largest discrepancy between claimed and actual mpg involved city driving. Some models we tested fell short of claimed city mpg by 35 to 50 percent.
- Hybrids, whose selling point is fuel thriftiness, had some of the biggest disparities, with fuel economy averaging 19 mpg below the EPA city rating.
- The EPA ratings are the result of 1970s-era test assumptions that don't account for how people drive today Automakers also test prototype vehicles that can yield better mileage than a consumer could get.
- Despite federal certification, it appears that U.S. vehicle fleets, all cars and light trucks produced in one model year, don't meet government fuel-economy standards. For example, fleet mpg for 2003-model-year vehicles we studied was overstated by 30 percent.
For consumers, the news means that their vehicles typically cost hundreds more per year to operate than they were led to believe. Put another way, when gas in August hit $2.37 per gallon, the mpg shortchange effectively boosted the price for some motorists to $3.13 per gallon.
For the nation, where the fleet average fuel economy is near its lowest point in 17 years, the findings suggest that the country is far short of its energy goals.
"We are concerned about the differences," Margo Oge, director of the EPA's Office of Transportation and Air Quality, said of our study, "I think we can do a better job to help consumers assess actual fuel economy."
VEHICLE TYPE MAKE & MODEL CITY MPG . EPA CR EPA mpg mpg shortfall Small SUV Jeep Liberty Diesel Ltd 4WD 22 11 50% Hybrid Honda Civic sedan 48 26 46% Large sedan Chrysler 300C 17 10 41% Midsized SUV Chevrolet Trailblazer EXT LT 4WD 15 9 40% Minivan Honda Odyssey EX 20 12 40% Luxury sedan BMW 7 Series 745SLi 18 11 39% Pickup Dodge Ram 1500 SLT crew cab 4WD 13 8 38% Family sedan Oldsmobile Alero GL 21 13 38% Large SUV Dodge Durango Limited 4WD 13 8 38% Small sedan Ford Focus ZX4 SSES 26 17 35%
HOW THEY TEST, HOW WE TEST
Almost from the dawn of EPA testing in 1975, automobile buyers have complained that the government rating was impossible to achieve. In 1984, a rising clamor from consumers prompted the EPA to shave its test results by 10 percent for city mpg and 22 percent for highway mpg. But the agency did not change its test protocols and rules, which the Government Accountability Office had criticized in 1981. This "adjusted mpg" is what you find on a new car's window sticker today.
In the two decades since, the driving world continued to change, and the EPA rating again drifted from real-world mpg.
For one thing, Americans drive more miles in dense traffic. "Idle times are longer in real life than in the EPA test cycle; you're stopped at traffic lights longer," says Mike Duoba, who is an engineer at the Department of Energy's Argonne National Laboratory near Chicago and has studied the EPA test.
Many automobiles today spend 62 percent of their annual miles in city stop-and-go traffic, where fuel economy is the lowest. The EPA formula still uses a 55/45 percent city/highway ratio to calculate combined fuel economy.
Vehicles have also changed. Computerized engine systems have improved efficiency, but the potential fuel savings has been traded for increased engine horsepower. Since 1981, horsepower is up 89 percent for cars and 99 percent for trucks. Automatic transmissions, air conditioning, four-wheel drive, and bigger and heavier vehicles are also more common, all of which burn more gas. Moreover, vehicles burn up to 10 percent more fuel per mile simply by traveling at today's faster highway speeds.
Automakers conduct the government fuel-economy tests on a laboratory dynamometer. They can use hand-built prototype vehicles, within the EPA rules, to maximize miles per gallon in simulated city and highway driving. "Anybody taking a test, you're going to figure out what the rules are and figure how to optimize your chances of passing that test," says Reg Modlin, director of environmental affairs for Daimler-Chrysler. "So in that sense, yes, everyone attempts to put their best face on for the test."
By contrast, Consumer Reports testers check fuel economy on roads and on our test track. We buy models anonymously from dealers, as consumers do.
We gauge overall fuel economy from our city, highway, and mixed-driving tests. Overall, the gas-powered vehicles we studied delivered 9 percent fewer mpg on average than their EPA stickers claimed; diesels and hybrids, 18 percent fewer mpg than claimed. The numbers ranged from 21 percent better than the EPA sticker to 28 percent worse.
The discrepancy between our numbers and the EPA's is increasing. For gas-powered vehicles, the shortfall was 6 percent for 2000-model-year cars that we tested, but about 12 percent for 2005- and 2006-model-year cars.
Big differences between claimed and actual city mpg were the main reason for the discrepancy in overall mpg. Our city mpg figures ranged from 13 percent better than the EPA sticker to 50 percent worse. On average, our highway mpg more closely reflected the EPA rating.
Ironically, six fuel-thrifty hybrids we tested had some of the largest discrepancies, mostly on city mpg, where real fuel economy ranged from 11 to 25 mpg below EPA ratings. City traffic is supposed to be the hybrids' strong suit, but their shortfall amounted to a 40 percent deficit, on average. Still, hybrids won three of the best five spots in our tests for overall mpg, along with the diesel Volkswagen Golf and the all-gas Toyota Echo.
HOW YOU'RE SHORTCHANGED
The EPA ratings do allow comparisons among models, so that consumers can trust that the fuel economy of the Honda Civic EX (33 claimed mpg, 29 actual) is considerably better overall than that of the BMW X5 (18 claimed mpg, 17 actual). But as a predictor of real miles per gallon, if the EPA ratings are exaggerated, they are a deceptive sales tool.
Consumers are clearly frustrated. "According to Honda, the Element gets 21 mpg," says Tom Mannino, a retired firefighter from Staten Island, N.Y., one of many readers who have complained to Consumer Reports about being shortchanged. "My Element, however, gets 14 mpg. Isn't this false advertising?"
Our study found that only 10 percent of vehicles achieved fuel economies as good as or better than EPA estimates, including the 2003 Infiniti FX35, the 2004 Chrysler Crossfire, and the 2000 Honda S2000 convertible.
The EPA's estimates can cause real pain at the pump over the five years you're likely to own the vehicle. The extra fuel cost depends on make and model: $1,316 more for a Nissan Quest, an extra $1,742 for a Mercury Grand Marquis LSE, and $2,558 more for a Dodge Ram 1500. That assumes driving 12,000 miles per year and no further rise in gas prices.
Bigger problems emerge when incorrect fuel-economy numbers are used by Congress and the National Highway Traffic Safety Administration to set U.S. energy policy and enforce fuel-economy standards. Here, the distortion is magnified to the benefit of three groups:
- Automakers, who get false passing grades on fuel-economy standards.
- Government, including lawmakers, who can point voters to their "successful" standards, and regulators, who can appear to crack the whip while actually going easy on a powerful industry lobby.
- Oil interests, which benefit from the seeming energy efficiency of what appears to be a responsible national energy policy.
Federal fuel-economy requirements were enacted in 1975 in response to the 1973 Arab oil embargo, which sparked fuel shortages and sent gas prices skyrocketing. The requirements, known as Corporate Average Fuel Economy (CAFE, pronounced cafe) standards, are national goals designed to prod automakers to produce more fuel-thrifty vehicles.
In 1975, passenger cars got only 14 mpg on average, light trucks just 10.5. By 1985, CAFE required the fleet of passenger cars to average 27.5 mpg and light trucks, 19.5. The different standards for cars and trucks can be traced back to the late 1970s, when the auto industry pressured Congress to cut the mileage requirements for light trucks, which included mainly pickup trucks and cargo vans used commercially.
That move had unanticipated consequences when light trucks, including pickups, SUVs, and minivans, began to take off in sales as passenger vehicles. Today, that segment accounts for about half of all new vehicles sold. In addition, CAFE standards don't apply to vehicles that exceed 8,500 pounds when fully loaded, such as GM's Hummer H2 and the Ford Excursion.
NHTSA uses the EPA ratings, automobile manufacturing data, and a set of formulas to calculate the average fuel consumption for the entire fleet of cars and trucks sold each model year. By doing so, NHTSA ensures that automakers meet CAFE standards.
Automakers that don't comply are subject to fines; since 1983, they've paid more than $625 million. But CAFE credits and loopholes allow many automakers to reduce or avoid payments. For example, Subaru raised the ground clearance of its 2005 Outback sedan and wagon by about an inch. That change qualified the vehicles as light trucks as defined by CAFE, meaning they could meet that category's lower fuel economy standards.
Because EPA ratings are inaccurate, resulting national fleet estimates are wrong, too. In fact, NHTSA's national estimate is farther off-base than the EPA sticker mpg. That's because Congress requires NHTSA to use the unadjusted EPA test results. They are higher than the adjusted mpg and thus more inaccurate.
Why the congressional mandate? Automakers argued that if the lower, adjusted EPA ratings represented real-world fuel economy then the CAFE standards should have been relaxed accordingly to reflect the new reality. "Nobody wanted to go to that trouble," says Michael Love, national regulatory affairs manager for Toyota. But Russell Long, founder of Bluewater Network, a San Francisco-based environmental group that has petitioned the government for more accurate new-car fuel-economy stickers, has a different explanation. "Automakers give terrific amounts of money to members of Congress," he says. NHTSA officials declined to be interviewed for this report.
If more-accurate mpg figures were used to rate CAFE compliance, most automakers would fail to meet the standards, our study shows. For example, the fleet fuel economy for the 2003 model year was off by 30 percent when calculated using our road tests. By NHTSA's reckoning, the fleet of 2003-model-year passenger cars we tested averaged 29.7 mpg. We got no better than 22.7, below the federal target of 27.5 mpg. NHTSA says the fleet of light trucks in our tests averaged 21.4 mpg; we got only 16, below the 20.7 mpg target.
The mpg inflation has allowed automakers to trade fuel economy for performance features that draw buyers. Between 1987 and 2005, car and light-truck manufacturers slashed 0-60 acceleration time by 24 percent and bulked up average vehicle weight by 27 percent. Consequently these vehicles got 1.1 fewer miles per gallon in 2005 than they did in 1987.
Automakers have lobbied against tougher standards, saying that higher mpg is technologically difficult to achieve and that they're making vehicles the public wants. If consumer demand were not a consideration, light trucks could be getting 28 mpg and cars, 38, says John German, manager of Honda's environmental and energy analysis. "The role of government is to create mandates or incentives so some of the ongoing engine-technology-efficiency gains go to fuel economy and not just more horsepower," he says.
Automakers have also stirred fears that to achieve greater fuel economy, vehicles would have to shed weight, which would lead to more traffic deaths because occupants in lighter vehicles have a lower survival rate in crashes. Some of the horsepower gains, however, could be traded for improved fuel economy without lightening the vehicles, German says.
Inflation of mpg has also let Congress stave off public pressure for better fuel economy, More than 80 percent of 1,221 adults, in a nationally representative sample surveyed by Consumer Reports in May 2004, said the government should raise fuel-efficiency standards. Congress, however, has kept CAFE standards at the 1990 level of 27.5 mpg for cars. For light trucks, the standard was frozen at 20.7 mpg from 1996 through 2004 and will be raised to 22.2 mpg by 2007.
Finally, mpg inflation has helped energy policies. The exaggerated EPA and NHTSA estimates forestall demand for more fuel-efficient cars and alternative fuels. And the country gets a distorted view of U.S. energy needs.
REGAINING LOST GROUND
Consumers Union, publisher of Consumer Reports, supports raising CAFE standards and revising EPA's test.
The EPA says it will propose changes in how it reports fuel economy to the public. But Congress voted to cut back on tax breaks for motorists who buy fuel-efficient hybrids. For more about fuel-economy claims, see Viewpoint on page 65.
FOR CITY DRIVING, CLAIMED MPG IS WAY OFF
The biggest gaps between claimed and actual miles per gallon are in city driving. That's because the federal test protocol is far afield of how people really drive. For the vehicles listed below, the shortfall was 35 to 50 percent of claimed mpg.
VEHICLE TYPE MAKE & MODEL CITY MPG . EPA CR EPA mpg mpg shortfall SMALL SUV Jeep Liberty Diesel Ltd. 4WD 22 11 50% HYBRID Honda Civic sedan 48 26 46 LARGE SEDAN Chrysler 300 C 17 10 41 MIDSIZED SUV Chevrolet TrailBlazer EXT LT 4WD 15 9 40 MINIVAN Honda Odyssey EX 20 12 40 LUXURY SEDAN BMW 7 Series 745Li 18 11 39 PICKUP Dodge Ram 1500 SLT crew cab 4WD 13 8 38 FAMILY SEDAN Oldsmobile Alero GL 21 13 38 LARGE SUV Dodge Durango Limited 4WD 13 8 38 SMALL SEDAN Ford Focus ZX4 SES 26 17 35
HYBRID HYPE Our road tests show that hybrid vehicles, especially, get fewer miles per gallon than claimed. We checked mpg for the Honda Accord Hybrid, at right, using a fuel meter attached to a readout, like the one below.
what you can do
BUY A FUEL-EFFICIENT VEHICLE
Look for a vehicle that gets good fuel economy for its class. The vehicles at right have provided the best and worst overall fuel economy within their class in our recent tests, and they are still sold. Some appear in more than one category, if appropriate.
SMALL CARS Best Overall mpg (manual transmission) Honda Insight 51 Toyota Echo 38 Toyota Scion xB 32 Toyota Scion xA 31 (automatic transmission) Toyota Prius 44 Honda Civic Hybrid 36 Toyota Scion xA 30 Toyota Scion xB 30 Worst (automatic transmission) Subaru Impreza 2.5 RS 22 Chevrolet Cobalt LS 23 FAMILY SEDANS Best Overall mpg Toyota Prius 44 Volkswagen Passat GLS TDI 28 Honda Accord Hybrid 25 Worst Subaru Legacy GT 18 Hyundai XG350 19 LARGE SEDANS Best Overall mpg Toyota Avalon 22 Mercury Montego FWD 21 Ford Five Hundred 21 Worst Mercury Grand Marquis LSE 16 Ford Crown Victoria LX 16 Chrysler 300 C 16 THREE-ROW SUVs Best Overall mpg Toyota Highlander Ltd. (V6) 19 Acura MDX 17 Honda Pilot 17 Worst Ford Expedition Eddie Bauer 12 Dodge Durango Limited 5.7 12 TWO-ROW SUVs Best Overall mpg Toyota Highlander Ltd. (V6) 19 Nissan Murano 19 Worst GMC Envoy SLT 15 Chevrolet TrailBlazer LT 15 Volkswagen Touareg 15 PICKUPS (4-DOOR CREW CAB, 4WD) Best Overall mpg Subaru Baja 20 Toyota Tacoma 17 Worst Dodge Ram SLT 5.7L 11 Dodge Ram SLT 4.7L 12 SMALL SUVs Best Overall mpg Ford Escape Hybrid 26 Honda CR-V EX 21 Subaru Forester 2.5 X 21 Toyota RAV4 21 Worst Jeep Wrangler Unltd. (6-cyl.) 14 Jeep Liberty Sport (V6) 15 Kia Sorento LX 15
OTHER WAYS TO SAVE MONEY ON AUTO TRAVEL
- Be skeptical of EPA ratings. The EPA sticker can help you evaluate relative gas mileage among vehicles, but not absolute mpg. Until the EPA ratings are made more realistic, discount the EPA sticker numbers for city travel as follows: conventional cars and trucks, 30 percent; larger hybrids, 35 percent; diesels, 36 percent; smaller hybrids, 42 percent.
- Buy regular. If your car owner's manual doesn't recommend a particular grade of gasoline, fill 'er up with regular. And don't waste money on so-called gas-saving devices; our tests have shown that they don't work.
- Walk, bike, or "chain." Fuel economy is worst on short trips. These trips also create the most exhaust emissions and cause the most engine wear. Half of all car trips are under six miles, within walking or biking distance. If you must drive, "chain" several errands into a single trip on a warm engine instead of making separate short trips throughout the day.
- Avoid highway drag. At highway speeds, where fuel economy is best, more than 50 percent of engine power goes to overcoming aerodynamic drag. Try not to add to that drag by carrying things on top of your vehicle. A loaded roof rack can decrease a car's fuel economy by 5 percent. Even empty ski racks waste gas.
- Keep your vehicle in top shape. A poorly maintained engine can cut gas mileage by 10 to 20 percent. A clogged air filter can cause up to a 10 percent increase in fuel consumption. Underinflated tires require more energy to roll and can reduce fuel economy by 5 percent. Follow the maintenance schedule in your owner's manual, and keep the tires properly inflated.
- Drive smart. As much as possible, avoid hard acceleration and braking. Once up to speed, maintain a steady pace in top gear; varying your speed a lot wastes fuel. A vehicle's gas mileage decreases rapidly at speeds above 60 mph. With most gasoline engines, it's more efficient to turn off the engine than to idle for any longer than 30 seconds. If you have air conditioning, use it sparingly.
Driving to Save Gas
Consumer Reports 29jan2006
Car buyers have started to reject large gas-guzzling SUVs in favor of more fuel-efficient vehicles, but Consumer Reports says if you don't pay attention to how you drive, you'll bring your mileage down anyway.
The four-cylinder Toyota Camry is pretty fuel-efficient. At 65 miles per hour, Consumer Reports' tests show it gets an impressive 35 miles per gallon. Engineers wanted to see what happens when you change how you drive. They used a fuel meter to track the amount of gasoline used.
If you slow down to the ideal 55 miles per hour, the mileage improves from 35 to 40 miles per gallon. But if you speed up to 75 miles per hour, the Camry's mileage would drop below 30 miles per gallon.
Put the air conditioner on and the engineers found you lose another mile per gallon or so.
Consumer Reports also checked what happens if you put a luggage carrier or pod on top of the Camry.
"You put on a pod, it breaks up the aerodynamics. It was six miles per gallon worse with the pod on than without at 65 miles per hour," said Consumer Reports' Rick Small.
If you use a pod and drive fast with the AC on, the Camry's highway mileage would plummet to a projected 22 miles per gallon— not much better than what you routinely get with an SUV on the highway.
Consumer Reports says regardless of what kind of car you drive, there are ways to stretch your fuel dollars.
"Keep a steady pace; don't be erratic driving," says Small. "If you use a cruise it will help. Some people pump the throttle all the time." Follow this advice and go 65 miles per hour or less in order to maximize the fuel efficiency of your car.
The most fuel-efficient car Consumer Reports has tested is the hybrid two-seater, the Honda Insight, which gets 66 miles per gallon on the highway. The Toyota Prius was the second best.
source: http://abclocal.go.com/kfsn/story?section=consumer&id=3857384 14mar2006
Get the most mileage for your fuel dollars Consumer Reports' tests show how to avoid money-wasting driving habits
Consumer Reports 1apr2006
How you drive your vehicle can have a big impact on fuel economy. That's the key finding of recent real-world fuel-economy tests performed by Consumer Reports' auto engineers. On the highway, driving smoothly and steadily and not carrying items on top of the vehicle are two of the most significant factors. In slower, city-driving conditions, driving with the engine warmed up and driving nonaggressively made the greatest difference.
We conducted a series of tests on two vehicles: a 2005 Toyota Camry sedan with a four-cylinder engine and automatic transmission and a V8-powered 2005 Mercury Mountaineer midsize SUV. We tested the effects of driving aggressively; carrying a car-top storage box; driving at faster and slower speeds on the highway; and driving with a cold engine, underinflated tires, a dirty air cleaner, and with the air conditioning running. Our results show you how to get the most for your fuel dollar.
Of course, the biggest factor in fuel economy is the vehicle. Even when we simultaneously committed a number of fuel-economy faux pas in the Camry, it still got better fuel economy than the Mountaineer did at its best. ConsumerReports.org subscribers can see the models we've tested that have provided the best and worst fuel economy in their classes.
DRIVING TIPS THAT MAKE THE MOST DIFFERENCE
Minimize driving with a cold engine. Engines run most efficiently when they're warm. In our city-driving tests, making multiple short trips and starting the engine from cold reduced fuel economy for both the sedan and SUV. Engines also produce more pollution and wear faster when they're cold. To minimize cold-engine driving, avoid making a lot of separate short trips with a cold engine. Instead, combine short trips into one so that the engine stays warm.
Drive smoothly. Avoid hard acceleration and braking whenever possible. In our tests, frequent bursts of acceleration and braking reduced the Camry's mileage by 2 to 3 mpg and the Mountaineer's by about 1 mpg. The harder you accelerate, the more fuel you use. Unnecessarily hard braking wastes the fuel you use to get up to speed. Drive smoothly and anticipate the movement of traffic. Use your brakes as little as possible, since every time you hit the brakes you are wasting fuel. Once up to speed on the highway, maintain a steady pace in top gear. Smooth acceleration, cornering, and braking not only save fuel but also extend the life of the engine, transmission, brakes, and tires.
Reduce unnecessary drag. At highway speeds, more than 50 percent of engine power goes to overcoming aerodynamic drag. Don't add to that drag by carrying things on top of your vehicle when you don't have to. We installed a large Thule Cascade 1700 car-top carrier on our sedan and SUV. Keep in mind, however, that the effect varies, depending on the model. Driving with the carrier cut 6 mpg from the normally aerodynamic Camry, dropping it from 35 mpg to 29. It only reduced the boxier Mountaineer from 21 mpg to 20. Even driving with empty racks on the car reduces its fuel economy.
Slow down. Aerodynamic drag exponentially increases on the highway the faster you drive. We tested our vehicles' fuel economy at 55, 65, and 75 mph. Driving at 75 mph instead of 65 reduced the Camry's gas mileage from 35 mpg to 30. For the Mountaineer, fuel economy fell from 21 mpg to 18. Slowing down to 55 mph improved the gas mileage by similar margins: The Camry improved to 40 mpg and the Mountaineer to 24 mpg.
IMPORTANT DRIVING TIPS THAT HAVE LITTLE EFFECT ON FUEL ECONOMY
Keep tires inflated. Our tests show that driving on moderately underinflated tires is more of a safety concern than a fuel-economy issue. We set the pressure in all four tires to 10 psi below that recommended by the automaker. This reduced highway fuel economy slightly, by about 1 mpg for the Camry and by a much smaller margin for the Mountaineer. But more importantly, underinflated tires provide much less grip for turning and stopping and run much hotter. Overheated tires wear faster and can lead to a blowout. Check the pressure of your vehicle's tires at least once a month, when the tires are cold. Also check the tires before and after long road trips. The recommended tire pressure is found on a label inside the car--usually in a doorjamb, inside the glove-box lid, or inside the fuel-filler lid.
Keep your air filter clean. According to our tests, driving with a dirty air filter in modern engines doesn't have a significant impact on fuel economy, as it did with older engines. While fuel economy didn't change, however, power output did. Both cars accelerated much more slowly with a dirty air cleaner. We drove both vehicles with their air cleaners restricted and found little difference in gas mileage with either engine. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow, therefore, caused the engines to automatically reduce the amount of fuel being used.
USING AIR CONDITIONING VS. OPENING THE WINDOWS
Air conditioning places more load on the engine, which can affect fuel economy. But some auto journalists say that opening the windows at highway speeds can affect fuel economy even more by disrupting the vehicle's aerodynamics. In our tests we found that neither makes enough of a difference in fuel economy to worry about. Using air conditioning while driving at 65 mph reduced gas mileage in both vehicles by about 1 mpg-it might make more of a difference if you drive faster. The effect of opening the windows at 65 mph was not even measurable. Because air conditioning can help keep you comfortable and alert and because most modern cars use it to keep windows defrosted, we suggest that the small trade-off in fuel economy for increased safety is worthwhile.
DON'T USE PREMIUM FUEL UNNECESSARILY
If your car specifies regular fuel, don't buy premium under the mistaken belief that your engine will run better. Most cars are designed to run just fine on regular gasoline. Furthermore, many cars that recommend premium fuel also run well on regular. Check your owner's manual to find out if your engine is designed to handle either grade. Think twice about using the more expensive gas even if your owner's manual suggests "for optimum performance use premium." We have found that the differences aren't perceivable during normal driving. However, if your car "pings" or knocks with lower grade fuel, buy premium.
source: http://www.consumerreports.org/cro/cars/maintenance-accessories/get-the-most-mileage-for-your-fuel-dollars-406.htm 14mar2006