Explained: 2008 EPA Fuel Economy Ratings

Car buyers shopping for fuel economy this year should expect to see some unexpectedly low mpg figures on 2008 models, some of which are hitting showrooms now. Don't get all stressed. New cars have not suddenly become thirstier.

Starting with 2008 models (which went on sale as early as January 2, 2007), the methodology for measuring and reporting fuel economy has been reworked to make published miles-per-gallon (mpg) ratings more accurate. The trouble for consumers is this: 2007 models and leftover 2006 vehicles on the lot next to the '08s will appear to have superior fuel economy. But they don't. This conundrum exists because 2007-and-earlier models use an Environmental Protection Agency (EPA) fuel economy measurement that is being phased out.

Your mileage has varied

For years, consumers have groused that their new car simply can't match the gas mileage estimates displayed on the window sticker or featured in advertisements. The typical gut reaction has been to blame the carmaker, attributing the shortfall to an overenthusiastic marketing department. We see this type of comment all the time in our forums, and it seems that no company — even hybrid car manufacturers — is immune.

What consumers may not know is that the U.S. government, via the EPA, specifies all aspects of fuel economy ratings and how the mpg tests are carried out, including the "City" and "Highway" driving patterns. Those patterns are simulated on machines called dynamometers (dynos) to make the results accurate and repeatable. EPA's influence even penetrates to the design of the window sticker itself, right down to the point size and fonts used. So if the numbers seem too "optimistic" compared to real-world results, the manufacturers themselves aren't necessarily guilty as charged.

The reason why fuel economy estimates have been coming out too high is simple: The EPA-specified testing and reporting method has not been updated since 1985. Since then, a lot has changed. For one thing, the former national speed limit of 55 mph has been abolished. So instead of topping out at 60 mph, the new highway rating test includes speeds up to 80 mph.

And then there are added scenarios the old tests never included. Things like very hot and very cold temperatures and driving with the air-conditioner and defroster on are now part of the testing regimen. Acceleration rates in the old tests were funeral-procession sedate, owing to the limits of the dynos used when the tests were originally conceived in the '70s. More spirited, but realistic, acceleration and braking rates are now specified.

Hybrids take the biggest hit

Taken together, the higher speeds, use of air-conditioning, hot and cold temperatures and more aggressive acceleration and braking will bring 2008 ratings down. According to the EPA, city ratings will drop about 12 percent, with some losing as much as 30 percent. Highway ratings should fall an average of 8 percent, but could drop by up to 25 percent.

Ironically, smaller cars oriented toward fuel economy take the biggest hit, with hybrids drawing the shortest of all straws. Why? Assuming that the fuel economy on both vehicles drops the same percentage, a car that gets 40 mpg will lose more actual miles per gallon than one rated at 14 mpg. Also, smaller-engine cars have to work harder to accelerate more swiftly, particularly with the air-conditioner on.

For hybrids it can be worse, as the additional demand caused by air-conditioner use, strong acceleration or high-speed driving reduces the amount of electric-only operation. And as you might expect, more fuel is consumed whenever the gasoline engine is required to power up.

On the braking side, the old test's mild deceleration rates allowed hybrid cars' regenerative braking systems to absorb more energy and store it in the batteries. The sharper deceleration rates now specified require more liberal use of the brake pedal, leaving less braking energy for the regenerative system to capture.

Some examples

Most cars on the market today are 2007 models, and we won't see their 2008 gas mileage numbers until September or October. Below are the current EPA ratings of some 2007 models, along with predictions of what their 2008 ratings will be under the new method. Of course these manufacturers might make vehicle improvements for 2008, so don't read too much into these figures.

Do you have one of these cars? Does your experience more closely match the "new" ratings? For the majority of owners, it should.

At this point it's important to reemphasize that the cars themselves have not become less economical. Sure, the 2008 ratings are lower, but the real-world performance that owners experience should be the same as with an equivalent 2007 model. The intent of this EPA revamp is to bring the ratings closer to reality.

One of the first 2008 models for which we have EPA ratings is the all-new Porsche Cayenne. A redesigned V6 engine has led to significant improvements in fuel economy for 2008, yet the new EPA mpg calculation method obscures that fact. Therefore, we've included figures showing what the 2008 engine would have produced under the old mpg rating system.

* Not on the window sticker** This early-release vehicle has no 2007 model.

The above examples illustrate the difficulties consumers will face when comparing 2007-and-earlier models against early-release 2008 vehicles such as the Cayenne.

The rating is the hardest part

About a year from now, when all the '07 models have been sold off, all window stickers and car advertising claims will be based on the new method and shoppers will again enjoy an apples-to-apples situation. But for the next 12 months or so, car buyers will need to do some arithmetic to keep things straight.

Have they gone far enough? Will your actual fuel economy now match the EPA label on your 2008 machine? Time will tell. There are still a lot of different driving conditions and driving styles across the breadth of this big country of ours. One system can hardly be expected to predict all of them with precision.

For now, two things are clear: 1) The new 2008 fuel economy ratings are a big step in the right direction, and; 2) Your mileage may still vary.