Yet another reason why you can't always get that great gas mileage

It's funny what happens when gas prices go up. All of a sudden, that new generation automobile - the same make and model one you have - now gets better mileage. Especially on the highway. Did it go hybrid? No. Is the engine smaller? Oh no, they'll never do that! Wow, something great must have happened right? After all, necessity is the mother of invention and soaring gas prices sure make better mileage a necessity. At least in the marketing brochures.

A lot has been said about the deficiencies of the existing EPA mileage rating system. Among the known problems - they don't represent real-world driving conditions where accelerations are quicker for instance. The EPA changed the measurement system once recently when mileage ratings for hybrids came out looking a whole lot better than reality. They're supposed to be looking at them again.

For me, the mileage I got used to be almost spot on with that on the window sticker. Not anymore. What changed? Was it the car or the testing? For me, the answer is a little of both. On the highway, my Civic can get upwards of 39 mpg or as low as 30 mpg; depending on which highway. What's the difference?

It all starts with where you're driving. To measure mileage, the testing is done on a dynamometer. The car itself doesn't go anywhere no matter how fast or slow the wheels are turning. And most important of all, it's on level ground. We all know that climbing a hill is harder work than walking on level ground; well the dynamometer only knows level ground. When I'm cruising down I-95 in the Carolinas, that's flat ground. When I'm traveling through the Berkshires, that's not. In my previous cars, this hardly mattered. The testing was no different - still on a dyno but the key is that my transmission never changed gears - it stayed in top gear.

To improve mileage, automakers have been adding gears. During the 80's and 90's a typical manual transmission had 5 forward speeds and an automatic had 4. In the US, automatics far outsell manuals so what has happened in recent years is that a 5th and sometimes 6th forward gear has been added. Talk about overdrive! Take the Hundai Sonata for instance. Starting with the 2011 model, the new 6 speed tranny bumped the highway mileage estimate from 32 in the 2010 model to 35. True, these are very different models - a new "generation" Sonata but the engine is the same and so is the rough overall weight. And it's not just Hundai of course. Just about every manufacturer across the board has done so.

Don't get me wrong, this is a good thing. If anything, it's too bad they waited until gas prices soared before adding more gears. But there's a potential downside. If you live or commute through roads that are not table-top flat, that great mileage suddenly looks deceiving. What happens is that the last gear - the one added to make the highway mileage spec look better, only works on relatively level ground. Put a load on the engine - like say going up a hill, and you drop a gear. Here in New England, there's hills everywhere and that means you can expect to shift up and down a lot even on the highway. Don't expect to hold top gear. And because that top-notch mileage number is the result of a dyno test, it bears little resemblance to hills. It knows nothing about downshifting on the highway. Testing and reality just don't see eye-to-eye.

But wait, if the top gear was added for mileage reasons, and you drop out of it, doesn't that mean that mileage should be at least as good as the previous model without the extra gear? One would think so, but that's not quite right either. The problem is that when a new gear is added, the other gears are often moved a bit. So the 4th gear out of 5 is not where it used to be when there were only 4 gears and so the mileage in 4th gear is lower when downshifted than when it's the top gear.

So the overall effect is both one of the car's design and that of the testing conditions. Again, there's nothing wrong with the better highway numbers - drivers living in areas of flat terrain certainly benefit from it all. But it'd be nice if the EPA could also provide another number that shows what happens in less than ideal highway driving conditions so that consumers are better informed, and have more accurate expectations, at the time of purchase.