Anyone who has paid close attention to the EPA ratings–or the real-world fuel economy figures on TrueDelta, has probably noticed that compact cars don’t tend to get much fuel economy on the highway than midsize or even large cars.

Take Toyota’s offerings, for example: 

Yaris – subcompact:  29/35

Corolla – compact: 27/35

Camry – midsize: 21/31

Camry V6 – midsize: 19/28

Avalon – fullsize: 19/28

So, the tiny, 106-horsepower Yaris manages to go only 25 percent farther on a gallon of gas than the large, semi-luxurious 268-horsepower Avalon. Compared to the also much larger, 158-horsepower Camry, the subcompact’s advantage is just 13 percent.

What gives? Well, aside from the fact that the main fuel economy hit of additional weight is getting that weight up to speed, not keeping it there, gearing makes a huge difference. Smaller cars usually have fewer gear ratios in their transmissions–four vs. give or six in this case. And to compensate for their relative lack of power the overall gear ratios (gear ratio multiplied by final drive ratio) in these cars are usually shorter (numerically higher). As a result, the engines in these cars turn more rpm on the highway. The EPA highway test includes a maximum simulated speed of 60 MPH. Even at this speed these smaller engines often turn around 3,000 rpm, while the larger engines when paired with taller gearing are spinning in the low 2000s. Engine efficiency starts to dive around 2,500 rpm.

Drive the 70ish speeds common on today’s highways, and the real-world difference likely shrinks even more. My 2.0-liter Mazda Protege5 gets 27 to 28 on the highway–because the speed limit where I live is 70. I’ve read that the same car, driven at a constant 55, gets close to 40 miles-per-gallon.

The obvious solution: transmissions with six-plus speeds for smaller cars. No doubt they’re coming. Each gear ratio adds about $100 to the price of a car, and given our recent experience with fuel prices many people should be ready to spend another $200 for a five-to-seven MPG bump in highway fuel economy.

Thanks to Matt K of The Auto Writer for asking the question that led to this post.