Weird But True Facts About Fuel Economy

Jim Motavalli

Jim Motavalli | Sep 09, 2011

It’s probably no huge surprise that it matters what kind of car you drive and how you drive it, but who knew that it matters this much? We can all guess that the guy in the Prius deserves the green star, especially when compared to the Hummer driver, but according to the University of Michigan Transportation Research Institute, the best American-market choice is nine times more fuel efficient than the worst choice.

There are 282 vehicle models for sale in the U.S., so don’t get seduced by a pretty fender. Michael Sivak, a research professor at the university, told me, “Clearly, the main message is that the biggest impact you can have on the planet is selecting a car with very high mileage, but if your car is not very good you can still do a lot by following eco-driving suggestions.”

I buy that, I really do. I’m off to a good start with my 2007 Honda Fit, which gets 31 mpg combined. I’d be much worse off if I drove my 1963 Dodge Dart (exactly like the one wrecked under Tom Magliozzi’s ownership) most of the time. The fuel economy, even with the most economical 170-cubic-inch Slant Six, is probably 20 mpg downhill. And even with the engine in tune, the smog emissions would make a Ford Excursion look like a Nissan Leaf. Even the rudimentary emissions control device known as the PCV valve wasn’t seen on most cars until 1964.

Now get this. The actual on-road fuel economy for all vehicles in 2008 averaged 17.4 mpg. That looks even worse when you consider that the average was 14 mpg… in 1923. The Model T got 25 mpg in 1908. When you take trucks out of the equation, we’re currently up to a much better 23.7 mpg average (from EPA figures). Pickups (averaging 17.2 mpg) and SUV/crossovers (19.2 mpg) really drag the team down.
Now on to the eco-driving thing. The Michigan survey is valuable because it doesn’t just advise you to check your tire inflation, it tells you what you’ll gain by being vigilant about it. If your tires are down just one pound per square inch, rolling resistance goes up and fuel economy drops by three tenths of a percent. Here are a few other points:

In Tune. Fixing that emissions test failure or simply getting a tuneup for the first time in years can improve gas mileage by up to four percent. And if the “check engine light” is staying on, a possible cause is the oxygen sensor, and replacing it can improve economy “as much as 40 percent.” That’s about four times what I would have guessed.

Oil’s Well. I do change engine oil fairly often, but I never thought it had anything to do with fuel economy. But if the owner’s manual says to use 5W-30 and instead you use 10W-30 (which I just did because it was the only weight on the gas station shelf) you can lower mileage as much as two percent.

Take the Long Way Home. Sivak told me that that the shortest route may be the worst one for fuel economy—if it has lots of hills, and the alternative is flat. “The penalty for going uphill can’t be overcome by the times you go downhill,” he said. The flat route can be up to 20 percent better for fuel economy, even if it’s a little longer.

The Road Less Traveled By. Everybody knows that congestion kills fuel economy, but by up to 40 percent? Yes, it's the difference between what happens with free-flowing traffic and start-and-stop near immobility.

The Weight. Add just 100 pounds to your car’s load—all that junk in the trunk—and fuel economy drags by two percent. Smaller vehicles are more dramatically affected by adding weight. And, of course, the weight can be on you. There’s a great visual gag in the movie Goodfellas—when an overweight mobster gets out of his Cadillac, the thing must rise two inches on its springs. In 2002, the average adult was 24 pounds heavier than in 1960. Just that gain in the driver cuts fuel economy by half a percent, but if there’s four heavyweights in the car then you’re up to that two percent sacrifice.

Idle Thoughts. An hour of idling uses up to a half gallon of fuel, so you should turn the engine off if you’re going to sit for more than a minute. No, it doesn’t use huge amounts of fuel to restart the engine—just the equivalent of idling for a few seconds.

Speed Demons. In a V6-equipped 2007 Honda Accord, fuel economy peaks at 31.6 mpg at 61 mph, but it drops to 21.2 mpg at 90 mph. That’s a 33 percent difference. Use cruise control at highway speed, and make a seven percent improvement.

Air Ride. Run the air conditioner—now standard in more than 90 percent of mid-class and premium cars (it used to be a rare option)—and mileage suffers from five to 25 percent. And to address another long-term debate, aerodynamic drag means its better to turn the air off and open the windows at low speeds, but on the highway close the car up and punch that button.

Road Hogs. Wow, this is a big one. Aggressive driving has a 31 percent mileage penalty compared to moderate driving.

Let’s say you do everything wrong—aggressive driving, speeding, taking the wrong road, failing to tune the engine and never checking the tires, idling excessively and using the air conditioner too often, carrying extra weight and even using the wrong oil. A 36-mpg car will become a 19.8-mpg car. That’s sobering, isn’t it? Let’s hope you’re scared onto the straight, the narrow and the level road. After all, you pay for the gas you waste.

Here's a British eco-driving video to drive these points home. Don't forget to avoid jackrabbit starts:

Get the Car Talk Newsletter