Imagine driving on a flat highway and approaching an overpass. From a fuel-efficiency standpoint, the best strategy is to turn off cruise control and forget about maintaining a constant speed up and down both sides of the grade. The theory predicts that, and our data prove it. The physics work like this: Lifting off the accelerator while traveling up the hill and allowing your speed to decay trades some kinetic energy (related to speed) for potential energy (related to the car’s tendency to roll downhill). You regain the kinetic energy—and get better gas mileage—on the backside. While hypermilers—who are obsessed with getting the best possible gas mileage—claim significant economy benefits from this technique, our results showed only modest gains. Two things did happen, though: (1) We drew the wrath of a lot of drivers following us, as evidenced by their single-​finger salutes; (2) We were nearly sideswiped by an impatient 18-wheeler. Yes, the method does work. But we’ll save it for lightly traveled roads.

Tactic No. 6: When Coasting Downhill, Leave the Car in Gear

There are those who refuse to be shaken from the practice of coasting downhill in neutral to save gas. This is a bad idea no matter how you look at it. Let’s set aside fuel economy for a moment. Coasting downhill in neutral is illegal in most states. And it’s dangerous in all states. In neutral, you have no way to accelerate to avoid a hazard, and if the engine stalls, you have no power steering or vacuum boost for the brakes. If the hill is steep enough to call for hitting the brakes to keep you from gaining speed, they’re more likely to overheat—and overheated brakes lose effectiveness until they cool off. They’ll probably do that right around the time the police show up to take the accident report.

Here’s the surprise: There’s no tradeoff between safety and fuel economy in this case. Leaving the car in gear while coasting downhill actually is more efficient. Why?

Most fuel-injected engines today use computer-controlled Deceleration Fuel Cut Off: When you lift your foot from the gas while leaving the car in gear, injectors shut off automatically, and the car’s rotating tires—which are connected to the engine via the transmission—keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

In contrast, the fuel-consumption rate for an engine idling in neutral falls between 0.2 and 0.4 gallons per hour (gph). Splitting the difference and using 0.3 gph for our example, idling in neutral down a ½-mile-long hill consumes fuel for 30 seconds, for a total of about 0.32 ounces of gas. Popping the car into neutral actually wastes gas.

This may seem counterintuitive, but that’s what data are for—replacing good guesses with solid facts. Watch the data, and over time the savings will take care of itself.