6 Driving Tactics to Save Gas This Summer

Gas is near $4 per gallon, but you don't need to buy a new car to get better mileage on your road trip this summer. PM put fuel-sipping advice to the test by outfitting an ordinary ten-year-old car with an extremely accurate fuel economy gauge and trying out all the gas-saving driving tactics we could think of. We bring you the six strategies that work, plus more quick tips for better MPG.

Tactic No. 1: Coast to a Stop

Brakes are necessary (duh!), but they're inherently wasteful: They take the kinetic energy of a moving car—energy it took pricey gasoline to generate—and turn it into heat that's lost to the air. Everyone knows that accelerating until the last moment then braking hard to stop is less efficient than slowly coasting to a red light. But PM's test data (illustrated below) prove what a huge difference coasting makes. The lesson: Whenever possible, anticipate that a light will turn red and ease off the gas. Generally, the less you have to brake, the better your fuel economy.

Tactic No. 2: Avoid Slowly Crawling Up to Speed

Conventional wisdom says that jackrabbit starts consume more fuel. But it turns out that nursing your speed up to the limit too slowly also lowers mpg. How can that be? Cars get poorer fuel economy in lower gears, and accelerating too slowly prevents upshifting at an efficient rate. The best acceleration rate varies with the vehicle, gear ratios and weight. But in our testing we found that taking 15 seconds to accelerate to 50 mph used less fuel than taking 30 seconds to reach the same speed, because the car entered its top, fuel-​saving gear sooner.

Tactic No. 3: Close Windows and Use a/c at High Speeds

It's a fierce efficiency debate: Open the windows in summer to avoid running your energy-intensive air conditioner, or keep the windows closed and the a/c on to preserve your car's aerodynamic profile. (We'll leave aside the option of sweating it out.) PM's testing settled the issue. Driving at 55 mph with the a/c running, we got 24 mpg; turning it off bumped us up to 28 mpg. Then we opened all four windows, one at a time, and lost 1 mpg per window until we were back at 24 mpg. So at that speed, it's a wash. But aerodynamic drag rises exponentially with speed­—the faster you go, the more the open windows hurt efficiency. The answer? Below 55 mph, open the windows and leave the a/c off. But at 60 mph or higher, keeping them closed and the air conditioning running will burn less fuel.

Fuel-Sipping Basics

Monitor Tire Pressure
Keep your tires properly inflated, because low pressure increases rolling resistance. Few drivers check and adjust their tire pressure often, but it's a good idea to do it once a week.

Plan Errands Carefully
Reduce the miles you drive by running all your errands in one trip. Making a run to the dry cleaner and then picking up the kids after soccer practice? Don't make separate outings. A little bit of foresight will stretch your fuel economy.

Warm Up the Engine
Cars get better fuel economy when the engine is warm. So if you have a three-stop run, hit the farthest destination first, then work your way back home. A fully warmed-up engine will remain at an efficient temperature even if it's parked for 30 minutes.

Make Right Turns Only
FedEx does it, and the MythBusters proved it works: When city driving, make as many right turns as possible, even if it means going a few hundred yards out of the way. Reducing loiter time—or idling while waiting for traffic to clear—saves gas.

Avoid Ethanol When Possible
Gasoline that has been cut with 10 or 15 percent ethanol, called E10 or E15, is an mpg killer. Why? Gasoline stores more energy than ethanol (119,000 Btu per gallon vs 80,000). So it takes more ethanol than gasoline to go the same distance.

Tactic No. 4: Cruise at a Slower Speed

Since the power required to overcome aerodynamic drag is a function of the velocity cubed (in other words, it shoots up quickly), a car's jump from 40 to 60 mph requires less fuel than the increase from 60 to 80 mph. (As the graph above shows, the hit to fuel efficiency is roughly twice as severe in the higher range.) So go slower, right? Well, yeah, but fuel efficiency isn't the only thing that matters. Some studies suggest that the old 55-mph limit saved fuel but cost us more in terms of lost work hours. Then there's safety: Going 55 mph when traffic is cruising at 70 can be dangerous to everyone. Just don't go 80. That will drain your tank quickly—and the costs add up if you also have to pay for a speeding ticket.

Tactic No. 5: Climb Slowly (When It's Safe)

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.

PopMech Test Tools

To develop the best tactics for fuel-efficient driving, we instrumented our car, then hit the road. To settle the long-standing question of whether it's better to coast along in neutral or in gear, we tapped into the fuel injection harness on the engine, hooking up to an oscilloscope to capture the opening and closing of the injector. For most of the other tests, we used Palmer Performance Engineering's DashCommand and ScanXL Pro software running on a generic Windows netbook computer, with Palmer's cable plugged into the OBD-II port under the dash.