6 Driving Tactics to Save Gas This Summer
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 above) 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 up-shifting 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.
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. (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 hyper-milers—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.