Does Cruise Control Save Gas? AC On or Windows Down? We Tackle These and Other Fuel-Saving Questions. - Motor Trend

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Is cruise control better or worse for gas mileage? Does running the A/C or opening the windows use more gas? And what about those fuel line magnets and tornado gizmos for the intake and exhaust pipes—do they work? No matter the price of gasoline, we owe it to Mother Earth to do what we can to go as far as possible on as little gas as is practical, so let's dig into these questions and more. Oh, and let's just lay out this spoiler out at the top: To date, scientific testing has yet to measure a meaningful benefit to fuel line magnets, aftermarket intake/exhaust swirl inducers, or "gas pills" as I explained in this Technologue column. (If any of these "technologies" worked, automakers would be incorporating them on new vehicles.)

Does Cruise Control Save Gas?

Absolutely. If you're cruising along a mostly level highway, electronics generally are far better at avoiding the little speed-up/slow-down events that are inevitably caused by a weary accelerator ankle or an inattentive mind. It's those changes in momentum that waste fuel, because force equals mass times acceleration. Using cruise control is beneficial on mostly level ground, as well as on long, steady uphill or downhill grades. On highways traversing rolling hills, however, you're better off targeting an average speed and then allowing the vehicle to slow down by 5 or 10 mph when climbing and then rise by a similar amount during the descent. Note that most of the gas-saving hints here also apply to extending range in an electric vehicle, but in this case, because much of the energy an electric car expends climbing a hill gets recovered by regeneration on the way back down, feel free to leave the cruise on in the hills, as well. (We first verified this theory on an early Tesla Model S drive from L.A. to Vegas.)

How much fuel does cruise control save?

How jerky is your ankle? A Natural Resources Canada study found that setting the cruise control at 80 kph (49.7 mph) versus cycling from 75 to 85 kph (46.6 to 52.8 mph) every 18 seconds consumes 20 percent less fuel (and dramatically reduces motion sickness). Studies by CNN/Money and others quote more modest claims in single-digit percentages.

Is cruise control bad for your car?

Absolutely not. Acceleration that costs fuel also causes wear and tear on the driveline, so cruise control is good for the car. BIG CAVEAT: Cruise control should not be used in heavy rain, snow, or at temperatures where ice could form, because any acceleration on slick surfaces or while hydroplaning can induce an under- or oversteer event, and/or a resulting stab at the brake pedal to unset the cruise control can result in loss of control and an accident.

Does A/C or open windows use more gas?

The engineer's answer is that the energy required to overcome aerodynamic drag varies with the cube of the vehicle's speed, so for every vehicle there will be a particular speed at which opening the windows causes more drag and energy use than powering the air-conditioning compressor would. On older, less aerodynamic vehicles with primitive A/C systems, it's generally best to lower the windows at city speeds and run the air at highway speeds. But new vehicles feature highly efficient climate-control systems, employing variable-output compressors, humidity sensors, and in some instances heat-pump technology. Newer vehicles also tend to have more aerodynamic body shapes that are compromised more by opening the windows. So if they're driving a modern vehicle, even the most avid green-niks should feel no guilt driving in climate-controlled comfort.

Should I turn off my car to save gas?

If you've clicked and scrolled this far down and your vehicle is equipped with auto-stop/start, then absolutely leave it on! Without that system and the fortified battery and starter that comes with it, it's not wise to shut off the engine every time your vehicle stops. But if you're likely to stand still for most of a minute or more—at a notoriously long red light or while waiting in a drive-thru line or for a fuel pump to open up—and you last switched the engine off several minutes ago, then by all means key off. There are smartphone apps that can help. EnLighten counts down the time remaining before a red light turns green in the cities of Portland and Eugene, Oregon; Pasadena, Arcadia, and San Jose, California; Salt Lake City and Provo, Utah; Garland, Texas, and Las Vegas, Nevada.

No-cost gas-saving tips

Without renting wind tunnel time you can easily improve your vehicle's aerodynamics by removing roof rack cross bars and mounts for occasionally carrying bikes, kayaks, skis, or luggage. If you've added those dope-looking aftermarket wings, air dams, and spoilers, removing them will boost efficiency. Without fabricating a new aluminum or carbon-fiber hood or doors, you can "lightweight" your own car by jettisoning all of those "just in case" items in your trunk and just carry a charged phone. Look farther ahead than the car in front of you to try and anticipate lights and stop signs so you can coast longer and brake less. And of course the most effective, least heeded or appreciated advice is to simply drive slower (see the first line of the paragraph above).