Does driving faster really waste gas? Speed and fuel economy

Concerns & risks

You drive 60 highway miles a day at about 75 mph and wonder whether 65 would matter.

Causes & evidence

Fueleconomy.gov says that while each vehicle reaches its best fuel economy at a different speed, gas mileage usually decreases rapidly above 50 mph. It estimates that each 5 mph you drive over 50 mph is like paying an additional $0.30 per gallon for gas. It also says aggressive driving, meaning speeding, rapid acceleration and hard braking, can lower your gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. Idling can use a quarter to a half gallon of fuel per hour, depending on engine size and air conditioner use. Cruise control on flat highways helps hold a steady speed. Exact savings depend on your car, so the site's calculator lets you plug in your own speed and fuel price.

Edge cases: the $0.30-per-gallon figure is a rule of thumb from fueleconomy.gov and varies by car. EVs and hybrids also lose efficiency at high speed. Always follow posted limits and traffic conditions.

Focusing only on top speed while still accelerating hard and braking late wastes much of the gain from slowing down.

What to do

1. Note your usual cruising speed and current fuel price.
2. Use the fueleconomy.gov speed calculator for your situation.
3. Try 65 mph for two weeks, where legal and safe.
4. Accelerate gently and look ahead to coast toward stops.
5. Use cruise control on flat highway stretches.
6. Compare fill-up costs or trip computer averages before and after.

Common mistakes:

• Judging from one tank
• Ignoring hard acceleration
• Using cruise control in hilly terrain
• Idling long periods to warm up

ReportSpeedMan tip: Leaving five minutes earlier is often what makes slower driving stick; most speeding on a commute is about the clock, not the car.

What I would actually do

I'd drop from 75 to 65 for two weeks, leave five minutes earlier, use cruise control on flat stretches and compare fill-up costs.

The trade-off people miss: slower driving adds a few minutes, which some people value more than the fuel savings. That's a fair choice, but it should be a choice, not a guess.

Signs it worked

Your fill-up costs drop over several tanks.

Quick FAQ

Is 50 mph the sweet spot?
Mileage usually drops above 50; exact peak varies.

Does cruise control help?
On flat roads, yes.

Does idling waste much?
About 1/4 to 1/2 gallon per hour.

Wrap-up: Above 50 mph mileage drops quickly, roughly like $0.30 more per gallon for each extra 5 mph. Slow a bit, drive smoothly and measure your own result.

Wiki: Fuel Economy Basics and Sleep Basics and Insomnia.

Does driving faster really waste gas: where the facts come from

• Fueleconomy.gov: Driving more efficiently
• Fueleconomy.gov: Keeping your vehicle in shape