The Ideal Vehicle Speed for Best Fuel Economy
February 25, 2010 at 12:01 am Chad Upton 6 comments
The precise number varies by car and environmental conditions, but the sweet spot is generally between 40-60 mph (65-95 km/h). Most small and medium size cars get the best mileage at about 50 mph (80 km/h).
A lot of factors affect the fuel economy of your car. Some of them don’t vary much with speed, such as the resistance of the engine pumps and accessories. Other factors, like the size of the frontal area and the drag coefficient create increasing resistance with speed.
The faster you drive, the more energy is needed to overcome the aerodynamic resistance of the car. Up to 40 mph, that isn’t really even a factor. So, if you’re driving a box then your best fuel economy is likely closer to 40 mph than 50. If you’re driving a teardrop shaped eco-car, then you’re likely closer to 60 mph. Of course, engine size and other factors are involved too.
If you’re in a hurry, then going faster than 60 mph (95 km/h), presents new problems. Getting above this speed, your transmission is starting to run out of gears, so the engine has to keep turning faster and faster without getting any mechanical advantage from the transmission — that burns a lot of extra fuel. This also hurts your ability to overcome the exponentially increasing resistance — your engine can only turn so fast. But, if you’re in a small car then your fuel economy at 70 mph is likely still better than a large SUV traveling 60 mph.
If your car has a fuel economy display, be sure to take notice at many different speeds to find the best speed for your car. If your car does not have a fuel economy display and you do a lot of driving then it might be worth getting a device that will read your vehicle data and calculate fuel economy accurately. Try: Car Chip Pro ($75-85) or ScanGuage II ($160).
Broken Secrets
Written By: Chad Upton
Sources: ThinkQuest, How Stuff Works
Entry filed under: Automotive, Be Efficient, Be Frugal, Be Green, Gadgets and Toys, ProTips. Tags: auto, automobile, Automotive, car, economy, efficiency, fuel, green, obdii, odbii, pro chip, scangauge, speed.
Broken Secrets 
1.
Niel Loeb | April 17, 2010 at 11:19 am
Here are two easy ways to figure this out:
If you have a tachometer, find your best fuel efficiency by noting the speed at which the tachometer is showing the fewest RPMs. Note there will be a number of speeds at which you get this number. See the second way for the reason.
Way number two is if you don’t have a tachometer. Listen to your engine as you accelerate, immediately after each gear change, you will hear and feel the engine slow. It’s at that point the engine is turning the slowest and therefore using the least amount of fuel.
2. How I achieved 17 km/liter : Marcus Can Blog | October 1, 2011 at 4:20 am
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3.
Mr. P | December 12, 2011 at 9:02 pm
Actually the transmission can’t ‘run out of gears.’ If there were any more the car’s engine would stall or refuse to run. Imagine you pedaling on a bicycle, and every 1mph faster you go you add another higher gear. You will soon learn that @ 20mph unless your pedaling increases (you get more horsepower as your RPM increases) or you shift down it will be very difficult to pedal. Same with the engine.
You have to rev more to go faster. I will not disagree with burning more fuel. You notice that at 20mph on a bike that pedaling faster will indeed spill out calories
Most cars do get best mpg @ 40-60, I have exp.
4.
Chris | October 17, 2013 at 10:50 pm
Actually, lower RPMs doesn’t always mean better fuel economy.
In town I can shift into 5th gear at 35 to get my RPM’s down to 1200 if I wanted, But if I leave it in 4th my revs settle around 1700.
So why do I get better fuel economy at 1700 RPMs instead of 1200? My car (mustang) makes more Torque in 4th gear requiring me to depress the gas pedal less, that is where the fuel savings are.
In short, fuel economy is relative to how far the gas pedal is depressed, not the RPMs.
5.
free | December 16, 2013 at 3:13 am
This blog was… how do you say it? Relevant!! Finally I’ve found something which helped me.
Thanks a lot!
6.
Tabitha Levine | June 17, 2022 at 8:31 pm
Thanks ffor a great read