# HP to maintain speed

Does anyone have an approximate figure for how much power it takes to maintain a modern sedan at cruising speed, say 65 mph?

I suspect you’ll get a lot of replies philosophizing about gear ratios, tire size, weight, wind, etc…

Personally, I have no clue, but a quick google search pulled up this:
http://www.insightcentral.net/community/viewtopic.php?f=4&p=66139

It all seems to revolve around the Honda Insight, but there also seems to be some good math formulas concerning your question that I suspect are applicable to most any vehicle.

As I remember from some rough calculations, I figured about 40 to 60 horsepower. A lot has to do with the frontal area of the car; its shape; how high it sits above the road; and devices available to keep the air passing through the engine and under the car to a minimum. I will respond later with the formula for translating MPG into average horsepower.

Depending on the vehicle, 17 to 25 hp will maintain 65mph, flat and level, no wind.

That sound about right, it takes surprisingly little power to cruise at a constant speed on level ground. I have a 240D (my wife’s car) that weighs about 3500 pounds and is about as aerodynamic as a brick; the car has a total of about 70 HP and will very easily cruise at 65 mph (at about half throttle). A newer, more aerodynamic car would require much less power.

Thanks, those are the kind of answers I was looking for. I probably did not use good search terms for Google. I was guessing 20-30 hp with no real basis beyond the throttle position.

The reason I asked is because I had a short discussion with a colleague about natural gas consumption in a whole-house generator. I said that if the gas was compressed, you could store enough natural gas to power it for quite a while since CNG cars will run for quite a few miles. He tried to tell me the generator takes a lot more power to run the generator than to cruise in an auto. I differed, but I need a little more info.

16 KW gen should take less than 25 hp to run = I win.

Before anyone chimes in, I know that CNG cars have a more limited range than gasoline power. I am willing to accept other criticism of my assumptions and reasoning.

Yup, 16 kW is about 21.5 HP, considering loses it still should be less than 25 HP; you win.