HP to maintain speed


#1

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


#2

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.


#3

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.


#4

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


#5

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.


#6

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.


#7

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