How Much Horsepower Required to Cruise Comfortably at 70 MPH?

Garry in AZ said

Yes there is a formula. I had to learn this for a Bonneville project I just worked on. You need to know the vehicles coefficient of drag, (Cd) the frontal area, rolling resistance on pavement, and total vehicle weight. Once you have those values the amount of horsepower needed to achieve a given speed can be calculated. The horsepower is independent of rpm and gear ratios. This is simply the power required to do the work involved.

For example, using approximated values, if your car has a Cd of around 0.44, and a frontal area of roughly 15 square feet, a weight of 4000 pounds, it will take 12.37 HP to overcome the air resistance, and 9 HP to overcome the rolling resistance. This means that your car needs approximately 21.4 HP to maintain 65 mph on level ground, in calm air. RPM and gear ratios only come into play if you can't make the required power.

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Oh! A Big huge like and agree for this post!
I was going to say less than 30hp. I don’t have any equations in front of me. Not even sure where to find them to be honest.

What I have found out is that the amount of power required to roll the car against the aero dynamic frontal are of the vehicle vs. the wind resistance is actually not a heck of a lot. This is also seen (not needing a lot of power) when a Prius strolls on by. While the car has a Aero dynamic adge, it still has to produce some power to roll down the Hwy at 60/70mph. And they don’t have jack for HP!

Also remember that if you dyno’d the engine, the power numbers are at wide open throttle. You don’t cruise at wide open throttle. Your carb is only opened up a little bit. Maybe, a tenth of full opening on the primary’s. So if your dyno read out said, oh, 320hp @ 3000 and your traveling @ 70mph, your not cruising at 300hp, your probably producing 40/50 hp with the closed up carb.

Are you worried about a powerful get up and go feeling? This can be addressed with more gear ratio. To high of a rpm on cruise? Add an overdrive transmission. Spinning tires to easy now? Add wider tires... LMAO...