How do RPM, MPG, and rotational force relate?

Question

My 6 Cylinder manual transmission sports car tacs out at 2.4K - 2.6K RPM when I'm driving 60 mph in 4th gear. If I speed up to 70 mph, it will tac out at about 2.1K RPM in 5th gear. ( I can't shift into 5th without straining the transmission until I get going near 70.)

Am I saving gas going 70mpg rather than 60mpg? How does the torque play into this?

Since it's an aerodynamic design, I'm considering that wind resistance is minimally different at the higher speeds, but correct me if I'm wrong about that too. Thanks in advance.

Answer 1

This is a good question.

For cruising at a constant speed, the only forces the engine needs to overcome is friction in the drivetrain, friction between the tires and the road, and wind resistance. At higher speeds, wind resistance dwarfs the other two and easily become the dominant factor. At the speeds you're talking about this will still be true even if it's a sports car with good aerodynamics.

The reason going fast is bad for MPG is because the wind resistance increases with the square of the speed. So, going from 60-70MPH is like increasing from 6 to 7, or 17%, but that's like increasing the wind resistance from 36 to 49, which is a 36% increase.

    So, you increase the rate you cover distances by 17%, but you're needing to overcome a resistance that's 36% greater to do it. That's not a good trade off, and that makes it less efficient.

I see what you're saying about the RPMs though, and that is true, but I just think that in this case the increased wind resistance trumps the fuel savings you get from operating at a slightly lower RPM.  

You could do a test. If you ever are on a long interstate drive, you could cruise in 4th at 60MPH for a while and then fill up and wite down the MPG, then do the same in 5th at 70MPH.

Answer 2

Torque curve on the internal combustion engine is not linear. It is related strongly to the engine RPM. Some engines top out at say between 4,000-5,000 rpm.  The less RPM  the less fuel the engine consumes. 2.6 K is a bit high to cruise on. Under 2 K is just fine. Most of the cars a re build to cruise at 55-60mph.  

So perhaps you saving fuel by going in the 5-th gear at 70 then cruising in the 4-th gear at 60mph.

And yes aerodynamic design does reduce torque demand on the engine. In general power is a product of force and velocity

P=Fv

and torque is related to force as

T= r x F (x is a for vector cross product)

r is the radius of the tire or the engine  flywheel, depending on where you measure your torque.