Is driving at 56MPH most economical due to being top of torque curve?

Question

It seems to be a well known fact that driving at 56MPH provides great fuel efficiency in 5th gear (I'm unsure of 6-speed cars).

I recently experiemented by driving between Glasgow and Edinburgh keeping at 56mph for the majority of the journey, and discovered that at this speed my revs were pretty much on par with 2500rpm - my car's maximum torque!

So I'm wondering, is there any connection between the two?

Also, my car seemed to love sitting at this 56mph/2500rpm, it didn't take much effort from me to keep it's speed steady, is this also due to being at the maximum torque at all?

Cheers!

Answer 1

Quite a lot of cars are the same, the 56mph is a mean figure for most cars where economical output equals drag, giving you best efficiency figures for fuel consumption.

Answer 2

In essence at the "top" of the torque curve you have the highest volumetric efficiency (VE):

http://en.wikipedia.org/wiki/Volumetric_...

And most cars are geared (transmission design) so that starting at 55 to 65 your CAR achieves max MPG.  After that speed the higher air resistance makes the MPG drop.  Below that speed, the lower VE also makes the MPG drop.

Good luck...