hi dodgeman.
I kind of don't understand, if you have a engine reving at 4,000rpm, compared to a engine reving at say for example 3,000rpm
in the same conditions at both the same speed on the road, the lower reving engine wouldn't drink as much juice as it's not working as hard to keep up the fuel demands of the higher reving engine.

Also the lower reving engine would not wear out as quick as the higher reving engine as there is not as much load on it.
Thanks