the car companys usually quote a car doing x amount of miles at xxmph this is usally not too far away from 54mph.

But then as im sure you all know mpg is all about rpm the less rpm you can do in 5th gear will be better, unless youre putting unnesasary (sp) strain on the engine, so doing 1000rpm in 5th gear could probably give worse mpg than at 2000 rpm in 5th gear,