Mole wrote:
It doesn't work like that' I'm afraid. Your little oil burner is likely to do even better to the gallon at a constant 30 (and probably better still at a constant 20), I'm afraid!
Basically, the faster you go, the more energy you use. No amount of fiddling with engine output or gearing will change that. Some years ago, when petrol engines were more common, it was true to say that they were generally most economical at a particular speed - 50-ish, because of the dreadful inefficiency of a petrol engine at part throttle. That's not the case with diesels (and is less the case with current petrol engines than it used to be). Diesels are now so efficient that they're remarkably linear. Beyond about 20 or so, the faster you go, the more fuel you use - end of!
At 20 - doubt it, as 30 in 5th is difficult to maintain unless a totally flat road . That was my point about increase of power and an extra gear. Increase the power, but use only enough to take advantage of the extra gear. I.E INCREASE EFFICIENCY .
Something that a trip up north seemed to make sense. Last trip I hired an Insignia, 2.0L, 130BHP, six speed From memory, I possibly got a bit more on the wrong side of 70 than usual, but I stopped to refuel in Glasgow at usual place ,and got a pleasant surprise at the pumps. A lot less fuel used .
On some runs home at night ( not something I do regularly ,but done to keep mind occupied and me awake) was to see what conditions/speeds got me best instantaneous MPG. On the flat, lowest throttle setting did that( at times > 199) but hit a hill and it went right down . Even anticipating a hill dropped the figures. I found that a certain speed ( for mine circa 60) ,on a long trip gave me best results.