Quote: Originally posted by blink172 on 28 August 2005
Personally, 6 gears is good as it drops the revs when cruising at 70-90mph, meaning it will be more economic and quieter.
not always really true. quaieter but perhaps not as economic. the figurs below are just examples to illustrate the point, plucked out of my arse.
to maintain 70mph may take (plucks figures out fo the air) 20bhp.. there is no way round this. to do 70mph you need 20bhp at the wheels. it doesnt matter how the power gets there (e.g. via the 6th gear or 5th).
say that you are at 4000rpm at this speed and the max power at 4000rpm is 40bhp. to maintain 70mph you have the foot 50% on the throttle.
now mr new clio owner is in 6th gear at 3000rpm, however at 3000rpm the max power output is say 30bhp. now you will need the throttle at more like 66%.
the engine isnt pumping as quickly but each roatation sucks in bigger gulps of fuel. so to maintain your 70mph you may be using a less efficient part of the rev-range and using more fuel.
all this info is taken from an article about motorbikes in last weeks MCN but im sure the physics applies to cars too.
EDIT: saying all that i suppose the point is they pick the gear ratios so that the blaance between gears makes it worthwile acceleration and top end cruising.
[Edited by entjm on 30 August 2005 at 9:22pm]