"chaniarts" wrote in message
...


Well... That's really what the overdrive is for. Not below 30mph (I
think most cars kick them in around 50-55mph), but to reduce engine
RPM at high speed. And actually many engines are supposed to be most
efficient running around 3/4 throttle.

Puckdropper

3/4 throttle without specifying the gear doesn't mean anything. 3/4 throttle
in overdrive gear in my car is probably 130mph, which probably isn't it's
most fuel efficient.

The transmission gear is irrelevant when specifying engine efficiency. The
engine is more efficient at a throttle plate opening of 1/2 to 3/4. Below this
opening the pumping losses around the plate go way up. Above this and
the mixture gets richer on carbureted engines, FI may be different.

Back in the 70's one of the car mags ran an article on a BMW study of
the most efficient acceleration method. The end result was to accelerate
at 1/2 throttle opening up to 2,000 rpm and upshift. This is repeated until the
desired speed is reached then use the highest gear, without lugging the
engine, and the appropriate throttle opening. Their data showed that small
throttle openings caused low efficiency, and mpg, due to the high pumping
losses. Long sloooow acceleration times were almost as bad a jack rabbit
start.
Art