The new Consumer Reports has a write-up on this very topic. They
claim any gasoline engine when going faster than 60 mph uses up far
more gas than maintaining speed at 60 mph. The flaw I find here is
that they don't mention at what RPMs this 60 mph vehicle's engine is
running. Differing gear ratios can make all the difference and should
I have a 6-speed, I'm damn well going faster than 60! Back to
reality, I dont have a 6-speed but I've had plenty of sticks that vary
widely between speed/rpm ratios. Maybe I need to re-read as they may
be reffering to an auto but I'd thkn the same idea applies.
I would take Consumer Reports with a grain of salt.
Consider the following on their web page, amusingly dated Oct 2005:
The problem with new-car models
Think twice about buying a newly designed model
in its first year
When a new model is introduced, the media buzz and marketing
hoopla can tempt you to be the first on your block
to drive one home. But this can cost you in
dollars, time, and hassle.
I have owned a few new cars, some in the first year of a new model
and some in the last year. The car that gave me the most problems
was in its last year of production. The two cars that gave me
the least aggravation were brand spanking new to the market:
a 1982 Accord sedan, and the 6th Acura ever to be delivered in my
city.
Since wind drag increases with the square of speed, it becomes
significant more quickly. Stick your hand out the window. At
50 mph you can start to feel something. By 70 you have to be
careful how you shape your hand. By 120 you'll have trouble
doing much with it. Below 50, the gas engine isn't very efficient
because it hasn't been designed to be. It has too much available
power. You only use about 15hp in the average small car at 60 mph
to maintain a constant speed.
But to state that going over 60 is far more wasteful is too vague
a generalization. How about 62? Far better advice is to suggest
people travel at a speed that is close to normal speed on that
road for safety, but as slowly as practical for best efficiency.
I've done many long trips where the speed during the day was
fairly constant. At about 50 mph I get 40% better than the
overall yearly combined mpg, and about 27% better at 70 mph.
Different engines are designed to run at different speeds,
so unless you know the engine design, the rpm isn't going
to tell you all that much. A corvette running at 1600 rpm
at 60 mph eats more than my car at 3000 rpm at 60.
The number of gears doesn't tell you much either, since most
6 speed transmissions, auto or manual, are designed to
be more closely spaced ratios than a 5 speed to keep the engine
in the power band. The top gear is often virtually the same
ratio as it would be in a 5 speed or even a 4 speed auto.
Maybe one thing we'll see on newer cars is a taller top
gear for fuel efficiency with much lower revs at highway speeds.
I would assume most manufacturers
don't like to put too high a ratio because it makes the car
rather slow to respond. Ideally there should be almost no power
left to accelerate the car - you should be using almost
full throttle to keep a constant speed.
