When a gasoline engine runs, does it use the same amount of gasoline per stroke in each cylinder, or will higher speeds change the amount of gas used?
As an example, straight level ground, cruising happily at:
35mph @ 2000rpm
65mph @ 2000rpm
Will the same amount of gas be used over the same time period (since each piston is going up and down at the same rate), or will different amounts be used since you’d be in a lower gear and/or at a different speed?
Hope I explained that well enough…
Your engine is using more fuel at 65 mph for one main reason - it takes more power to maintain the higher speed than the lower speed. By far the main thing that you are fighting here is the aerodynamic drag of the car, which roughly increases as the square of the velocity increase. The piston may be going up and down at the same rate, but your throttle position is different, and thus at 65 mph the cylinder is filling with more fuel/air mixture.
Another intersting question is what happens if you are maintaining a constant speed, but in different gears. In my Mustang:
65 mph in 3rd = about 3300 rpm
65 mph in 4th = about 2400 rpm
65 mph in 5th = about 1700 rpm
(if I recall correctly)
In this case, as in that of most all cars, the 5th gear option will end up saving more gas for the reason of less geartrain loss and the engine is operating in a more efficient portion of it’s power/torque curve.
Anthracite–nothing I have so say here is intended to question your answer in anyway, but you said something that made me think (a dangerous thing, for me 
). I recall being told years ago that aerodynamic drag increases with the cube of the velocity. I have no recollection of the reasoning behind using the cube–the square makes more sense to me at the moment–but maybe somebody can reason it out for me.
Back more to the OP–how does the engine know to use more gas per cycle at higher speeds? If I built a car it would be too dumb to have a clue!
So Far So Good asks,
Actually, you’re the one that controls how much gasoline is used per cycle. That’s what your accelerator pedal is for. Stomp on the pedal ->more fuel in the engine ->more torque output ->accelerating the car ->higher speed. So if you don’t use the correct amount of fuel (i.e., have your foot in the right place), the car will slow down/speed up.
SFSG, the drag force goes up as the square of velocity, but the power required to overcome said force goes up as velocity cubed. For most practical purposes, the power is what we care about, rather than the actual force - that’s where I feel the confusion comes from.
Basically, that comes from the definition of power:
Power = Force * Velocity
This got talked about in more detail in this thread (http://boards.straightdope.com/sdmb/showthread.php?threadid=28114) a few weeks ago. Check out maybe 2/3 of the way down.
Thanks zut–I must have been thinking with my foot instead of my head. And also thanks brad_d, the cubed part is clearer now.