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Question
Does a car use the same amount of gas traveling a given distance at 60 mph than it does traveling the same distance at 10 mph (or any other speed)? And if so, does a person burn the same amount of calories walking a given distance as running it?
Asked by: Bob Darcy
Answer
With respect to automobiles, there are several factors affecting mileage. Transmission
gearing is one. Lower gears require the engine to turn more times than higher gears
for the same distance traveled, consuming more fuel per mile. At higher speeds, air
drag supplies a force that must be counteracted by the engine, again consuming more fuel.
(Drag, in fact, increases with the SQUARE of speed so that resistance increases four times
every time speed doubles.) Engine design can also determine the most efficient RPM for fuel
consumption. Given all of that, yes, speed DOES impact your car's fuel efficiency.
(Remember the original reason for a nationwide speed limit of 55 mph--conserving gasoline?)
Optimum fuel efficiency should be experienced somewhere just above the speed at which the
highest gear becomes useable.
The human body, on the other hand, is not affected by the same factors. Legs have only
one 'gear', and only very fast runners should experience air drag of any significance.
Except for the different mechanics used in walking vs. running, therefore, and how that
might affect muscle efficiency, I would not expect speed to significantly impact the
number of calories burned per mile.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
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