How is it much more fuel efficient to travel at say 50 mph for longer than 70 mph for a shorter period of time. Like if I was going a set distance, if I drive slower I’m using the car longer so why isn’t it the same fuel being used.
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Basic physics come into play. The faster you go the more drag (wind hitting your car) your car has to overcome. It goes up exponentially, so the difference between 50 and 60mph’s wind drag isn’t 20%, it’s substantially more (sane with 60 to 70). You can test this by rolling down your window and feeling the drag on your hand at different speeds.
There is also engine efficiency – your car was built to go specific speeds. Some older cars that might be 30 or 40 mph (think a 4 gear VW van). Racing cars may run most efficiently at much higher speeds with great aerodynamics and larger gears.
The reason most cars max out at 55mph for efficiency (or around there) is because there is a complicated formula used to establish MPG advertised on cars. If you hit your max MPG at 55 you can actually maximize the rating you get. This goes back to cars being designed to drive on highways/roads in an era where 55mph was a pretty reasonably fast speed.
However, your car may vary where it does best. Tracking mileage (or using a MPG display) can help you figure out where you car runs best. My friends 1990 volvo shook at 55-65 and for some reason burned gas faster than 50 or 70. So, 70 was ideal (though that seemed really fast in a tin box). My 2001 VW Jetta ran best at 65 exactly. My 2014 impreza does best around 55-60. The 2000 Jeep Cherokee Sport topped at 50ish I think (gas was cheap back then so I didn’t track it as closely).
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