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.

Question

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.

In: 15

Answer 1

2 reasons –

1. Air resistance is substantial. It’s harder to push through air going 70 MPH then when going 50. That means the engine needs to work harder. It’s kind of like going 1 mile flat or 1 mile uphill. Same distance, harder to do.
2. Engines are not “universally” efficient. There are tons of variables that determine the efficiency of an engine and you need a multidimension chart to pin-point “optimized” sweet spots. A car manufacturer will target the 50-60 mile per hour range for optimization because that’s where most people drive (at least, it’s around the speed limit). Since 70 MPH is beyond that space, they don’t care as much and the engine might get lousy fuel efficiency at that speed.

EDIT – just because this is gaining some speed (see what I did there) I’ll add that the “multidimensional chart” involves things like car load (weight), aerodynamics, internal friction of the engine, fuel types, even air pressure. All those factors play into the engine’s efficiency so they create these wild “maps” of efficiency. If you imagine the map looking like the surface of the Earth you’ll see occasional mountains, those are the areas where the engine is performing really well. Manufacturers will then try and “move those mountains” so they sit right where the car is going to perform the most. Something like average air pressure, normal fuel, weight = 2 passengers, driving at 65 MPH in 4th gear. Or something like that.

EDIT 2 – This is why Hybrid cars can be so efficient as well. You can use the gas engine not to drive the car, but to recharge the battery, which is a really constant and specific performance on the engine. Drop the efficiency mountain right where engine needs to power the battery and you have a really high performance engine operating at maximum efficiency to reduce emissions and waste.