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Comment by mrfusion | Hacker News Reader

I thought the optimum speed was 45 or 50?

~55 mph is the speed where aerodynamic drag starts to exceed mechanical friction, becoming the primary negative force and it exponentially increases at higher speeds.

Drag increases quadratically with speed.

Going too slow hurts mileage too right? I’m not sure how exactly.

No. There is no speed* such that going slower will decrease mileage.

* Unless we're talking about some sort of pathological edge case in a vehicle without a properly specced transmission or something, or driving an automatic so slow it's just idling against the torque converter or something.

I guess for modern vehicles though you want to be in top gear. Otherwise it’s more revolutions of the engine for a given distance. So the minimum speed to get into top gear.

Revolutions and fuel consumption don't really correlate. It's not like the engine injects a fixed amount of fuel every cycle.

When acceleration isn't happening (i.e travelling at a constant speed under constant load), it largely does correlate.

It's not a straight line. Brake Specfic Fuel Consumption (gal/hp-hr) is often higher in the lower 3rd of the rev band. Across the bulk of the RPM range the BFSC curves are basically flat for most engine. Throttle position matters much more than RPM.

No. I checked multiple sources with actual test results. Most cars get peak MPG at over 25 MPH.

For traditional ICE cars, the amount of gas you burn is largely correlated to the RPM's of the engine when acceleration isn't happening.

Since ICE cars have gears, you'll have worse MPG efficiency while idling in 1st gear going 3 MPH, when compared to idling in a higher gear at a faster MPH. Most ICE cars can almost (but not quite) idle in a high gear at around 40MPH. So yes, going too slow can hurt gas mileage.

Its a different matter for hybrids and EVs though...

My car can get best fuel efficiency when I drive between 40-60km/hour with constant cruising speed and no sudden acceleration. I am wasting a lot of gasoline in downtown traffic jam, speed below 20km/hour.

The optimum speed can't be higher than 6mph, any faster and I'd ask you to leave my living room.

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