does driving at a constant speed (pressing the accelerator constantly but with varying amounts) give better gas mileage or does getting to a certain speed and coasting for a while before pressing the accelerator do better? Why?

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I feel like the first one should do better, but I don’t get why, and while I’m driving I keep thinking the second version will somehow end up better – but how do hills affect this?

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Increasing speed uses substantially more gas than maintaining speed. Therefore, constant-speed driving is more efficient than coasting/accelerating. However, the most efficient is to put the accelerator at one position and hold it. Speed will decrease going up hills and increase going down.

Staying constant is going to be the best bet all other things removed.

In an automatic vehicle, Losses in the drivetrain and increased engine load make coasting and accelerating less efficient.

In a standard, slowing down causes the engine to lug, forcing the computer to put more fuel into the engine to keep it going. If you shift into a lower gear, you negate any winnings you may have got by going slow at the time.

Alternatively you can accelerate up to a speed, shift into neutral, and coast for a while with the engine at idle. Some jurisdictions have made operating a vehicle not engaged in a gear illegal, as it is unsafe and may make a vehicle less manoeuvrable. IANAL your gas mileage may vary.

Generally you save the most fuel by accelerating normally and evenly and keeping a constant speed. Not by floring it everywhere.

Driving consistently will give the best results and finding the fastest speed you can travel in the highest gear with the lowest rpm (speed of the engine)

In your second scenario it sounds like you’re describing [Pulse and Glide ](https://en.m.wikipedia.org/wiki/Energy-efficient_driving#Pulse_and_glide) which has been shown to be more efficient than maintaining a steady speed.

The reason being is that gas engines are most efficient at wide open throttle.

The more consistent you are with the pedal, the better mileage you get. This is why cruise control is recommended during long drives.

Your engine has a specific set of conditions to be in for maximum efficiency. The transmission tries to keep it in this set of conditions. By accelerating more or less, the engine is forced to leave this efficiency zone, so it is less efficient when you’re accelerating *and 100% inefficient when coasting*. The result is a significant drop in gas mileage.