On cars with manual transmissions, when in low gear (typically 1 or 2), why does accelerating and then taking your foot off the gas make the car lurch forward with that uneven, jerking motion?

Question

Why wouldn’t the car just decelerate smoothly when you take your foot off the gas? And why does it often continue even if you step on the gas again?

In: Engineering

Answer 1

The engine is the single most inefficient part of your car. When it’s pushing the car, it is throwing away most of the energy it makes. If you stop giving it gas, it still wastes a lot of energy (though considerably less), but now that energy is coming from whatever else keeps the engine turning. If you’re in gear, then “whatever keeps the engine turning” is the car’s movement.

Low gears give the engine better leverage over the car, which is great when the engine needs to give the car a push, but when the engine is actively sucking energy away from the car then giving it “better leverage” means you slow the car down way more.