I drive down the Hamilton Escarpment fairly regularly at night, without many people around, on the 403. It's a fairly steep (for an expressway) 2km long downhill. Just out of curiosity, which of the following would use less fuel?

Situation 1

- Start at 120 km/h at top

- Push in clutch, let car idle to bottom where it would be at about 100 km/h

- Accelerate back up to 120 km/h gently

Situation 2

- Start at 120 km/h at top

- Leave in 5th, take foot off gas, going about 85 km/h at bottom

- Accelerate back up to 120 km/h gently

I usually just leave my cruise on, because I don't want to slow down just to save a little fuel.

On a side note, I found out that my Cobalt really doesn't like to run with the engine off - I coasted down this hill a couple of times when I first got it (shut off engine, restart at bottom with clutch). After I did this, both times, the fuel gauge showed Empty the rest of the trip. The next day, it reset to normal.

Best situation is #3... leave the cruise on.

Otherwise situation 2 saves the most fuel.

Situation 420 is the best. have friends go with you shut car off put in neutral tell friends to push the car.

should use less fuel when coasting in gear than idling. When in gear, the momentum of the car keeps the engine turning, so it doesn't have to feed the engine much if any gas. Idling requires fuel to keep the engine turning.

should use less fuel when coasting in gear than idling.  When in gear, the momentum of the car keeps the engine turning, so it doesn't have to feed the engine much if any gas.  Idling requires fuel to keep the engine turning.

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but it depends what rpm it's turning, i'd think as long as the engine is turning it'd have to give it at least idle ammounts of gas, so w/ it turning fast i'd think it'd be using gas faster, and thus in more amounts over that distance... and with the engine drag...