[mirimir]

Good point. I should have noted that I've only done that with a 60s VW Bug. Just to see, I have turned off and restarted my ~10 year old Civic, going down a hill. But not with the battery disconnected. Maybe I'll try it. Not now, though.

Edit: And upon reflection, maybe not. Because I'd have to pay someone to reset the computer.

Edit: But wait a minute. If the vehicle is moving downhill, and you engage the clutch in first gear, the engine will be turning. And even if it's not firing, the alternator will also be turning. So you should have enough power for the computer, ignition and fuel pump. Or am I missing something?

[Someone]

It wouldn’t surprise me if newer cars require a higher battery charge to start, which means you would need to roll the car for longer, or (worse) don’t effectively charge the battery at all at lower speed because the partially booted system takes too much power, so that you need a higher speed to start charging the battery enough to power the ignition system.

Also, are there cars where you need the electronics to disengage the brakes?

[jacobush]

Don’t do it with battery disconnected, it can cause the voltage to go too high and you may fry things. Back in the day only bulbs, nowadays who knows.

[mirimir]

Thanks. I'm not going to try it.

[jotm]

The alternator needs to get some decent RPM to generate enough power. You'd have to go downhill quite fast.

[mirimir]

In first gear, so not that fast.

[drfuchs]

Careful! Shutting off the ignition will lock the steering wheel on older cars.

[ryacko]

That’s called powered steering.

[mirimir]

It depends on the vehicle. Power steering can either be electric (EPS) or mechanical, with a compressor that's belt-driven off the engine. My Civic has rack-and-pinion steering with EPS. When the engine is off, it's still steerable, but there's no power assist. But you don't want to turn the ignition lock all the way off, because that does lock the steering wheel.