Will putting a vehicle in reverse guarantee that it won't roll down a hill?

Dear Car Talk

Dear Car Talk | Mar 01, 2008

Dear Tom and Ray:

Several years ago, my Ford Ranger coasted out of my driveway at about 2 a.m. and rammed a minivan across the road. (I parked on a very steep driveway.) I learned to drive on my grandparents' cherry farm and was taught to put everything -- tractors, pickups, cars, anything with a standard transmission -- into reverse when I parked it, because that would keep it from rolling backward. It seemed to work for many years ... until that night, at least. My question is: Will putting a vehicle in reverse make it less likely to roll backward down a hill? It seems counterintuitive. When I got to my truck that night, the door was locked and it was in reverse. -- Brendan

RAY: Will parking a manual-transmission car in reverse make it less likely to roll down a hill? Yes. Will it guarantee that the car won't roll down a hill? Absolutely not.

TOM: Whether you put the transmission in a forward or reverse gear makes less difference than the ratio of the gear (the compression in the cylinders will make the engine resist moving in either direction -- forward or backward). You want a high gear ratio in order to give the wheels a mechanical disadvantage. First and reverse have the highest gear ratios.

RAY: By putting the vehicle in gear, you're making a direct mechanical connection between the engine and the wheels. You're making it so the wheels can't turn unless they force the engine to turn.

TOM: So, what you've done, when you put the car in gear and connect the engine to the wheels, is made a bet that the compression inside the cylinders is strong enough to counteract the effect of gravity trying to pull the weight of the car down the hill. And on mild-enough hills, you'll win that bet.

RAY: But you could lose the bet on steep hills -- or if your engine's compression is low, or if your clutch is slipping, or if the transmission slips out of gear, or if there's ice on the ground.

TOM: That's why we have this relatively new invention called -- pay attention, Brendan -- the parking brake.

RAY: Putting a vehicle in gear -- even the most mechanically disadvantageous gear -- doesn't guarantee that your car won't roll down a hill. So we always suggest that you put the car in first or reverse AND apply the parking brake.

TOM: If your Ranger used to hold on that same hill, under all the same conditions, then something may well have changed, and you'll want to have a mechanic check out the condition of the clutch and get a compression test. But regardless, even when everything else is working perfectly, you still should make a habit of using the parking brake.

RAY: It's good insurance, Brendan. It's why my brother's dates always bring two paper bags with them -- one to put over my brother's head, and one for themselves, in case my brother's falls off.

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