A friend and I were driving through rugged mountains with...

Dear Car Talk

Dear Car Talk | Sep 01, 1994

Dear Tom and Ray:

A friend and I were driving through rugged mountains with grades up to 15%. We were in my 1991 Toyota Camry V6 with automatic shift. While going down grades, I suggested that she put the car in second gear to help with the braking. I have been told by several people that this is the best way to handle steep downgrades. She said she had been told never to do that because brakes are cheaper to replace than transmissions. I'm in a quandry. Who is right?

RAY: You're right, Rose. It's true that brakes are cheaper to replace than transmissions, but that's irrelevant here.

TOM: First of all, you wouldn't be doing any damage to your transmission, so it won't need to be replaced.

RAY: Second, if you overuse the brakes, they won't work at all. And then you WILL have to replace your transmission...and your arms and your legs and your skull etc.

TOM: When you overuse the brakes---like by counting on nothing but the brakes to keep you under control down a long, 15% grade--those brakes will overheat. And when they overheat, the brake fluid boils. And when the fluid boils...no more brakes!

RAY: 15% (that's 15 feet of drop for every 100 feet of length) may not sound like much. But over some distance, you'd be amazed at how much speed your car will build up.

TOM: So you're absolutely right, Rose. On a long, steep downhill, you want to use a lower gear to keep the car at a comfortable speed so you can be stingy with your braking.

RAY: By the way, we don't recommend downshifting to slow down in normal stop and go driving, or on hills of a couple of hundred yards or less. It's not necessary there and it just wears out the clutch prematurely. But we absolutely recommend it for long, steep hills.

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