Q. Dear Tom and Ray:

My wife and I have a bet about my 1995 Olds 88 with a four-speed automatic transmission. When descending a hill, the car picks up speed as if there is no "engine braking." My wife says this was designed into the transmission; I say it's a problem of some kind. Who's right?

-- Laurence

A. RAY: Oh, Laurence. We hate to embarrass you right here in front of 270 million of your fellow Americans--not to mention our 14 readers in Kuala Lumpur--but your wife is 100 percent right about this.

TOM: When you're traveling in high gear (overdrive), there's very, very little resistance from the compression of the engine. And the transmission is designed that way so you can do what? Go fast!

RAY: If you had to overcome significant engine braking while going downhill at 60 mph, we'd all be getting four miles per gallon on the highway.

TOM: Wait. What's wrong with four miles to the gallon? I wish I was getting four!

RAY: When going downhill, Laurence, you have to specifically take action to get engine braking. If you're going down a long, steep hill and you want to use engine braking to avoid riding the brakes (which is absolutely the right thing to do), then you have to downshift into third, second or even first gear until you get enough engine braking to keep your speed under control.

Dear Tom and Ray:

I need you guys to set my friends straight about my lousy truck. They seem to think that because it has four different-size tires (five, if you count the spare), it's somehow "unsafe." I am an artist, so I have an abundance of free time and very little money. So every time I see some tires out on the curb, I stop and check them out. People throw away tires with many, many miles left on them, and since I know how to mount a tire, my tire cost is zero! My tires are generally within a few sizes of each other--say, a 175/85 R14 on one side and a 165/75 R14 on the other. Please tell my friends that used tires are not inherently unsafe and that I'm not crazy.

-- Rod

TOM: Actually, you're right that used tires are not inherently unsafe. People sometimes do throw out pretty good tires. But, generally speaking, you do want the tires on each of your four wheels to be the same size and type.

RAY: Here's why. If you have tires that are different diameters (such as 85R and 75R), you're going to be putting stress on your drivetrain. On the rear wheels of your truck, for instance, the spider gears in the differential are always going to be working. That's because having different-size tires mimics the dynamics of constantly turning a corner. And eventually that'll burn out your differential.

TOM: But even more important are the traction issues. Tires have different traction qualities. Compromises are made in each tire to emphasize things such as ride, braking, longevity, cornering or snow traction. And if your tires are reacting differently to the road at each wheel, you could get some very peculiar handling.

RAY: For instance, one tire could brake better than another and could cause the truck to pull or skid. Or one tire's traction could be weaker than the others and cause the rear end to come around in a corner.

TOM: The handling would probably be most peculiar when you need good handling the most, as when you're cornering and braking in the rain.

TOM: Nonetheless, it's still better to have the same size and type of tires on each of the wheels of your truck. So if you really have that much free time, just scavenge a little longer, and try to find four of the same used tires.

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(C) 1999 by Tom and Ray Magliozzi and Doug Berman