Is driving with four different sized tires unsafe?

Dear Car Talk

Dear Car Talk | Nov 01, 1999

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. People don't want to ride with me when they hear this. I never drive over 55 due to other
"performance-related peculiarities." Please tell my friends that used tires are not inherently unsafe and that I'm not crazy. -- Rod

RAY: I suspect that the reason your friends won't ride with you has nothing to do with unsafe tires, Rod. They're just tired of having to stop at every festering junk pile
you pass and wade through it with you looking for tires!

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 (like 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 like 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: While you probably wouldn't notice that at slow speeds on dry roads, the handling would probably be most peculiar when you need good handling the most, like
when you're cornering and braking in the rain.

RAY: Different size tires could also mess up an ABS (anti-lock braking system). But based on the description of your truck, your ABS is probably "Almost a Brake

TOM: Nonetheless, it's still better to have four of 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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