Explained, at long last: all those tire dimension numbers!

Dear Car Talk

Dear Car Talk | Dec 01, 2006

Dear Tom and Ray:

I recently bought a 1966 Volkswagen Bus Sundial Camper and noticed that it has a set of 205/75/R14 tires on the front and a set of 185/75/R14 tires on the back. I don't know how long it was driven this way by the prior owner. Is this dangerous? Could it have caused damage to the Bus? Of course, I am not going to leave them that way! The info I can find says the correct tire size is 195/75/R14. If you put on tires bigger or smaller than recommended by the manufacturer, will the odometer be able to accurately record the number of miles you've traveled? One revolution of a larger tire will cover more ground than one revolution of a smaller one, and it's the revolutions that make the odometer rack up the miles, right? Not that it matters much in a car of this vintage, but over 40 years, the extra miles on or off the engine could really add up. Just wondering. -- Joan

TOM: Joan, this is like worrying about the curtains on the Titanic. This should be the least of your worries.

RAY: In terms of the width of the tires, if the vehicle calls for 195s (195 millimeters of tread width), that's what we'd generally recommend so the handling and stopping of the vehicle are what the vehicle engineers had designed them to be.

TOM: But there are two qualifications. One is that it's generally acceptable to go 10 (and sometimes even 20) millimeters in either direction and still be OK. As long as you're not mixing sizes on the same axle, and as long as the width of the tire is not interfering with the front wheels' ability to turn from lock to lock and steer the car, a few extra millimeters of width shouldn't be a problem.

RAY: But more importantly, this Bus handles like a vertical pizza box on a skateboard. So precision handling is not something to lose sleep over. Ideally, you'd have 195s on all four wheels, yes. But in reality, it's not going to improve or degrade the handling by a measurable degree on this vehicle.

TOM: And like I said, Joan, the tires are small potatoes. The thing most likely to affect the longevity of this car is a brisk wind and an oncoming UPS truck.

RAY: You're right that the wrong-size tires can affect the odometer (and the speedometer). But those are affected only when the diameter of the tire is wrong, not the width. And since your tires are all the correct diameter (they have the same inner opening, 14, which is the wheel size, and aspect ratio, 75, which essentially is the height of the sidewall), your gauges shouldn't be affected at all.

TOM: So I'm sure you'll sleep a lot better tonight knowing that when the speedometer says you're doing 35 in the breakdown lane of the highway, you're really doing 35!

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