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It's dangerous to drive on mismatched tire sizes

Dealership installing 3 new tires on Kia a big mistake; properly fitting wheels from different car almost impossible; get rid of 15-year-old tires

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Q: My elderly mother-in-law took her car into the Kia dealership and they installed three new tires. Not two or four but three. The three tires installed are P195/65/R15. The remaining tire is P195/60/R15. Is this acceptable to be running different size tires on the same (rear) axle? She is driving a Kia Spectra.

FYI, she paid $120 plus installation for each Solus KH16 tire. I notice they are available on-line for $50 (U.S.) so she certainly didn’t get a bargain. I am concerned about the effect of running different size tires on same axle. The tire left on is a Solus KH14.

A: The mismatched tire is a dangerous situation. It should be replaced with the proper size immediately. And they have placed the tire on the worst possible end of the car, the rear. Mis-sized rear tires can lead to a loss of control under hard braking (the car can spin).

The one tire left on the car is almost an inch smaller in diameter. It has a different size contact patch. If the vehicle has ABS, the ABS computer will be confused by the different size tire.

The tires should be replaced in pairs or all four. If just two are replaced the newest best tires should be on the rear for safety and stability under braking. You are also correct, the price is a rip-off. They charged you full list price, these tires are usually discounted between 30-40 per cent at independent tire dealers.

Worst of all, they have done a dangerous thing by mismatching the sizes.

Q: Will the rims from a 1999 VW Cabriolet fit onto a Ford Escort or a Mazda, as long as they are the same size lug four-hole? Are there books that can be purchased or computer programs for the layman to find out which parts from certain vehicles say Ford with Chevy or Dodge with Ford and if so where can they be purchased?

A: Sorry, I am not aware of any one book that lists all that information. It is more complex than just comparing bolt patterns. Each year the major tire companies produce a book called the Fitment Guide. It lists the tire sizes for every model of car on the market as well as the recommended pressures, torque of the wheel nuts or studs, wheel widths and bolt pattern for the wheels.

So that would be a start but you still need more information before making a swap. For instance if you have two wheels with identical bolt patterns, say 5/120. That means five bolt holes on a 120 mm circle from the centre of the wheel. You now also need to know the size of the hole in the middle of the wheel for the axle hub. This hole must fit exactly or the wheel can move around and seem out of balance. So for a swap, bolt pattern and hub hole must match — not many do.

Even if they match, the next information requirement is the wheel offset. That is how much of the wheel is offset towards the outside or inside from the centreline where it bolts onto the hub. This is usually expressed in millimetres. There are very, very few cars that have zero offset. If the offset is too great, the wheel will stick out from under the fender of the car; if it is too small, the tire may rub on the inner fender lining.

Chances of all three measurements being ideal for a swap are remote. And not all three pieces of information are ever found in one location. You need to get the wheel info from the auto or wheel manufacturer, the tire info from the tire fitment guide.

Q: I have a sports car that I drive only in the summer. As a result, the tires are 16 years old but only have about 5K on them. The car is always stored in the garage so the tires don’t receive much direct sunlight. A visual inspection reveals no obvious cracks. Should I be worried about driving the car with these aged tires?

A: Those tires have got to go before they kill you. Fifteen years! They must be fully petrified fossils by now. Tires age just the way we do, and their normal life expectancy is about five to six years, Four new tires will give you some extra grip, and driving your sports car in the summer is more fun than an ambulance ride.

Email tire questions to John Mahler at thetireguy_1@hotmail.com. Please include vehicle’s make, model and year, tire brand and size, as well as your name, address and phone number. Due to volume, personal replies cannot be provided.

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