Question:

Spare tire size is quite different

by Guest6600  |  earlier

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Hi

I have always been told that a spare should never be of a different tire than the other three on the vehicle. Radial/bias, tread, or size. New vehicles (Chevy) are being delivered with 17 main tires and spare tires that are 16. They are full size spares (not donuts) P245/65R, so it is conceivable the driver can/will drive on this spare for more than emergency means. IS this not unsafe, and especially damaging to the different and/or transfer cases on AWD cars? When did this practice start? It is a surprise to me.

Thanks

 Tags: size, spare, tire

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1 ANSWERS

  1. John

    Hello

    Here is the rule. All tires ON THE GROUND are supposed to be the same, with the exception of the temporary use of the spare.

    The reason for this is 2 fold

    1) so the vehicle handles properly. An odd tire tends to make the vehicle pivot around it in emergency maneuvers, just when you would want it to behave in a straight and true manner.

    2) So the tire diameters are the same, so the differential is not overworked. In FWD and RWD, an odd sized spare is not much of a problem unless the temporary spare is mounted on a drive axle. But the vehicle manufacturers always design the overall diameter of the tire (the rim size is unimportant) so that the differential can accommodate the temporary use of the spare. In the case of 4X4s and AWD, the spares are usually the same as the ground tires, but sometimes they are not. Sometimes they are different, but the diameter is very close. And usually it is obvious that this is a temporary spare because the rim is ugly black steel, where the other rims are aluminum.

    But with the advent of plus sizing (larger rim diameters) many vehicle manufacturers are using the base tire size as the spare and sizing the optional tires so they are the same overall diameter. That eliminates the problem for 4X4s and AWD.

    Thanks
     

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