Wheels and Tires

Wheels and Tires

When your car was designed, certain performance objectives, such as responsiveness, ride comfort, controllability, rolling resistance, and grip are set as targets. The wheel/tire combination helps the manufacturer meet those objectives. Early in the suspension engineering process for a given car, the overall diameter (OD) of the tire is determined. In the late 1960's and 1970's tire designers developed successful wider and lower profile tires. When vehicle manufacturers utilized these wider low profile tires, they used tires with the same OD as what the car originally was designed for. When using a wider tire, the vehicle manufacturers selected wider wheels to support the wider tire cross section to optimize the benefits of the new technology.

As for the X 1/9, here is how Fiat/Bertone approached the technology:

Details

Year	Tire size	Details	Tire OD	Wheel width	Details
1974	145HR13	tube type(TT)	22.29	4 1/2	wheels have no safety humps
1975 - 1978	145SR13	some TT	22.29	4 1/2	some X had tubeless(TL) wheels and TL tires
1979 - 1981	165/70SR13	Tubeless (TL)	22.29	5	wider wheel, all are TL
1981 onward	165/70SR13	TL	21.37	5 or 5.5	optional is 185/60HR13
1983 onward	185/60HR13	TL	21.37	5 or 5.5

I worked for an Italian tire manufacturer as a factory liaison with our dealers and consumers in the 1980s. BY TIRE INDUSTRY STANDARDS, it is not correct to put a tubeless TL tire on a TT wheel. Under certain circumstances like high side forces, it is possible for the TL tire bead to walk across into the drop center of the wheel. When this happens, there is something called an explosive decompression. The tire instantly loses all its air and the rim digs into the pavement. There are problems when you put a tube in a TL tire that have to do with incompatible textures and also, heat build up. At times, it was difficult to persuade tire retailers to actually follow industry standards or to do the right thing. At the retail level, it is easier to sell the tire you have in stock and not to take the chance you will lose the sale when you have to special order the correct tire or wheel.

Finding the correct size 145R13" (also designated as 145/80R13) tires is an expensive and difficult process. It is hard to find 80 series these days. Same for 13" 70 series tires like 165/70SR13 and 175/70TR13. 4 years ago, I found four 175/70TR13 Michelin Defender tires at Discount Tire. I mounted them on Fiat Speedline Iron Cross 5 1/2 x 13 alloy wheels. Today, on 5 1/2" wheels, I would look at 175/70TR13 Hankook Kinergy tires at Tire Rack for $80 each.

In conclusion, just follow Fiat's lead. If you have 4 1/2" wide wheels, stick with 145 or 155 wide tires. If you want to make life easy on yourself, you should follow Fiat's move to wider wheels(with the correct backspace) with the right size wider tires. Any wheel wider than 5 1/2" or tires wider than 175 or 185 may rub stock fenders or fender liners. What works is shown in the chart above.

(content from johnph)