Are Fiero Spare Tires Usable? (Page 4/4) theogre DEC 15, 07:05 PM quote Originally posted by Spoon: A non Fiero friend of mine had a flat on rear and put on his donut spare. Not sure if that was factory issue or not. After a while of steady driving he noticed smoke coming from rear of vehicle. His differential overheated because of unmatched tire size and the distance he had to travel. He said it was too hot to touch. I guess continuous differentiating was not part of the design. It was an older Jeep late 80's. It was a 4x4 so even if he had it up front he may of had the same issues plus the steering to deal with. My last 2 Jeeps had full size spares from the factory. Just thought I'd mention this for you guys with oversize tires and a donut spare. Anyone w/ "posi" differential of most types can heat up or outright fail fast like that. "Car" tries to lock/bind the diff same as 1 wheel on ice for miles. I bet he's rear lost the posi at minimum. 4WD can have posi front axles too so possible can have same problem. Most FWD and Many 4WD front axle from the factory don't have posi fronts because can affect steering and diff wear. Fiero drive train is just transplanted FWD and no "posi" from factory for this reason but can still be problems w/ taller tires. IOW Simple diffs like Fiero can have problems even fail driving w/ mismatch tires, worse if diff/trans is low on oil, even tho nothing inside should heat like above. So if you have taller tires on Fiero and others that can reduce speed and distance allowed using OE spare before you have problems. Taller tires and factory spares can cause problems for brakes (More so w/ cars w/ ABS) suspension etc. They do make bigger donuts but likely don't fit in front of Fiero or many other cars w/ same size spare.