The little doughnut spare for my truck is old and cracked on the sidewall and looking like it wants to be replaced. The size is T155/90D16 and the other tires are P205/75R15. Little doughnut spares are hard to get and expensive so I hit the junk yard and came home with a T145/90D16.
According to this tire size comparer, my new tire is .8" (3%) smaller in diameter than the other tires.
How big of a problem is this difference if I ever have to use the spare? Obviously, if it’s used, it will only be for a short distance at speeds under 50 mile/hour.
It can be hard on a differential to run different tire sizes on each end of it - instead of the drive gears remaining relatively static, they have to turn continuously. Low speed and short distances would probably keep the wear and overheating down. That’s only on an open differential - never do anything of the kind on a locker or limited slip. You’ll total it.
If you ever have to use an odd-sized tire, you’re better off putting it on the front, even if that means swapping a front to a rear flat first. The vehicle steering will be a little off, but the diff will remain happy and unstressed.
If a vehicle is 4WD, disengage the front hubs or drive; if it’s AWD… call a tow truck. Never drive an AWD vehicle with mismatched tires on one axle. It’s serious enough that if you ruin a tire, you have to replace at least that axle pair to keep the slight size difference from wear from wearing out the axle and center differentials.
Did a bit of research, and the stock spare is 26.5" diameter, the 145, 26.3, and the regular tire 27.11 on a run of the mill '97 S-10. Open differentials really don’t mind differentiating- that’s what they’re for. 0.2 inches difference between the stock and new spare isn’t a big deal for spare use as described by the OP. I wouldn’t sweat it.
An anecdote, just for fun- drove from south of Louisville to north of Indianapolis with an FR78-14 and a CR78-14 on the back of a '67 Dodge once. No differential problems then or later whatsoever.
Can the spare tire carrier on an S-10 hold a full size spare? If so, take it back to the junkyard and get a steel rim and then look for a used tire of the correct size to serve as the spare.
Cars sometimes have limited space for spares, trucks usually can carry a full size.
But what about load capacity ? Well it seems like that the small tyres can carry the load, as the small tyres are T for truck,
and the larger tyres are P for passenger… the T for truck tyres will be quite strongly built and rated to carry a larger load than the same size P tyre…
You shouldn’t do highway speeds with mismatched tyres like this, the emergency spare is so you can probably get home or get to some place where you can stay overnight and get the tyre fixed in the morning…
But at least it can carry the load while do you 30 mph …
If the difference isn’t much, yes you can, but you should keep the driving on it from the place you break down, to the tire store, period. Don’t run around town with the spare on (you shouldn’t really be driving around with a donut spare anyway).
If you drive a lot on the highway, so that there is a fair chance you might blow a tire on the highway, or out of town, and really want to drive home to get the problem taken care of, then buy a full-sized wheel, and put a used tire on it. I have one for my car that I put in the trunk any time I go on a trip. If I might blow a tire in town, I use the donut, but if it happens on the highway, I can put on the full-sized, and drive on my way, taking care of the problem when I reach my destination, or even when I get back home.
They are fairly rare, but the latest in limited slip diffs are based on non-reversible gearing principals (see Gleasman patent) rather than friction. With these, mismatched wheel diameters are not terribly worse than with an open diff. Still a good idea to limit speed and distance to the nearest shop that can supply a wheel of the matched size.
On a front wheel drive car, an open diff will split the torque equally, and this will cause the car to pull toward the larger wheel (less force to the ground). A RWD car will do it to, but the effect at the steering wheel won’t be as extreme as with FWD.
Thanks everyone. Looks like it’s not a very good practice but not fatal.
Yes. I’ve been using one of the winter tires as a spare since I got the truck this spring but I don’t want to carry a good aluminum wheel in the under-the-bed spare tire carrier all winter and ruin it. What I’ll do is get a full sized spare but since no one’s too freaked out about the size difference, I won’t worry about getting it done today.
The amount of air you would have to remove would result in a (probably) severely under inflated tire, which would increase the chances of a blowout. Then you would have two flats and no spare for the second flat tire.