Question for Tire EXPERTS

Ed_Gee wrote:
Looking for facts - not guesses.

LOL

Grit dog wrote:
You guys realize the OP is talking about a tire that is 1/4" or less different in radius, right?

NO sir!.....don't think so.....

MDKMDK wrote:
This will be fun. Release the hounds......

Indeed.

You guys realize the OP is talking about a tire that is 1/4" or less different in radius, right?
Only 1/2 to 1/3 the difference between a new and wore out tire of the same size and model.
Steer axle, won't notice anything.
SRW drive axle, won't notice anything or cause any harm.
DRW axle, I'd make sure the smaller tire is at max pressure and maybe soften up the taller tire a bit if you're loaded pretty heavy, but I'd run it, maybe not indefinitely. I've put a new spare on the back of a dually with worn tires a few times. Didn't result in any problems.

i think i would buy a spare identical tire

why keep a spare the wrong size, when you know there could be a problem

if i absolutely had to keep an undersized spare or run with out a spare

i would make sure it went on the drivers side front, and use that tire to replace any drive axis tire that had gone flat

even keeping an unmounted spare tire is a better option than NO spare and buying the wrong diameter for a mis-match

If the tire will be used as a single front (steering) no problem other than POSSIBLE minor alignment error.

If it will be used as one tire of a dual set that is a other problem. Remember your HS Physics 101 the relationship between the diameter of a circle to the circumference is- Pi= 3.1416. (D x Pi= C) So the smaller tires circumference is approx. 1.5 inches less or the larger tire circumference is 1.5 inches larger. Which means the larger tire will have more weight on it, now you have to assume that the larger tire also will have ZERO slippage. Now with both rims/tires bolted together and the larger tire having zero slippage the smaller tire will have rotated 1.5 inches less/revolution than the larger tire, ie there will be 1.5 inches of slip for every one revolution of the set! So the smaller tire will wear out faster than the larger tire. Now we all know that there is no such thing as ZERO slippage, so it is a complex issue.

A truck driver years ago told me that when they pick tires that they use as a set of duals they measure the diameters and that they have to be within a certain ?? limit, I cannot remember for the life of me what it was! Now, maybe with radials and new manuf. processes maybe that is not required now.

So to answer the OPs question it will wear out faster or maybe not, it just depends. If it depends on calling out a service truck or wearing out a tire I would sacrifice at tire. At least for hundred miles but keep my eyes on it!

IMHO it depends

1) does the vehicle have ABS or any speed sensors that would trigger a fault if the tire diameters aren't close enough

2) Does the vehicle have positraction or auto locker rear diff? again difference could cause premature where on the clutch plates

3) Again as mentioned, might be an issue on a dually.

If none of the above apply I can't think offhand of an issue on a short term basis.

And in an emergency, again even with any of the above you'd probably be ok to get to a repair point.

Remember, even if you started with exactly the same tires all around, once the ones in use are worn to near wear limit, the overall diameter of the tires will be smaller than the un used spare anyway.

As an example suppose a tire has 8/32 of useable tread depth (in the range of a new tire) when that it worn away overall diameter is down by 16/32 = 1/2 inch.

IMHO on a motorhome putting a different size tire on the drive axle is a bad idea. With dual rear tires this would put excess load on the bigger tire next to it which could cause it to run hot. It shouldn't cause any physical damage on the front axle but handling might be unpredictable. Even having a few pounds different air pressure between front tires can cause handling issues. But in an emergency I'd do what I can to get to civilization.

This will be fun. Release the hounds......