Question for Tire EXPERTS

[Ed_Gee]

Looking for facts - not guesses.
In the event of a flat tire, What are the ramifications of using a same spec tire of a different brand but that has an outer diameter up to 1/2 " less than the original?
Specifically on my small motorhome I have Hankook Dynapro HT RH12 195/75R16 tires, ( which in remote areas are hard to come by in a timely fashion). Other manufacturers of this size tire with same load characteristics all seem to be spec'd at .4 to .5 inches smaller outer diameter. What I am asking is what is the harm, if any, of installing the slightly smaller diameter tire in an emergency and running for a few hundred miles? What is the harm if I leave said tire on indefinitely?

[richardcoxid]

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!

[ktmrfs]

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.

[TheBar]

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.

[MDKMDK]

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