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Hello - I'm trying to match up a set of 8.25x20 tires for the rear of a 58 Chevy Spartan 80. I have 2 size wheels and diff. tire brands. I have 3 matching tires which are ready to bolt on - they are on 20x6.5 split rims. I have a nice tire of a diff. brand mounted on a 20x7.5 split rim. It is shorter in heighth (diam.) by about 3/4". Can I assume that's because it's on the wider rim? If, on the other hand, it is in part due to being a diff. brand tire, is there a way to measure it while off the rim to determine if it will air up to a matching heighth on a 20x6.5 rim? Thanks !!

7.5" wheels are for 10:00x20 tires and shouldn't be used with an 8:25x20 tire. All four of the tires need to be the same size and matching them is important. Maybe you can trade the 7.5" for a correct wheel. They are plentiful. 6.0 will also work for 8:25 tires.

Depending upon how you use the truck running two different brand tires side by side won't matter much. When running a heavy load though one tire can/will carry more load than the other if they are not matching tires.

The rims do ALL have to be the same(height & width), and the tires ALL have to be the same size. Your one tire is going to be shorter due to the wider rim. There will also be some difference in height due to brand and wear. This is why it is recommended to always run matching tires.

You would have to look at the tire specs to see the width of rim that the tire is allowed to be installed on. I have not checked large truck tires, but I know smaller car/truck tires have a range of rim widths that they will fit...often about 1-2in variation. Of course the rims all have to be the same on the same axle though in any case.

Tad

Thanks for the responses....interesting tidbit that I also noticed on the rim markings: some are marked 20x6.5 and another is marked 20x6.55...all Firestone....am I seeing things? Why would there be such a slight difference?

Running different height tires on duals will cause the shorter tire to be turning more RPM than the taller one next to it. The short one will wear out in no time.

That doesn't add up.... both dual tires on the same side of the truck will turn exactly the same rpm no matter the sizes.
The much taller tire in a side by side pair will wear faster because it sees more load and road. If the difference is not great I bet they both wear faster than expected trying to be the same size eventually.


Grigg

I think what he is trying to say is that if tires are mismatched, the smaller one will wear out in almost no time because centrifugal force will cause the smaller one to stretch to touch the road. This is why we use a caliper to match used tires on our trucks. One would think that the taller tire would wear faster because it carries the load, but it flexes like it was designed to do and wears normally. On the smaller tire, the carcass stretches abnormally and the tread is traveling less feet per second than the larger tires. Since the larger tire is carrying the weight, it gets traction and the smaller tire scuffs trying to keep up with the larger one. I have had drivers buy an old lug type tire on the road to replace a non repairable tire and mount it along side of an almost new shallow tread tire and the almost new tire is down to the wires in less than 500 miles. Its rather difficult to explain, but it is common knowledge to commercial truck operators.

For example, we set our ECMs for 513 revolutions per mile for low pro 22.5 drive tires. Low pro trailer tires can be about 516 revs per mile. If you do the math, you can figure how much the smaller tire will be scrubbed in 500 miles.

Make sense to me. In your example the smaller tire would be "dragged" for 3 revolutions per mile. (516 vs. 513) That's a long distance if you consider the circumference x 3 every mile. No wonder it wears so quickly.

Sims

I guess I should have explained in this case RPM means revolution per mile. I owned trucks for several years and have seen it happen. The smaller tire is not dragging but trying to spin.

Thanks for the clarification and explanation.

Grigg

You'll find the same problem when running an under inflated tire next to one properly inflated. The under inflated tire, being smaller in circumference, will be covered in dips after running just a few hundred miles. Identical tire size, along with proper inflation, are both important when running duals.

James