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New tire time coming up and I have a dilemma. When I bought the coach it had 9R22.5s all around. I couldn't find any 9Rs that weren't Chinese locally so I went with the alternate 25570R22.5s as specified in the chassis manual.

I want to go back to the 9Rs (a few manufacturers are offering them now), but I had planned to do the steers this years and the rears next. What are your thoughts about running 9Rs in the front for a year?

The only issue with the 255s is that the speedo is off by 4 MPH.

If you are asking if one can use a different size tire on one axle vs the other-- should not be a problem.

BTW, a much closer match for the 9R22.5 is the 255/80R22.5.

Thanks for the reply Mr. Wolfe. I am going from memory on the metric size, but I did get in from the chassis manual. The docs are all in the coach at the airport. I'll grab them Monday and check.

What are you running on your Alpine?

BTW, it was exactly 6 years ago we were at HoHum.

I pulled out my Michelin RV Tire Guide

Revolutions per mile:

9R22.5 543
255/80R22.5 538 so, these two pretty close

255/70R22.5 563 this one-- not so much--engine RPM faster for a given road speed. Same as numerically higher rear axle ratio.

I believe Brett is right for Michelin tires, but you would need to double check for other brands. I believe Brett once recommended 265/75R22.5 as the best match.
Specifications for the Toyo M120Z 9R22.5 133/131 F/12
Max Speed – 75 mph
REVS per mile – 544
OAD – 38.1
Max Single – 4,540
Max Dual – 4,300

1Lotosrggp wrote: ↑Sat Jan 22, 2022 8:31 pm I believe Brett is right for Michelin tires, but you would need to double check for other brands. I believe Brett once recommended 265/75R22.5 as the best match.
Specifications for the Toyo M120Z 9R22.5 133/131 F/12
Max Speed – 75 mph
REVS per mile – 544
OAD – 38.1
Max Single – 4,540
Max Dual – 4,300

That is correct, Michelin uses an odd number for this size (255/80R22.5), while most other tire manufacturers use tire size numbers 265/75R22.5

Goodyear's chart shows the 9R22.5 with a diameter of 38.7 inches, and revs per mile of 537.

The 265/75R22.5 with the diameter of 38.5 inches and revs per mile of 539, which is pretty darn close. The 265/75R22.5 can carry a lot more weight at a lower psi compared to the 9R22.5. With the 9R's you are likely close to maxed out in the rear, also this tire size is not common, so if you have a blowout, availability would be sparse. The max weight the 9R's are rated for at 105psi, the 265 tire can carry that same weight (actually a bit more) at 90psi - single and 85psi - dual, so you would have a much smoother ride and more of a safety cushion. The 265/75R22.5 also rides much smoother than the 255/70R22.5 low profile tires that you have on now. Being that the 255 is a shorter tire, that is the reason why your speedometer is off, the rev per mile is much higher 566 vs 537 that the 9R's originally came with.

Correct-- Michelin is the only one who makes the 255/80R22.5.

Other brands have it as the 265/75R22.5

Yes, the 265 is slightly wider than the 255 which is slightly wider than the 9R. SO, verify both rim width and dual spacing. If rims are 7.5", you should be fine, but check.

wolfe10 wrote: ↑Sun Jan 23, 2022 6:47 am Correct-- Michelin is the only one who makes the 255/80R22.5.

Other brands have it as the 265/75R22.5

Yes, the 265 is slightly wider than the 255 which is slightly wider than the 9R. SO, verify both rim width and dual spacing. If rims are 7.5", you should be fine, but check.

Just an FYI, I have the 265/75R22.5 tires on rims that are 6.75 inches wide and there are not issues with dual spacing, mounting, or odd wear. Of course, like Brett mentioned, dual spacing must be always checked. That said, sometimes people don't know how to properly check for dual spacing and end up making errors.