Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 11, 2025 ...

Two different size tires on our coach

September 19, 2013, 09:32:20 am

Does anyone else have two different size tires on their coach? Our steer tires are 275/80/22.5 and the rear tires are 235/80/22.5. All are Michelins. The speedometer reads 61 MPH while the GPS reads 56 MPH. I realize the tire size causes the discrepancy in MPH, but why would anyone put the larger tires on? What height change would occur with putting 235s  on the front? Right now the coach appears to be level with the larger size tires on it.

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Re: Two different size tires on our coach

Reply #1 – September 19, 2013, 09:45:30 am

My ORED had 275/80R 22.5 all around which I replaced with 295/75R 22.5 which is the same diameter tire.  Putting these tires on the drive axle will correct the speedometer reading.

Hope this helps.

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Re: Two different size tires on our coach

Reply #2 – September 19, 2013, 10:12:04 am

Having 235s on the back will not handle as well, extend braking distance, turn more rpms, use more fuel and require you to carry two spare tires. We have 295/75s (same as 275/80s) all around.

Now you are turning 540 tire rpm per mile. Changing to 295/75s will drop that to 505 per mile or a 7% decrease in rpm. The 295 is almost 25% wider.

Here is a good truck tire computer I set up for your two different sizes: 235/80-R22.5 vs 295/75-R22.5 Tire Comparison - Tire Size Calculator

And yes, your speedo/odo will read much more accurately will the 295s on the back as Rudy has posted above.

Pierce

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Re: Two different size tires on our coach

Reply #3 – September 19, 2013, 10:35:31 am

WOW, thanks guys!! Now I know what to shop for and what size to get. BUT, do I have the sidewall clearance with the wider tire?

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Re: Two different size tires on our coach

Reply #4 – September 19, 2013, 11:00:07 am

OK, now we have to figure if you have the same size Alcoa wheels that our later rigs have. Looks like you do have Alcoa wheels in your photo. It's possible the smaller tire size was fitted to give a little more power on hills towing a trailer.

Here is the official Alcoa size chart in PDF format. You should be able to compare part numbers on your wheel to get the exact wheel you have. With the tires mounted, the little bulge in the radial sidewall should not touch the other wheel's bulge as it can cause failure. http://www.alcoa.com/alcoawheels/catalog/pdf/brochures/SpecGuide11x17.pdf  You can also just take down your wheel part number and call your local Alcoa dealer and ask about maximum tire sizes, clearance, etc.

All maximum air pressures for Alcoa wheels are cold pressures, just like maximum tire pressures. Alcoa wheels are also forged wheels not cast like most cars have. This means they will bend if they hit anything and not break off chunks like most do. Porsche also uses forged aluminum wheels. Check the price on one wheel compared to Alcoa. A new 22.5 is dirt cheap in comparison.

Here is a great site for all Foretravel members to read. It shows how the forged wheels are made, how they do in accidents compared to cast wheels, etc. An excellent read: http://www.alcoa.com/alcoawheels/europe/pdf/en/facts_figures_2013_EN.pdf

Pierce

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Re: Two different size tires on our coach

Reply #5 – September 19, 2013, 11:30:49 am

ah, learn something everyday pm the Forum.....thanks Pierce for the wheel size discussion and the alcoa web site. 

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Re: Two different size tires on our coach

Reply #6 – September 19, 2013, 12:37:57 pm

Thanks for those details... I was also not all that savvy about the tire sizes and had no idea that the Alcoa wheels were forged.

Someone needs to write a book; a compendium of Foreforum knowledge.

Craig

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Re: Two different size tires on our coach

Reply #7 – September 20, 2013, 08:45:59 pm

Craig, that would probably turn into an encyclopedia of several volumes.

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Reply #8 – September 21, 2013, 06:50:14 am

                   By going to a smaller drive axle tire , you gain pulling abilty. Maybe the previous owner pulled a large heavy trailer. That would be the main reason for a switch of this type ----or he had a friend where he got free tires of that size . How about midnight auto parts ?? Hope this isn't political or it will get scratched   Brad Metzger

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Reply #9 – September 21, 2013, 08:42:52 am

Another possibility: better toad towing capability, seeing as how the 8.2 DD isn't exactly a mind-boggling powerhouse!

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Reply #10 – September 21, 2013, 11:15:59 am

Thanks Pierce for the info on the Alcoa Wheels.  I found it interesting.  I just wish I could make my wheels dura bright.  I like the easy care issues.  What I did not know was that steel wheels have less strength.  That surprised me. 

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Re: Two different size tires on our coach

Reply #11 – September 21, 2013, 12:18:45 pm

I would not want "dura-bright" wheels, because it is a clear coat on the polished alloy wheels. Over time, this clear coat gets cracks which allows water in and turns into spider webs under the clear coat. The fix is to remove the clear coat and polish the wheels. Just apply wax to non dura-bright wheels every six months and polish with a high speed buffer wheel.

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Reply #12 – September 21, 2013, 12:29:51 pm

When I bought my wheels they had some moderate oxidation.  I have tried the mother's wheel polish and they are better.  I have gone on the internet and searched and found out how to do it with progressively finer grit sandpaper then finally polish but just have not gotten around to doing it yet.  I have seen them polished and like the look.  When I was in dental school they used to say if it shines it flies.  I think it meant if it had a nice shine the instructors liked it.  I don't think that was the whole story. I guess I just need to get on that wheel polishing thing.  Raining today here, so not likely today.  LOL

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Re: Two different size tires on our coach

Reply #13 – September 21, 2013, 01:34:55 pm

Nitehawk:
On my 1996 U320, the coach identification plate beside the driver shows the original OEM tire sizes for both front and back tires. Perhaps older coaches also have this ID plate, in which case you could determine the tire size the speedo was calibrated for.

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Re: Two different size tires on our coach

Reply #14 – September 21, 2013, 01:51:08 pm

That s the beauty of the GPS, it gives you the correct info on flat straight roads.

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Re: Two different size tires on our coach

Reply #15 – September 26, 2013, 11:03:11 am

Mine has the Tyre sizes, Pressures, axle weights, all on the placard beside your drivers seat Norm, Left hand on the wall, Under the window, Just under the Ashtray, Hahahaha,

Mine has a non smoking sign near the exit door, Hahaha I use the ashtray,

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Re: Two different size tires on our coach

Reply #16 – September 26, 2013, 02:18:44 pm

Same place on ours, but someone put different size tires on. Wondered why.

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Re: Two different size tires on our coach

Reply #17 – September 26, 2013, 02:43:09 pm

OK, let's look at the major differences to you (from  Michelin's website):

https://www.michelinb2b.com/wps/b2bcontent/PDF/RV_Tires_Brochure.pdf

The XZE series comes in both sized.

                                235/80R22.5                          275/80R22.5

Revolutions/mile            555                                          516
Max load duels              4410 lbs                                  6610 lbs
Proper rim                    6.75/7.50"                                8.25/7.50"

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