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Old 07-25-2014, 10:21 PM   #1
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Tire revs per mile and speedometer error

New member here and just purchased our first DP. It's going to need new tires and I'm leaning toward the Michelin XZA2 Energy but I have a question on sizing if I could get your opinions. The coach came with 12R/22.5 tires and the equivalent size in the XZA2's is 295/80/22.5. From reading the specs the revs per mile on the new tires are 503 and the 12R's are 486. Would the 17 rpm make a appreciable speedometer error? If it does can it be changed electronically through the Cummins computer?
Thanks
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Old 07-25-2014, 11:28 PM   #2
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Please tell us make/model/year/chassis builder of your coach, it will help you get more specific answers to your questions.

Never mind what tire size is on the coach currently, buy the tire size that is spec for the coach. Is the coach builder still in business, or is it an orphan? I suspect that the 12R/22.5s were probably replacements tires and not OEMs

That said, first drive the coach with current tires and get a GPS speed... you will probably find some variation in speed +/- between the GPS and the speedometer, indicating perhaps that the 12Rs are the wrong size. A difference in revs between 503 and 486 is about 3.5%, which at 60 mph is only about +/- 2 mph... not enough to worry about, just make note of that variation. You might also shop different brands/models of the 295s, and you will probably find some variation in revs per mile within that same tire size. Finally, if increasing tire size/width check first for adequate clearance between wheelwell/fender and the tire, esp. when maximum turned, considering that when turning into a campground or gas station(I know it's a DP) your front wheels may be fully turned and bouncing up/down over the curb cut/swale/apron and thru some potholes. You don't want to be paying for fiberglass repairs on your fenders. Don't know about changing readout. Enjoy your new coach.
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Old 07-25-2014, 11:50 PM   #3
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There should be a Federal Certification Tag near the drivers seat or in a closet that will give you the original tire size. It was mentioned in the video I suggested yesterday. That will make sure you get the correct size tires for your coach based on the GAWRs.

It would help if you gave us the year, make, model of the coach too.
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Old 07-26-2014, 12:26 AM   #4
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I went from the 12R 22.5 Toyo to the 295/80s Michelein and the difference in speedo is about a needle width which is probably 2 to 3 MPH. I don't worry about it.
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Old 07-26-2014, 12:43 AM   #5
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This helped me and may help you with difference in tire sizes.

Tire Size Calculator - Compare Tire Sizes
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Old 07-26-2014, 01:35 AM   #6
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Originally Posted by TurtleKent View Post
Please tell us make/model/year/chassis builder of your coach, it will help you get more specific answers to your questions.

Never mind what tire size is on the coach currently, buy the tire size that is spec for the coach. Is the coach builder still in business, or is it an orphan? I suspect that the 12R/22.5s were probably replacements tires and not OEMs

That said, first drive the coach with current tires and get a GPS speed... you will probably find some variation in speed +/- between the GPS and the speedometer, indicating perhaps that the 12Rs are the wrong size. A difference in revs between 503 and 486 is about 3.5%, which at 60 mph is only about +/- 2 mph... not enough to worry about, just make note of that variation. You might also shop different brands/models of the 295s, and you will probably find some variation in revs per mile within that same tire size. Finally, if increasing tire size/width check first for adequate clearance between wheelwell/fender and the tire, esp. when maximum turned, considering that when turning into a campground or gas station(I know it's a DP) your front wheels may be fully turned and bouncing up/down over the curb cut/swale/apron and thru some potholes. You don't want to be paying for fiberglass repairs on your fenders. Don't know about changing readout. Enjoy your new coach.
It's a 42' 2005 Country Coach Allure 470, ISL 400 Cummins. The 12r22.5's are the original tires, I bought the coach from the original owner and that's also the tire size listed for that year model.
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Old 07-26-2014, 01:36 AM   #7
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I went from the 12R 22.5 Toyo to the 295/80s Michelein and the difference in speedo is about a needle width which is probably 2 to 3 MPH. I don't worry about it.
In that case I won't either. Thanks
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Old 07-26-2014, 07:05 AM   #8
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To answer the other part of your question. The next time you are in for service at a qualified shop, they can reprogram the ECM to fit the tire size. Years ago, I went from a 255 to a 275 X 80/22.5 on our 03 Phaeton. If they are hooked up to it for other reasons, it won't cost you anything. Otherwise they might ding you for a fee.
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Old 07-26-2014, 07:13 AM   #9
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The Cummins computer does take tire size into account. The engine computer communicates with the transmission computer, tire size is important and changeable in the Cummins computer. It doesn't take long for them to input the new tire size. Less than 5 minutes to hook up and make the required change. In fact it will take longer for them to find the data port. So if you know where it is it would save a little time. There will be a hook up for the transmission too. They are different (at least in my coach). A Cummins dealer in our area found numerous computer data inputs that were incorrect in our coach. Once the proper data was entered a number of issues I was having was corrected. So have them verify all settings while they are making tire size entries.
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Old 07-26-2014, 10:03 AM   #10
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Rpm increase indicates shorter tire so height should be fine but it may be wider.

Post this in cc department as likely been done already by someone and you can get better detail specific to your coach.

Be careful of weight ratings as I think front axles a bit heavy on yours.
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