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Old 01-17-2014, 06:42 PM   #1
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Odometer & speedometer error

I recently replaced the Goodyear 670's with Michelins. Michelins were the original tires on this RV. Now with the Michelins I seem to be having speedometer error and odometer error that I did not have with the Goodyear 670's. Comparing the speedometer with the GPS 62 mph on the speedometer is 58 or 59 on the GPS.

For the odometer part my wife keeps a log and writes beginning and ending mileage. Again with the Goodyear the odometer was very close to her mileage at the end of the day but now the GPS will show we travelled 309 miles but her total is 326. I realize there is a size difference but would have thought with Michelins being the original tire (I have the original size Michelins back on it) would be more accurate.

So does anyone else notice this and second is there some type of adjustment that can be done to correct the speedometer/odometer error.
Thanks
Bob
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Old 01-18-2014, 07:36 AM   #2
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I believe a transmission shop that works on Allison or other big transmissions can change the gearing for you. I think this is a common problem. Michelin makes a special size tire for RVs. When you change brands that size is not available. Check with a Freightliner Oasis RV repair center. They work on most models and might be able to help you.
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Old 01-18-2014, 09:13 AM   #3
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Sorry, can't help with the speedo calibration, but that is usually a direct output of an RPM monitor or in the old days, a direct mechanical linkage with sometimes a servo to move that data to a remote location via wire with the "pick off" usually at the transmission output from any I have seen, nontheless an interesting observation given the original tires were Michelon.

This difference is ~5.5%, hence one would think this is a direct result of the Michelon tire having a tread circumference that much less than that of the previous tire. Did you calculate any MPG difference? Just curious.

With smaller tires, hence more revolutions for the distance traveled could mean tire wearout 5.5% sooner, however there are so many other factors at play here that it might not make any significant difference. Different tire brands have different wear rate and tread resistance on the road etc.

Is it possible that the speedo was inaccurate right from the start with the original tires or the new tires have different outer dimensions than the original tires?

Of the 5 vehicles I own, only 1 has an accurate speedo that has a linear speed comparison with the GPS. My Van reads about 5MPH high @ 70MPH. I am really only going 65 MPH. I have calculated against mile markers and the GPS is right on but the speedo is not. My RV reads about 2 MPH higher than the GPS @ 60MPH.

Not certain what the legal tolerance is for speedo/odometer readings but one would think there would be one.

Good luck, please post your findings re speedo/odometer calibration.
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Old 01-18-2014, 09:22 AM   #4
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Quote:
Originally Posted by jerichorick View Post
Michelin makes a special size tire for RVs. When you change brands that size is not available.
I"ve had tire shops tell me that about my 275/80R22.5's ..."that's a unique Michelin size". But I found that was not true ...they just didn't sell what I wanted. I went from Michelin to Continental and stayed at identical size. That said, my speedo with either tire is apx 4mph faster than gps at highway speed. Didn't know that until coach was a few years old ...oem high $$ Alpine gps didn't show speed. One day I used the toad Garmin and saw that ...Rand McNally I put in the coach shows the same difference.
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Old 01-18-2014, 04:03 PM   #5
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I spent a bit of time this morning searching Michelin tire sizes trying to find the size difference or number of revolutions per mile on 255/70R 22.5 tires. My new tires are XZE not XRV and as best I can find out the XZE revolves 563 times in a mile and the XRV 541 revolutions. If what I am finding is correct that would probably account for the difference.
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Old 01-18-2014, 05:04 PM   #6
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I spent a bit of time this morning searching Michelin tire sizes trying to find the size difference or number of revolutions per mile on 255/70R 22.5 tires. My new tires are XZE not XRV and as best I can find out the XZE revolves 563 times in a mile and the XRV 541 revolutions. If what I am finding is correct that would probably account for the difference.
You are close. Based on my very quick.calculations : @ 563 Revs/mi means the tread circumference is about 9.4' with an OD diameter being about 36". So it all sounds about right.

If the difference in odometer reading is about 5.5% off from the readings on your old tires. One would think your old tires would have an OD circumference of ~9.4' x 1.055 = ~9.9' thus =~533 rev/mi with a diameter of ~37.8".

Note the 5.5% delta is a rounded number based on milage calculations so it may not be absolute.

@541 rev/mi the circumference would be ~9.75' and a diameter of ~37". So you can see a 1 or 1 1/2 in diameter makes a significant difference on milage measurement. And likely fuel economy.

I think I got the math straight. I only use pi to 2 decimal points.
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Old 01-18-2014, 05:09 PM   #7
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Thanks for the input. I live very rural but will be going to the "big city" Wednesday and will stop by Allison and/or Freightliner to see if they have a way of adjusting the signal output. I would presume it is electronic and not cable and gears like the good old days.
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Old 01-18-2014, 05:20 PM   #8
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Thanks for the input. I live very rural but will be going to the "big city" Wednesday and will stop by Allison and/or Freightliner to see if they have a way of adjusting the signal output. I would presume it is electronic and not cable and gears like the good old days.
Oh I suspect it is an electronic pickup by magnet somewhere off the driventrain, and perhaps there is a way to calibrate the instrumentation. Will be interesting to hear what your findings are.

Just by chance did you record RPM vs MPH on old and new tires?
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Old 01-19-2014, 07:03 AM   #9
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Oh I suspect it is an electronic pickup by magnet somewhere off the driventrain, and perhaps there is a way to calibrate the instrumentation. Will be interesting to hear what your findings are.

Just by chance did you record RPM vs MPH on old and new tires?
Me too. I was very general in my first post about the "gear", as am old school thinking. Yes, magnetic pickup.

When I had the od repair done on my last coach, Freightliner flat our refused to set the mileage to the known correct value. "It is against the law" according to them. (Never knew telling the truth was against the law) Cost me $4K on trade to this coach. But, with that said, the correction may be possible but they may be mandated by company rules not to do it.

Another thought. There are big discussions on irv2 about fuel mileage. The above math exercise indicates that this may be a wast of time. We really don't know how far we are traveling on a gal of fuel. I would really like to know. At the moment all I can tell you is that it costs big bucks to put 60+ gals in the tank . Also, with this latest info, I only know for certain where I started and where I finished. I have no accurate way of knowing the distance. It is all a duck shoot at this point and not worth fussing over.

Happy trails all.

Rick
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Old 01-19-2014, 07:12 AM   #10
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A 5 Star or simular tuner has a option for changing to Od and Speedometer settings by inputting the correct revolutions per mile for your tires. One of many reasons to buy one.
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Old 01-19-2014, 11:34 AM   #11
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Originally Posted by Libero View Post
Oh I suspect it is an electronic pickup by magnet somewhere off the driventrain, and perhaps there is a way to calibrate the instrumentation. Will be interesting to hear what your findings are.

Just by chance did you record RPM vs MPH on old and new tires?
I did not record the difference between the old and new tires but I have two GPS units and they both track mileage. As mentioned in post 1 the GPS mileage was very close to the odometer mileage. With the new tires it is about 5% off.


Quote:
Originally Posted by jerichorick View Post
Me too. I was very general in my first post about the "gear", as am old school thinking. Yes, magnetic pickup.

When I had the od repair done on my last coach, Freightliner flat our refused to set the mileage to the known correct value. "It is against the law" according to them. (Never knew telling the truth was against the law) Cost me $4K on trade to this coach. But, with that said, the correction may be possible but they may be mandated by company rules not to do it.

Another thought. There are big discussions on irv2 about fuel mileage. The above math exercise indicates that this may be a wast of time. We really don't know how far we are traveling on a gal of fuel. I would really like to know. At the moment all I can tell you is that it costs big bucks to put 60+ gals in the tank . Also, with this latest info, I only know for certain where I started and where I finished. I have no accurate way of knowing the distance. It is all a duck shoot at this point and not worth fussing over.

Happy trails all.

Rick
I think my previous mileage estimates were reasonably accurate based on odometer & GPS miles driven between fuel ups. Now if I use the odometer mileage the MPG will be off.
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Old 01-19-2014, 11:43 AM   #12
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A 5 Star or simular tuner has a option for changing to Od and Speedometer settings by inputting the correct revolutions per mile for your tires. One of many reasons to buy one.
I am not familiar with the 5 star tuner - does it change the speedometer/odometer reading on the vehicle?
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Old 01-19-2014, 11:57 AM   #13
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I found this document for my Freightliner, Cat, Allison chassis. It may not apply to yours but it's worth looking into.
https://skydrive.live.com/view.aspx?...=WordPdf&wdo=1

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Old 01-19-2014, 12:12 PM   #14
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Dick, Thanks
The question then becomes do I have that type of speedometer so I'll go out and see if I can make that determination.
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