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Old 01-19-2014, 02:46 PM   #15
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No identifier on the front of the speedometer except the word Freightliner. So a call to Freightliner might provide information.
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Old 01-19-2014, 04:52 PM   #16
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In the old days you had to change the gears on the tail of the tranny.

Today, I think they use a computer and pop in a new "RPM to MPH" factor.

But in either case.. You need an Allison man.
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Old 01-19-2014, 07:52 PM   #17
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If you have a electronic controled engine the speed-o calibration is set in the engine computer with pulses per mile. The formula is RPM (tire rev per mile) X Gear ratio (differential gear ratio) X output gear teeth (tone wheel in rear of transmission) = pulses per mile
Each tire size and brand have its own revolutions per mile
example 474 rev per mile gear ratio 3.73 16 tooth tone wheel
474 X 3.73 X 16 = 28060.8 rounded off to 28061 pulses per mile.
This is set in the engine computer parameters and needs to be done at a dealership ( engine or truck) or shop with access software on a computer.
If it is a manual controlled engine with no computer there are dip switches in the rear of the speed-o head to set the pulses per mile, and the pulses per mile are converted to dip switch settings on or off of each switch, there may be 8 or 10 switches depending on the speed-o head.
Hope this helps

Also I have a conversion book to change pulses per mile to dip switch settings for the 8 or 10 switch speed-o meters if anyone needs numbers converted.
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Old 01-25-2014, 11:15 AM   #18
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FYI - The local Allison shop says the speedometer is not adjustable at the transmission but can be adjusted using the ECM. (The Allison man on the Allison Forum on IRV2 confirmed this) I have not contacted Freightliner but plan to do so.
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Old 01-25-2014, 12:28 PM   #19
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I worked for a trucking company eons ago and on the trailers we used what was called a hubometer to keep track of mileage. Must be pretty accurate as we computed the taxes paid off the miles on those. You have to manually read them or did back then. I'm not sure if you could use one on these with the fancy wheel covers no way to read them
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Old 01-25-2014, 12:54 PM   #20
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I worked for a trucking company eons ago and on the trailers we used what was called a hubometer to keep track of mileage. Must be pretty accurate as we computed the taxes paid off the miles on those. You have to manually read them or did back then. I'm not sure if you could use one on these with the fancy wheel covers no way to read them
Revs per Mi would still be contingent on tire outer circumference.
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Old 01-26-2014, 06:48 AM   #21
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Quote:
Originally Posted by 1ciderdog View Post
FYI - The local Allison shop says the speedometer is not adjustable at the transmission but can be adjusted using the ECM. (The Allison man on the Allison Forum on IRV2 confirmed this) I have not contacted Freightliner but plan to do so.
This is good to know. Thanks. Now to find someone willing to do this on a RV. We are often lepers when it comes to stuff like this. And we get charged a premium because it is a RV. Hard to work on in all cases. No discount for ease access. Ahhh. The joys of being free to travel.
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Old 01-26-2014, 07:39 AM   #22
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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 found the difference. As those are not the same size tires.
The XRV are 255/80/22.5 38.2" diameter 541 RPM
The XZE are 255/70/22.5 36.7" diameter 563 RPM

The G670 255/70/22.5 are 36.4" diameter 566 RPM
If you replaced the 275/70/22.5 G670's they had a 37.6" diameter 548 RPM

There should be a label on the drivers wall that will have what size the OEM tires were.

To check the odometer. On the Interstate or any other road with mile markers.
Check the odometer against 10 mile markers and see what the difference is.

I went from XRV 235/80/22.5 to G670 275/70/22.5 and the G670's in 10 road marker miles show 9.9 miles on the odometer. Or it loses 1 mile for every 100 driven.
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Old 01-26-2014, 09:30 AM   #23
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Bob, your speedometer reading is adjustable by computer entry, one of the programmable parameters is tire size, as well as being able to control how cruise control and the exhaust brake functions. The gearing and sensor are not changed...

This calculator makes it easy to compare the change in dimensions for various tire sizes. 255/80-R22.5 vs 255/70-R22.5 Tire Comparison - Tire Size Calculator

The closest tire size to Michelin's 255/80R22.5 is everyone else's 265/75R22.5
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Old 01-27-2014, 10:09 PM   #24
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Don't feel so bad. If I wanted to do 55 in our '78 Tioga, the speedo had to be at 75! Good thing we never drove it much, or else the "mileage" on it would have been much higher! It was always way down on the to-do list, so it never got done.
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Old 01-28-2014, 02:00 PM   #25
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I talked to Freightliner today. They say the speedometer that can be adjusted by using dip switches on the back side of the speedometer was discontinued several years ago. The only way to adjust the is to take it to CAT and they will do a formula something like tires revs per mile, times the rear axle ratio times 16 (16 has to do with the gears in the transmission). You end up with a huge number they use in the ECM. At least that's what I think he said.
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Old 02-07-2014, 06:46 PM   #26
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I started this post: I stopped by Caterpillar in Medford today and they programed the ECU for the correct speed. I had called Freightliner and got the axle ratio (4.79) and the transmission gear (16) and along with the tire revolutions per mile. Using those numbers Caterpillar in Medford took about five minutes to do the adjustment. I drove about 150 miles today and the speedometer showed the same as the GPS. GREAT THANKS TO PETERSON CAT - NO CHARGE - Shop foreman just asked me to bring it in if I have any other problems.
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Old 02-07-2014, 07:38 PM   #27
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Thank you for the update.
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