Community Forums

jerichorick · 04-08-2016, 08:08 AM

I heard somewhere that the transmission could be calibrated to clock the revs per mile to give a more correct speedometer reading. I am changing my drive tires soon from 275/80R22.5 to 295/75R22.5. My speedometer is not that accurate now. I don't know what it will be when I change the tires. Is this correctable through the Allison? If so, where do I get this done?

Rick Y

Skip426 · 04-08-2016, 09:14 AM

The programming would have to be done at an Allison repair facility, or possibly, your chassis builder's dealer .
They will need to know your gear ratio ; info available from the chassis builder; and the tire RPM,( Rev,Per,Mile) from the tire manufacturer.

C-PHartley · 04-08-2016, 09:15 AM

I recently replaced my speedometer and learned that the Allison has a pulse signal to the speedometer. I had to calibrate the speedo to get it right. No simple task. I ended matching my gps indicated speed to the speedometer to get it close.

Sent from my iPad using iRV2 - RV Forum

Happycarz · 04-08-2016, 09:20 AM

On many of the later model coaches, the instrument cluster gets its information from the J1939 buss. The J1939 buss is the wire bundle that connected to the ECM, TCM, diagnostic connectors, shifter panel, and instrument cluster.

The way speedometer/odometer changes are made is by connecting a laptop to the diagnostic port to update the revs per mile on the size of tire you are using, into the ECM. Revs per mile are available on the tire manufactures website.

Cummins Insite was the laptop program that was used on my coach to update the rear end ratio when I changed the differential gearing, as I have a Cummins powered coach.

A phone call to the coach manufacturer or shop that services your particular engine would be able to confirm the above.

Hopefully, you will get lucky and the new tires will correct your speedometer error without spending an hours worth of shop time updating the parameters.

Theberrys · 04-08-2016, 09:22 AM

Hi
Here is a real SWAG. If you have a Freightliner Chassis and it's of the configuration that has adjustable speedo then this document may be of use to you. It would really help this thread if you would disclose your Chassis type and Model.
https://nczkyw.dm2302.livefilestore....Doc.pdf?psid=1

Dick

1ciderdog · 04-08-2016, 11:58 AM

A couple of years ago when I changed tires (same size different brand) the speedometer was off buy about 5 mph at 60 according to the GPS. As mentioned above I wrote down tire revolutions per mile, differential gearing and took the RV to a CAT dealer - they plugged in a laptop, hit a few keys and it was done. This is on my 2004 with a CAT engine. Didn't seem complicated at all - first was a call to the local CAT shop to find out what information they needed, then getting the information and going to the CAT shop. (The document mentioned in post 5 would not come up on my computer)

jerichorick · 04-09-2016, 08:42 AM

Quote:

Originally Posted by Theberrys

Hi
Here is a real SWAG. If you have a Freightliner Chassis and it's of the configuration that has adjustable speedo then this document may be of use to you. It would really help this thread if you would disclose your Chassis type and Model.
https://nczkyw.dm2302.livefilestore....Doc.pdf?psid=1

Dick

Thanks for the nudge, Dick. I updated my signature which should help. The answers are so mixed. Once the new tires are on I'll know more. I have decided on the Toyo M154 H load. That tire gives 514 RPM and the OEM Micheline XZE2 G load is 517 RPM.

If I recall correctly, at 65mph on the dash the GPS sees about 63mph. Less revs on the new tire will bring this down even more. Good for reducing speeding fines??

(Try not to.

)

I hope this helps. We will be near Gaffney, SC this summer. This is where the chassis is manufactured. I think I may call them to see what can be done. From the above posts it sounds like my original question can be answered with a YES, but....

Thanks all.

Happy trails,
Rick Y

Old Scout · 04-09-2016, 08:49 AM

If Cummins--its the "tire revs per mile" setting on the ECM--any Cummins shop should be able to change it--at a charge of course.....no biggie--I let it go--a few less MPH on the speedo, a few less miles on the odometer.....

DGBPokes · 04-09-2016, 09:38 AM

IMHO.....sounds like a real pain in the pocketbook for an accurate number.

Years ago, before GPS, and when MPG became a big thing in cars, I found that auto manufactures "cheated" on the speedometer calibration big time. Helped boost a poor MPG in people's mind. Like I said before GPS. So how do you get a "true" sense of MPH?

I live in a flat part of the country. I would find a straight, flat, length of interstate, turn on the cruise, and watch the sweep second hand of my watch. Set the cruise at 60 mph, then look at the watch as I passed a mile marker. Then the second mile marker, then the third, etc. Not all mile markers are the same, so 4-5 miles was a good distance. I would then adjust the cruise till I was passing mile markers at the same point on the watch second hand mile after mile. Now what does my speedometer say? That way I knew that 64 really was 60. Hence my speedometer was always off X mph at 60. The difference between 60 and 70 or 60 and 50 was almost mute.

A great exercise on a long boring drive. 😋😋

dbarton291 · 04-09-2016, 10:45 AM

Quote:

Originally Posted by jerichorick

I heard somewhere that the transmission could be calibrated to clock the revs per mile to give a more correct speedometer reading. I am changing my drive tires soon from 275/80R22.5 to 295/75R22.5. My speedometer is not that accurate now. I don't know what it will be when I change the tires. Is this correctable through the Allison? If so, where do I get this done?

Rick Y

The transmission control module only provides a signal from the output speed sensor based on the number of teeth on the tone wheel. It cannot be changed. The calibration of the speedo is done elsewhere depending on the vehicle configuration, usually at the speedo head itself or the engine ECM.

kketterling · 04-09-2016, 11:16 AM

Sorry don't mean to take this off topic but Happycarz what gear ratio did you change to? I currently have 4.30 to 1 gearing and upsized the tires to 315s and unless I'm doing about 67 mph the engine doesn't run in the sweet spot recommended by CAT. Realize the tire change affected part of this issue but even before the change seemed it was hard to stay in sweet spot and within the speed limit.

I'm wondering if you notice an improvement in gas mileage as result of your ratio change as well?

PM me if you get a chance.

Thx

grashley · 04-11-2016, 03:32 PM

All of the above are good. Personally, I pretty much ignore the speedometer and look at the GPS for an accurate speed.

jerichorick · 04-12-2016, 06:32 AM

Quote:

Originally Posted by grashley

All of the above are good. Personally, I pretty much ignore the speedometer and look at the GPS for an accurate speed.

I agree. I am interested in mileage and fuel consumption. I know how much fuel I use, of course, but the info center on the dash is providing false information. As the years and miles tic buy this error grows. Is it important? I guess not. For me it is some sort of mental comfort.

Happy trails,
Rick Y