Speedometer is slow after tire replacement

[jbell]

I have an 2002 Allegro Bus on a Freightliner chassis. Just replaced my old tires (255/80R-22.5) with (275/80R-22.5) and now my speedometer is running about 3 to 4 mph slow. It's not that big of a deal, but I was just wondering if anyone else has experienced this, and if they know how to correct the problem. Would the correction be made in the Allison transmission software or on the Freightliner ECM software?

[wolfe10]

It could also be in the speedometer. I know the VDO gauges have a calibration function. That may be the easiest/least expensive. So, start by calling your speedometer manufacturer.

It is also possible that your chassis maker has a module that can provide this function. So if the speedometer does not have the calibration function, call your chassis maker.

I confirmed with Allison that the calibration feature is NOT in the Allison ECU.

I also confirmed with Caterpillar (you don't state what engine you have) that the speedometer calibration CAN be done in the engine ECM on electronic engines.

Brett

[jbell]

It could also be in the speedometer. I know the VDO gauges have a calibration function. That may be the easiest/least expensive. So, start by calling your speedometer manufacturer.

It is also possible that your chassis maker has a module that can provide this function. So if the speedometer does not have the calibration function, call your chassis maker.

I confirmed with Allison that the calibration feature is NOT in the Allison ECU

I also confirmed with Caterpillar (you don't state what engine you have) that the speedometer calibration CAN be done in the engine ECM on electronic engines.

Brett

Thanks Brett,

All I need is a little direction and I become dangerous. I'll have the Caterpillar folks check it out at my next service date.

And by the way, I should have mention it is a Cat 330, and again thanks for the help.

Jimmy

[hermanmullins]

Thanks Brett,

All I need is a little direction and I become dangerous. I'll have the Caterpillar folks check it out at my next service date.

And by the way, I should have mention it is a Cat 330, and again thanks for the help.

Jimmy

Jimmy, You have one of the best go to guys in Brett. He will research your problem and get you the correct answer. I have a Trip-Tec in my coach between it the coach speedo and my GPS I have 3 different readings. I will have Cummins check mine the next time I have it in for service. In the mean time I set my speed by the GPS. It may be slow some times to react up or down but it is right on the money for speed readings.

Happy RVing