Community Forums

mlh2013 · 05-06-2017, 05:32 PM

My speedometer is 8 mph slower than what my gps reads. My question does this affect my odometer? And is there a calibration somewhere on the speedometer?

Unplanned Tourist · 05-06-2017, 05:49 PM

Quote:

Originally Posted by mlh2013

My speedometer is 8 mph slower than what my gps reads. My question does this affect my odometer? And is there a calibration somewhere on the speedometer?

If your speedo is off, pretty good chance the odo is too. I suspect that's how some people get wonderful fuel mileage numbers, they use the indicated mileage not the real mileage.
Mine is out by right around 10%, and so is the trip computer that says I'm getting 12 MPG! It's really 10.8!

But those are all fun miles.

I like to think of it as SPG (Smiles Per Gallon) Can't put a value on that!!

CadillLinc · 05-06-2017, 06:45 PM

IMHO it would be rare for the speedometer to read low and the odometer to be correct. Both are driven off the data link from the engine computer. Highway mile markers are reasonably correct so you can use them to check the odometer. I suspect you can also check with your odometer from your GPS.

There are 2 parameters that are programmed into the engine computer - rear axle ratio and tire revolutions per mile (RPM). Your rear axle ratio should be shown on your build sheet and probably also on a tag somewhere on the differential. Check your tire brand, type and size and then look up the tire revolutions per mile on the Internet. Then take your coach to your local Cummins dealer and have him hook up his scan tool. He can read the axle ratio and tire RPM that are programmed into the engine computer. He can also change them if incorrect.

Gene M
2004 Monaco Knight

Rburian · 05-06-2017, 07:19 PM

Did you change the size of your tires, if you have larger than OEM,s you will have a difference between your actual speed and indicated on the odometer.

J2Jake · 05-07-2017, 06:27 PM

Do the larger tire speed up or reduce indicated speed? On my coach, spedo indication is 5-7 mph faster than my GPS and OBDII readings. Jake

Rburian · 05-08-2017, 04:05 AM

You will be driving at a higher speed than indicated and your distance will be farther than indicated. When I changed from 255 to 275 on my previous coach, Multiplied 104.5% to the mileage indicated to get the actual distance.

Bob B
2005 HR SCEPTER

mlh2013 · 05-08-2017, 06:53 PM

I haven't changed tire size. What is weird is it has always been within maybe a mph or 2
at the most of gps, we took it out one day and all of a sudden it was 8 mph slower than gps

BigPapaYams · 05-10-2017, 08:56 PM

Hi mlh2013,
I had a similar situation last year that was very puzzling to me. I am not sure that there is a direct correlation but my throttle control / cruise control died a slow death over a period of more than 6 months and I noted that the speedo reading was more and more "off" from the speed reading on my GPS.

Read my post http://www.irv2.com/forums/f258/no-t...er-331521.html in the Class A Motorhome Discussions forum.

My speedo has been "healed" since the rebuild of the throttle control box. Not saying that this is your issue but it's something to consider if you have any of the other indications that my coach was exhibiting.

GOOD LUCK

R2Home · 05-11-2017, 06:58 AM

We just changed tire size, went from 275/80R22.5(39.8"tall) to 275/70R22.5(37.6"tall) and the speedo is off by about 3 mph. For the time being I can live with it, but when we return home will take the MH to our Cummins dealer and have the engine ECM re-programed.

Triker56 · 05-11-2017, 07:50 AM

Almost all roads have mile marker signs.

Run 10 miles of them and check what your odometer read.

Mine with larger tires that replaced OEM tires. Odometer shows 9.9 miles for every 10 miles of road markers. Or 99 miles for every 100 driven.