On a recent trip, I took along my Pioneer GPS unit. I noticed that the indicated speed on the GPS was at 65 when the coach speedometer was at 70.

The Data tag shows original tire size in 1993 as 255x80x22.5. The coach now has the lower profile 255x70x22.5 tires.

My question is could this possibly account for that much difference in the speedometer readings?
Nick

Yes, it could. Assuming 9" rims, roughly, the 80 tires would have a 7.2" depth while the 70s would be 6.3".

Adding each to the rim radius (11.25") results in 18.45" for the 80s and 17.55" for the 70s. The difference is 0.9", about 4.9%, which would result in a speedometer reading about 3.2 mph high. Not all the difference, but obviously significant.

Rusty, I'm going to soon have to buy tires. Ouch or double ouch. Any advice on going back to the original size tires or trying to go to a larger 70 series to correct the speedometer. Do You think the 70s ride and handle that much better?
Nick

I can't address ride and handling, since I've only used 80 profile on my coach, and the Regency has a different suspension than my Breakaway.

However, I have one consistent reservation about the lower-profile tires (especially the 40s and 50s folks use to rice up SUVs), and that is reserve capacity - the 70 profiles carry less air than the 80s, and if one has a leak...

Nick, I had the same problem but it was not tire related, my speedometer was off. When I got my GPS unit the speedometer showed 70 and GPS showed 67.

On the back of the speedometer (VDO unit) there was a set of 10 calibrating switches, they are either off or on. The sequence of 1s & 0s (on or off) that they have been set determines what the speedometer reads.

I recorded the switch settings (MUST DO before changing) then I got the code table from VDO and did a little math to determine what switch settings I needed to have the speedometer read correctly and agree with the GPS.

If you could get the sequence of switch settings that it is now set to me I could tell you what the switches need to be based on your 70 vs. 65

I used a magnifying mirror to look at the back of the speedometer to record the switch settings.

Don't need to change the tires to get the speedo on track.

I just put the VDO speedometer PDF up on my website for your reference, look it over and if you can do by yourself, do it, if you need help let me know.

VDO speedometer info

Thanks very mucho Ed for the education on the speedo, will try that on mine when I have the chance. Thanks.
Happy camiping. sky

I'm never ceased to be amazed at the info on this web site. Thanks Ed, I think I'll get the new tires then calibate the speedometer. If I need your help, believe me, I'll ask for it.
Thanks,
Nick

Me Three, awesome! The accuracy of mine is suspect. I'll have it checked and correct it.

I wrote up a quick method to re-calibrate your VDO speedometers using my settings as an example, I didn't want to post it directly here because I thought the columns might not line up an would be confusing. If anyone attempts this and has trouble, contact me and we can work it thru!

speedometer calibration

Enjoy