For Xmas I'm driving out to LA from Vegas cuising along the empty I-15 at 90 mph according to the speedo. Just for kicks, I reset the average speed counter in the OBC and watch it jump up to ... 85 mph. The car was in cruise control on a flat surface and the speedo was clearly at 90. Computer says 85 - speedo says 90.

Hmm...

 

there's a bunch of threads on this (use your search function)

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Yup. That is true with all of us. There have been a few threads regarding this...I could find them for you, but I'm lazy. If you put on larger/smaller wheels you will see more/less discrepency. I have been able to convince my wife that there is an even larger discrepency that there really is. When the big 'ol speedo says "90", I have her believing we are really only going about 70-75! Use it to your advantage!

 

The downside is our cars are going to show more miles than we actually drove, so our odometers will be quite high. Bummer.

 

>>The downside is our cars are going to show more miles than we actually drove, so our odometers will be quite high. Bummer.

This is not going to be the case because of the Speedo Discepancies. Yes, the Speedo reads high but that does not carry over to the Odometer. If you change wheels from stock you may have some Odometer distortion but it should be minor.

 

"Yes, the Speedo reads high but that does not carry over to the Odometer. If you change wheels from stock you may have some Odometer distortion but it should be minor."

I would be surprised if the odometer had a different signal source than the speedometer. Hence, whatever error is evident on the speedo will be reflected in a corresponding error on the odometer.

btw, in my 04 MCS, the digital speed readout perfectly matches the analog speedo reading. They may both be showing me the wrong speed but at least they track each other!

 

>>I would be surprised if the odometer had a different signal source than the speedometer. Hence, whatever error is evident on the speedo will be reflected in a corresponding error on the odometer.

I would also be suprised if the signal source were different. It is not the source that makes the difference it is what is then done with the signal. Euro car Speedometers are "calibrated" to never read slow so the signal is multiplied by factors that are different than those used for the Odometer. If you check your Odometer against mile markers you will most likely find a different "error" ratio than that of the Speedometer.