Speedometer recalibration problem

[jrtexgunner]

I have a 2004 Chevy 4500 with the Duramax LLY motor, using a U7185 programmer (I believe v2.02?). I just changed tires/wheels from the stock 19.5" to a set of 22.5". Tried to use the programmer to recalibrate the speedo, it takes the tire size fine, loads the tune, but the speedometer is always off about 8 mph, using anything other than the stock tire size. This is based on a GPS unit speed readout, and a trailing vehicle's speed.
Any ideas what the deal is here, I thought the Predator would fix the speedo with a tire size change?
thanks,
JR

[mikel]

Sounds like you are not entering the stock size first followed by the new actual tire size.

[jrtexgunner]

Mike,
I think that you mean the tunes weren't originally modified to input the stock tire size, axle ratio, etc. Did I understand you correctly? I did input that information when I started using the programmer, and it was there when I modified it for the new tire size a few weeks ago. The thing is, the speedometer doesn't seem to be "off" any differently no matter what tire size I put in. Any ideas with that information at hand?

[mikel]

Try resetting all parameters and reinstalling the tune, see what changes.

[neonhomer]

I think the tuners between GM gas and GM diesel are different, but on my tuner, I just put in the tire height, and that puts me within 1 MPH.... close enough for me.

[jrtexgunner]

I reset the tune, but no change. It doesn't appear that the gear ratio or tire size setting have any effect.
Neonhomer, on this programmer I have to input the complete tire size, rather than just height.

[neonhomer]

My programmer has both it in... tire size and tire height.....