OBD Roadspeed very inaccurate - advice from experts?

[HarryMat]

Hi, I am an engine noob, but am an electronics control guy, so I maybe know enough to be dangerous...

Summary - I have just had an L76 + 6L80E put into my 4wd, also has altered diffs and tyre size. Basic idea is that the ECU runs driveline and the original BCU is kept separate to run everything else. Builder has done some integration to get cruise working and has repurposed steering wheel buttons to do tap-up and tap-down shifting. All this works nicely, a speedo corrector was put in and the original speedo/odo are tested and correct.

So I put an Ultragauge on the engine OBD to help track fuel use in particular. RPM and most else reports correctly, but roadspeed is 40% high. Ultragauge can have correction factor put in, and this works, now roadspeed and fuel consumption in Ultragauge are correct.

Question for experts - is there an issue that roadspeed from the OBD is so inaccurate? 40%+! I'm guessing that parameter could be important to engine/trans management.

I'm trying to get some information so I can discuss with builder - should the speed input to the engine have been corrected (for diffs and tyres), or is there a calibration in the tune that should have been adjusted for this. Or is this no big deal, just correct the Ultragauge and all OK.

Thanks for any info!

[blindsquirrel]

You would need to look at the data direct from the ECM, as of now it's impossible to say if it's the calibration or how the OBD thingie is (mis)interpreting things.