Duramax and allison swap

I'm going nuts trying to calibrate my speedometer, so far the closest i can get is a 5 to 7 mph diff( speedometer to fast for actual). I keep changing or lowering the revs per mile but nothing seems to get it any closer and i'm a bit baffled as to why up to 55 on speed reader it's 5 to 7 mph fast but above that it's 12 to 13 mph fast what's the deal there?. This is a 2007 LBZ allison swap into a 1996 GMC sierra 2500. At first i was flashing the TCM but that did nothing so now i'm flashing ECM

I figured it out,even though I verified I have 4.10's I changed the tune to 4.56 this morning and it's now dead on