P10 ECM Strange Speedometer Values in VCM Editor - GMC Envoy I6

[KYSilverado04]

*** Edit.... I apologize this probably fits better in the transmission section.

Hey Guys,

I'm sure this isn't going to be a very popular platform so I have to wonder if maybe there are some wonky mappings / translations going on in the software. I just inherited a 2005 GMC Envoy with the 4.2L I6 and 4L60E transmission. It has RPO option code GT4 which is supposed to indicate a 3.73 rear axle ratio. This brings up the first bit of strangeness.... The gear / tire wizard does not report anything at all, it only has 1 option and says "manual scale" and gives me a multiplier. On every vehicle I've ever looked at before (including the tune from a 2007 Envoy downloaded from the website) these values exist. I didn't really think too much of it until I started looking at some other values.

I looked at Speedo and looked at Gear Ratio & Tire Size and found a values of Trans Revolutions / Mile. According to the description this is transmission output shaft revolutions per mile. Would this not equate to the TIRE revolutions per mile multiplied by the rear end gear ratio? Sure seems like it would to me, I've calculated the math manually many times in my life and have even written a spreadsheet to do it for me, I understand the concept, yet:

Tire size 245/65/17 = 29.54" Diameter = 682.75 revolutions per mile. 682.75 * 3.73 gear ratio = 2,546.65 driveshaft revolutions per mile. Ok, sounds great! Except for the fact that VCM editor is showing me a value of 4,244.8013. That's no rounding error. VSS Trans Revs per mile is 4,221.6984 which is also nowhere close. I vaguely remember one tuner on a youtube video state that the ECM actually often likes a tire size of about a half inch smaller than the actual calculated number. If I do that, AND assume 3.08 gears instead of 3.73, AND assume that the value in the ECM is for some reason doubled, then finally I can get kinda close.

29" Diameter = 695.45 revolutions per mile. 695.45 * 3.08 = 2,141,98 driveshaft revolutions per mile, multiplied by the 2x potato factor gives me 4,283.972 which might be close enough to be within the margin of error. That's all well and good except that I just don't buy it. I could almost accept that the value is doubled in the ECM and I could accept that the tire size vs calculated size is a touch off, but you will not convince me that GM put 3.08 gears in it and sold it as 3.73's. There's also the small little problem of the speedometer being accurate vs the GPS and the math proving it to be 3.73's.

If I use the mathematical tire size, I come up with 1500 RPM's in 4th gear should be 50.8 MPH, and I've got a data log showing it within 1 mph. If it had 3.08's in it, I'd be off by 10MPH.

At the end of the day it's not really all that important unless I change tire size or something but it just struck me as very odd. Thoughts? Comments? 4 Letter Words?