Final Drive Ratio - E38

[adunphy]

I've installed 285/60-R18 tires on my 2007 Silverado NNBS (GMT900). From the Speedometer--General tab I see 3 different "Final Drive Ratio" entries.

Ratio: 3.23
Err: 3.23
Trans: 3.73

In the Speedometer--Gear/Tire Calc tab it shows me a the current values of
Tire Size: 30.79
Gear Ratio: 3.73

Knowing my truck has the RPO code of GU5 (3.23 rear axle gear ratio) I'm unclear on what I should enter for the new Gear Ratio in this case. I have not changed the gears in this case. I know my new "Tire Size" is 31.5 but should I use 3.23 or 3.73 for the Gear Ratio?

[adunphy]

As an update, I tried using values of both 3.23 and 3.73 in the ratio and keeping the 30.79 diameter and comitting. Upon doing so the pulse counts changed so I'm thoroughly confused on what values to use for simply putting larger tires on the stock tune?

Any support is appreciated.

[07GTS]

leave the gear ratios standard and only change the tyre size by the same percentage the speedo is out, if there is a front and rear tyre size make sure u change both by the same percentage so it dosnt throw a code and then check with gps to make sure it's ok, if front and rear are different then change the error value to suit so it dosnt throw a code

[adunphy]

The software does not let you only change tire size. It requires you to enter (something?) for both. The problem I have is that if I enter the actual rear gear ratio of 3.23 and it changes my values and what the Trans ratio is on the other tab. If I use 3.73 which is what shows as "current" from the stock tune, it also modifies the values. In other words entering the current numbers and hitting commit makes modifications to the tables. That is why I'm very confused.

The Trans and rear axle are stock. All I want to do is adjust for my new 31.5" tire size and its not straight forward in the OS. Hoping someone from support can weigh in.