245/75R-16's to 265/70R-17's on a 2008 GMC LMM

[bsmall2800]

I
I know this topic has been discussed in lengths in the past but here is the latest as of May/2014, at least from my prospective, and knowledge.
I finally replaced my 245/75R16’s with 265/70R-17’s, which through off my shifting times with the Allison transmission, and the spedo no surprise there.
I read post after post after post about having the dealer re-flash the ECM/Computer on the truck and it will recalibrate things to match the 17’s. I heard everything from its not necessary, the dealer won’t do it, the re-flash only resets the Speedo and DIC (drivers information center) and not the transmission shifting points.
Fast forward too today, my dealer had to call GM to verify that my size tires were in their data base for my truck before they would send the program to have the truck re-flashed. It turns out GM recognized my size tires in their data and sent the program to the dealer/technician, which was downloaded to my truck.
If my new tire size wasn’t in their data base, then the dealer wouldn’t have been able to update my truck.

It turns out that the re-flashing does in fact re-set the transmission shifting points back to the speeds that it use to shift. It also re-sets the DIC to match the new tire size, and in addition, the re-flash of the ECM also will update any updates that GM has generated over the years, so now that’s up to date with all the latest updates.

http://i891.photobucket.com/albums/ac114/bsmall2800/2008%20GMC%20SIERRA/100_2400_zpsf8f3e9c1.jpg (http://s891.photobucket.com/user/bsmall2800/media/2008%20GMC%20SIERRA/100_2400_zpsf8f3e9c1.jpg.html)

http://i891.photobucket.com/albums/ac114/bsmall2800/2008%20GMC%20SIERRA/100_2399_zpscebff1bb.jpg (http://s891.photobucket.com/user/bsmall2800/media/2008%20GMC%20SIERRA/100_2399_zpscebff1bb.jpg.html)

[Kennedy]

Have you verified with a GPS?

This is something we in the aftermarket have been doing for many years. I'm surprised that GM did a VCI file which is why I asked about the GPS verification.

Typically they just use a Tech 2 which only really affects the ABS decisions. I'd expect a speedo only VCI file to be pretty spendy.

[bsmall2800]

Have you verified with a GPS?

This is something we in the aftermarket have been doing for many years. I'm surprised that GM did a VCI file which is why I asked about the GPS verification.

Typically they just use a Tech 2 which only really affects the ABS decisions. I'd expect a speedo only VCI file to be pretty spendy.

I will check/verify it with the GPS, and report back my reuslts. Thanks for the suggestion.

[More Power]

There was a time when some anal retentive GM dealers would void a truck owner's powertrain warranty if the owner installed non-stock size tires.

There was a time when Costco would not sell non-stock size tires to GM pickup owners because a small number of cheating pickup owners blamed an Allison transmission failure on Costco after they sold and then installed non-stock size tires.

[bsmall2800]

I will check/verify it with the GPS, and report back my reuslts. Thanks for the suggestion.

Ok just checked with the GPS speed against the indicated speed on the speedometer, and here's the results:

Indicated speed on the cluster panel / Indicated speed on the GPS

10 mph/ 10 mph
20 mph/ 20 mph
30 mph/ 30 mph
40 mph/ 39 mph
50 mph/ 49 mph
60 mph/ 59 mph
70 mph/ 69 mph
80 mph/ 79 mph

So it appears the spedo was set for the 17's in the process of the re-flash.

[Kennedy]

Curious how much they hit you to do the change?

I suspect that this will not be available to earlier models. The 2008+ trucks were available with 265's as an option on some models so the actual calibration from GM is available. Prior models were 245 only so I seriously doubt they can be done in this manner.

[bsmall2800]

Curious how much they hit you to do the change?

I suspect that this will not be available to earlier models. The 2008+ trucks were available with 265's as an option on some models so the actual calibration from GM is available. Prior models were 245 only so I seriously doubt they can be done in this manner.

I'd say your're spot on with that opinion sir. I think it was $126.00 total, it was split between the download itself and labor to have the tech download it to the truck. If you need exact figures I'll have to look up the receipt.