The speedometer has been slightly off since I bought the truck.
It came with 285/70R17 32.7" tires on Hummer H2 wheels that rubbed when I made tight turns. Those 33s are too big without a suspension lift and spacers... that ain't gonna happen.
I installed N89 17x7 wheels with Michelin Defender LT 265/70R17 Load Range E tires on the truck in September. Those tires are 31.6" diameter according to Michelin.
The stock tire size is 245/75R16, 30.5", on PY0 wheels. The additional 1.1" of tire diameter throws off the speedometer and ABS computer.
This is a two step process with two different pieces of hardware. I used a Tech 2 and a HP Tuners to do this. You could do a PCM tire size update via GM techline using the Tech 2 or MDI if the tires are an approved size. My tires are not GM approved.
I updated the tire size in the ABS controller using my Tech 2 back in September. 265/70R17 is not an available tire size so I set it to 265/75R16... they're both 31.6" outside diameter.
The brakes seemed to behave better after just the ABS change but that could be my imagination... I never trust the butt dyno... even a finely tuned instrument like mine
Unfortunately the ABS parameter change doesn't update the PCM (ECM/TCM) tire size on this truck. The speedometer was still reading several MPH lower than my GPS. That's definitely the wrong direction for this kind of error.
I have HP Tuners MPV2 registered to my Impala SS so I bought two Universal Credits and registered the 2005 Silverado to my MPV2 so I can modify parameters like the tire size.
I read the contents of the PCM and saved an un-molested copy to my laptop HDD and a thumb drive I keep in my garage laptop bag so I can go back to the as-shipped settings if I screw up. I have an otherwise un-molested copy with the modified tire sizes as well.
HP Tuners can't make changes to the ABS controller parameters so you need to make changes to that with a Tech 2.
Having the correct tire size in the PCM puts the fuel maps, transmission shift points, and TCC engagement back to the as-delivered from GM behavior. Fuel mileage has jumped noticeably and it no longer feels like I'm pushing it hard to go 65 on the highway.
After the PCM tire size change I see 32 on my GPS with 35 on the needle, 54 on my GPS with 55 on the needle, and 73 on my GPS with 75 on the needle. The rest of my cars and trucks read between 2 & 3 mph higher on the needle than the displayed GPS speed so this is well within what I would consider "normal" operation.
The odometer reads 1 mile in a measured mile so I'd call that good to go as well.
Not sure if I'll make any changes to firm up the mushy TCC and 4th gear engagement now or not. I'm going to drive it a little to see if this change alone made enough difference to make me happy.
It's worth noting that the chime replacer for the aftermarket radio can interfere with the Tech 2 and HP Tuners communication through the J1962 port. If you can't read the PCM unplug the radio harness and try again.
I'm going to experiment with putting a key switch to shut off the Class II data wires to the chime replacer when the weather gets warmer. Opening the dash to unplug the radio when I want to make changes to the PCM is a PITA.
If a switch in the data lines works I'll make a howto post... Honestly if the chime replacer was properly setup it wouldn't interfere with the Class II databus communications but these are built to a price point and proper engineering costs money.