Speed is off by 10 mph on new Used truck

[Lawman86]

So I bought the truck. Didn’t notice the speed was off by 10 until I got onto the interstate, and people passing me. I checked tire sizes, all factory sizes, hooked up my scanner and oddly enough the speed on OBD2 scanner read the same as the speedo. So after checking and verifying tire size was correct, and checking seeing that it was being sent via OBD false as well I am stumped. At 60 I’m going 48-50+/-, and at 85 it’s really going 75 +/-. What’s your observations? It’s on a 2011 GMC Sierra Cree Cab 4wd, all auto. In the photos I was going what was supposed to be 40 and it shows 50 on speedo, and on GPS it shows 45. So it’s + or - 5-10 at low speeds and at 75 it will show 85. What do y’all think. I’m retired ASE Master engine tech and can work on these but I was in the diesel industry. So while working on vehicles are nothing new. This speed has me stumped. I don’t want to take it to dealer if it’s the VSS on the rear of tail housing near transfer case, and I’ve also heard of a VSS Buffer? Someone elaborate a little on whether it could be related to that. There’s a ABS light flashing on OBD scanner but no active codes. 

Edited June 20, 2021 by Lawman86

[txab]

I'd think you're truck has been tuned for a different size tire. PO removed the tires and put stock back on

[Lawman86]

1 minute ago, txab said:

I'd think you're truck has been tuned for a different size tire. PO removed the tires and put stock back on

Somethings up too. I just realized this had the 6 speed. After looking at the RPO codes. I guess I’ll take it to the dealer to have them calibrate it back to the original tire size. It’s either that or the VSS on the trans is off. 

[Lawman86]

On 6/20/2021 at 11:21 AM, txab said:

I'd think you're truck has been tuned for a different size tire. PO removed the tires and put stock back on

Here’s what puzzling to me, when the truck is in gear, with the foot on the brake the needle is above zero. My Denali you can put it in gear and it won’t move anywhere above zero unless you’re going 2-3 mph, and is steady all the way. This one here shoots us like a rocket on the interstate. Here’s a pic of it on the interstate today (this is after I put in a VSS in the trans), and going 75. It’s showing 85 on speedo so it’s + or - 10 off depending on road conditions, and the needle goes above zero before you even move. When it’s in park it’s well below zero.