Wheel size and speedometer reading question

[TruckMeRunnin]

So amongst other issues with my 97 GMC 1500 4x4 (rod knock, manifold leaks, radiator leak) that I just picked up of Craigslist and got totally hosed on lol is that my speedometer reads about 10mph faster at lower speed up to about 45 or 50 actual mph it then reads about 15mph faster Ie: to go 60mph Speedo reads 75 but to go 25 Speedo reads 35 I have discovered this truck came stock with 16in rims and it currently has 15in rims I'm have heard this can affect reading does this sound like the issue at hand??

[Logan Lanfear]

Measure how tall the dire is (diameter) to see if they are different than stock size. Stock is somewhere around 31''.

[TruckMeRunnin]

All the tires are 31 except the rear driver side is 28.7 would one tire cause this?

[Logan Lanfear]

Is that tire at the correct inflation or is the tire size of that tire different than the others?

What is the tire size for all of your tires? a couple inches could be the result of a low tire or it could be a completely wrong size. It definitely would change the speedometer reading if you have the incorrect size tire. It would also probably cause ABS reading issues because the rear end would be spinning at a different rate compared to the front.

[TruckMeRunnin]

Yeah the one tire is completely different size

[Logan Lanfear]

I would probably blame that on your speedo issues