so my friend is lookin into buyin a chevy 1500, it has a 7 inch fabtech kit on sum 35" km2s. the spedo is off 20 mph and has stock gearing...normal?

 

anytime you change tire size or gears, it will throw off the speedometer.

 

Ya I kno but that much?

 

Friends don't let friends buy Chevys!

 

yes very much. I had 3.73s and 31s. swapped to 4.10s. My speedo reads about 10mph over at highway speed.

 

it wouldn't be off by a set MPH

it would be off by a percentage.....

i switched from 225/75/R14 to a 245/75/R16
its about an 10.5% difference
so i just do the math when driving down the road.... 60 mph X 1.10 = 66mph
so if my speedo says i'm doing 60 i'm actually doing 67
if it says i'm doing 65, i'm actually doing about 73

use a tire calculator online to figure out what percentage its off by, then just do the math on the fly

 

exactly! i just didnt want to get into all that crap. I believe mine is really 12% or so.