speedometer issues

well guys i have a 99 ext cab that i picked up a week ago, it has some odd issues. one being that it has a dang ol spedo issue. so basically its 20mph off...

i have 31x10.5 tires on it with a 2 inch body lift...so as far as i can think it should read a few miles to slow right...since i am covering more ground per revolution?

but my trucks spedo problem seems to be the opposite way im about 20 miles off the other direction. so when im doing 70 it reads 90??? and the governor kicks in early at 80 mph now because it thinks its doing 100mph

also the trip meter reads wrong it calculates milage way to fast when in reality it should be under calculating...wtf guys?

anyone have this issue and how can i start to try and correct this?

any ways thanks in advance for any input.

 

there is a thread on here someplace that gives tire sizes and itll show how much that size will effect speedo readings.

 

Sounds like the truck has been regeared. I'm having that problem right now since i have regeared my truck.

To fix it you need a new speedo gear. It goes in the transmission output where the drive shaft meets the tranny.