gears??

[DUMPD04]

soo on wednesday i blew the G80, but it just so happened i had a 02 silverado extended cab in the yard! so i went an pulled the whole rear end and put it in my truck!! i had 342s in my truck and the 02 was soppoased to have 373s..the glove box had the GT4 code and mine has the GU6 code so it should have 373s....buuut why when im doing 65 am i at 1800 rpm...is my mph off and they really are 373s orrrr are 342s...im kinda stumped cause it feels a hell of alot quicker off the line...i actually spin the tires bad off the line..so idk! anyones advice or input would really help me out lol

[sprayedenali]

going into OD instead of staying in 3rd maybe?

[DBRODS]

your speedo is definately off with the gear ratio change....

[GasGuzzler]

Gear and/or tire changes obviously make the speedo be off but the tach doesn't lie if it's working properly.