I just swapped my 2.73 gears for 3.73 gears, my speedometer is off and my tranny is responding a little strange. Do i have to change the VSS to make up for the rear axle? The tranny shifts into 1st and 2nd fine, but it doesnt seem to find 3rd or 4th. And where is the VSS? All this is on a 86 IROC

 

transmissioncenter.com has the speedo gears if its an electric speedo, and for the shifting, that a trans issue, maybe time for a rebuild

 

An 86 Iroc has a mechanical speedo, the only VSS is on the back of the speedometer, which you don't have to mess with. You'll need to get the correct gear (#of teeth) for the mechanical sending unit, which is located in the tailshaft housing of the trans, and you'll also have to change the trans output shaft gear to the gray 15 tooth one for any ratio that's 3.42 or numerically higher.