i have an 87 trans am with an 87 camaro rs posi 2.73 posi unit, at 60 mph, im doing 1500 rpms, i recently put a set of 3.73 posi unit in it as was wondering if anybody has done this and knows what the mph difference would be now, i was told around 15-18 mph off. Does anybody know exactlly the difference, and have you ever had it sent off to get it calibrated, my just saying, if i want to go 60, with 3.73, what mph will the speedometer read.

 

82 mph@60

3.73/2.73=1.366 or 36.6% off. It is not a specific mph off, just a percentage at any given speed.

If you have an auto with 215/65/15, 235/60/15, 255/60/16 tires or a roughly equivalent 26" diameter the correct speedo gears are 45 and 15.

 

should be doing 44mph @ 1500RPM with the 3.73s

Just a hair over 2000RPM is going to get you 60mph with the 3.73s