all depends on what year the trans is. the later (i think 91?) t5s have a 7 tooth drive gear, while the older ones have a six tooth. from what ive seen, the six tooth has more available combinations with driven gears to correct for rear-end ratios. theres charts around on the net (i know the frpp site has it somewhere) that show which gears you want for different ratios. i just ordered a kit from 50resto that has the correct drive and driven gears for the 4.10's ive got (6 tooth drive, 19 tooth driven gear). its not all that hard to do change, pop off the shifter base, pound out a roll pin, unbolt the tailhousing, and the drive gear is right there held on with a clip. the driven gear is even easier, just on clip on the end of the cable.