i put 3.73 gears in my 98 cobra and all i did was buy a $12.00 plastic speedo gear from summit. everything is fine. just make sure that u get the right speedo gear for the gears that you get. as far as the shorter gears needing a speed cal, i don't know, i didn't go that high but the 3.73 gear woke the car up. it is like night and day between stock gears and these.