Depends on the gears you are putting in...4.10's and higher (lower numerically) gears and you can just use a speedo gear...4.30's and lower (higher numerically) gears you'll need a speedcal or Abbott ERA or some sort of electronic compensator...
Those aftermarket speedo gears are plastic and rather cheap and they shread pretty easily...I know a few people that have gone through them like water and have switched to a speedcal for that reason...plus you can input different tire sizes into the speedcal...
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.
I used the speedcal in my wife's car. It's a bit more expensive, but you don't have to worry about shredding speedo gears. Also you can dial the speedo in in .5% increments, so if it doesn't read perfect you can dial it in.