An important bit of advice for anyone ordering a five speed based on T5 gear sets. Decide on your tire size and pick your speedometer. Then have the builder install the correct speedometer drive gear on the tail shaft (there are many options: 6,7, 8, 9 or 10 teeth). This is a simple choice while the transmission is a-buildin’. Once the box is assembled and in the car, it becomes a big, expensive job to change. If you pick the right drive gear, it’s a snap to recalibrate the speedometer by swapping driven gears in the the transmission pickup. If you pick the wrong one, you will never have an accurate speedometer.
I’ve put up an Excel spreadsheet that will help with the calculation. The green box allows you to vary input data. When it opens, it’s set up for stock tires and a 3.54 rear. To perform the calculation for your own situation, you need the tire width and aspect ratio, differential ratio, and the TPM of the speedometer head. If you want to check the accuracy of your speedometer using an electric drill, you can input the RPM of the drill. You can change the tooth count of the drive gear as well. Finally, you can experiment with % tire wear to see the effect of that. What you want is a drive gear that allows you to pick a driven gear towards the middle of the range…say a blue or white gear. That will allow you to easily recalibrate if you later play with tire size.
http://www.coolcatcorp.com/Speedometer/Speedometer.xls
Illustration of drive and driven gears, along with output housing and retainer:
