Speed gear?

Hey all. My speedometer is off due to larger tires. I found new gears at year1 kinda costy. I hope to find the color I need at the yard later. But how can I foind out what gear I need? I know the different colors have different teeth count.
I have a stock 904, 8 1/4 rear with 3.21, and tire size is 225-60/15. Thanks for any help.