It takes time to explain - and it doesn't lend itself to being written about. Talking live speeds up the process on this one.
You need to know 1) how many teeth on your drive gear (the one inside the tranny on the output shaft)? 2) how many teeth on your driven gear (the black one you installed)? 3) Exactly how far off is your speedo with your current gears and tires? (10-15 isn't close enough - use the mile markers along the interstate to check the odometer, preferably over a 10-20 mile run).
I'll make up some numbers to show you how to do the calculation. Let's say you've got an 18 tooth driven gear, and a 7 tooth drive gear. And let's say your speedo is reading 20% high - that is it's reading 72 mph when you're going 60 mph. So, you need to adjust the ratio of your drive to driven gears in the same proportion that your speedo needs adjustment. A higher number of teeth on the driven gear, or a lower number of teeth on the drive gear will speed the speedo up.
7/18=.39 You need a 20% adjustment in that ratio, in the direction so the speedo shows a faster speed. .39X.80=.31 So you need a set of gears that results in a .31 ratio - or something close. Keeping your 7 tooth drive gear, a 23 tooth driven gear results in a .304 ratio - pretty close. However, it's been my understanding that the 23 gear teeth don't always last a long time. So, an alternative is to switch to the 6 tooth drive gear. If you did that, 6 teeth on the drive gear and 19 teeth on the driven gear gives you a .315 ratio - pretty close.
Remember, all this was based on made up data. You need to know exactly how far off your speedo is, and what gears are causing that. Then you can use the methodology above to solve the problem. Capice? By the way, you can't count the number of teeth on the drive gear by shining a flashlight through the tranny hole. Use the color of it to match up the number of teeth. To count them, you have to be able to view the drive gear from the side - which requires pulling the tail shaft housing off.