Get a GPS or a friend with an accurate speedometer and find out what speed your speedometer is reading at a given speed. So lets say the GPS says 70 mph and yours says 65 mph. Then use this little equation with 20 teeth and it will tell you exactly how may teeth you need. If you need more help let me know. I also have a couple of speedo gears layin around.
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.