Transmission Speedometer Gear

I need to find out how many teeth I need on the gear in my transmission for my speedometer to read accurately. I have a turbo 400 transmission. 29 inch tall Mickey Thompson tires with a 456 gear ratio in the rear end. Can anyone tell me what size (how many teeth) gear I need?

a 15 tooth drive gear with a 45 tooth driven gear is as close as you can get

hey denny,,,how come my icon is showing up on the post above??i sure as hell didn't post that question about speedo gears.