Help me make a speedometer for my drag racing game

Hey, I got a decent start with a working tach and ability to go up in gears. But I can’t figure out how I can get speed based on the gear number and tach position? This game will be open source for others once it’s finished. Thanks for any ideas or help.

This is MX 2004.
Download what I have so far here: www.zeroentity.com/draggameshell.fla