Have a play with the calculator on the Speedo Healer site, here: http://www.healtech-electronics.com/. I can't link it directly as it's a Flash tool so select Speedo Healer from the menu on the left and then Online Calculator, tick the Estimated option and away you go.
Based on just dropping one tooth from the front, it'll over-read by 6.2%. The tool also allows you to compensate for the inherent speedo error, estimating at 5.5% so it could be over 10% out from actual