I used a website http://www.bikerenews.com/Stories_Archi ... ulator.htm to get some data points for gear and rpm, and plotted them in excel to get my equations for each line

basically for those equations, my code would be a bunch of if then statements. If rpm(x)/speed(y)=0.0042+.02 for 1st gear, for instance, I could always output a 1 on the gear indicator for that ratio
while in neutral the system might tweak out a little bit, would have to tidy up the code to get the least interference, but I think it could be done.
I just think the two hall effect sensors adds some extra wirings to the bike that would make it look messier, and there would be some issues since it would only count, so there would have to be a reset switch if you missed a gear or couldn't get into neutral right away. Anyway, let me know what you think.