When you say analog input are you talking potentiometer? I would consider 2 adjustments, a course adjust and a fine adjust. You could avoid this all together if comfortable with just plugging in a laptop and adjusting constants. Maybe a couple buttons and 4 7 segment displays could do what you want as well.
Have you got the pneumatic shifter connected and running? I would need to understand the code, but you could continuously measure RPM and when greater than a certain value trigger an interrupt and perform the upshift. I have no idea the data rate, but would want to make sure there is some averaging being done to reduce noise. Good chance the cars computer is doing some averaging.