OK, Looked at the code. By my very rough estimation, your ISR will take at least 28.125mSec to get through. That works out to around 3RPS or 180RPM assuming 12 interrupts per rev. For the purpose of debuging, How you tried to spin the wheel by hand and stopping between each magnet to see if the display shows the correct info?

Also it may be on interrupt messing you up. You may have better luck with DT_INT.