I wonder if the pumps in question are driven by electric motors under vfd control , if so the f data from the controller could be used to adjust the range of the 8051 to increase the accuracy.

In reciprocating engine is used the mag pick up should be reading the ring gear teeth giving from 90 to 450 pulses per revolution depending on the size of the motor.

rev limiters work from the assumption that the motor is the power input controling the revs. The human shifting the gears can downshift and drive the motor way over revs.

I saw a vivid example of this when a 4x4 service truck in low/low 4 wheel drive (red line speed about 6 mph) started sliding on ice comming down a hill. the driver pushed the clutch, the skid stopped , the speed went to about 35 mph,the clutch exploded and grabbed,just as the icy patch ended. Total cost $8500 US No I was not driving, I was operating a large excavator ( one of three machines while moving it to a new worksite. The truck was our ride home. Interesting thread with cheerful side issues for a change. Terry