I've been asked to find a solution to a problem my friend has created for himself. Searching didn't help so I'll ask

Vehicle is a '97 E350 ambulance in the process of a 4wd camper conversion. Powerstroke, automatic, NP271, dually, 60 and 80, powered by WVO

Old setup had a gear driven sensor on the output shaft, new setup uses an 18 tooth reluctor and a hall effect sensor in the extension housing

Speedometer currently reads about 5 times the actual vehicle speed

Only solutions I have come up with so far are

1) stacking about $600 worth of speed calibrators or

2) removing the reluctor and cutting it down to about 3 teeth which should make it read 1/6 what it does now, and dialing it in with a calibrator

3) external speed sensor that reads off the driveshaft???

Anyone have a different idea?

Anyone know about how many pulses per driveshaft revolution the old-style gear driven sensor generated (I say about because the driven gear can be changed). Maybe pulses per mile? I can work backwards through the math

Thanks!