Join Date: Jul 2003
Location: Memphis, TN
Bronco Info: '83 VIN & title on '93 frame/body w/'95 4.9L & EB paint
It doesn't actually CHANGE the shift points; it just makes them happen at the actual correct speeds, where they were always designed to happen.
And it won't actually change the mileage, either. But it will show the CORRECT mileage, and your MPG calculated off your (now corrected) odometer will be higher than it used to be when the odo was wrong. But again: the actual fuel economy isn't changing - you were just using bad numbers between going to bigger tires & correcting the PSOM.
If you went to SMALLER tires than the PSOM was programmed for, you apparent MPG would have gone up with those wrong tires, and by reprogramming, you'd bring it back DOWN to true.
Stock tires are ~29" tall, and we'll say you were getting ~15 MPG on them. If you swap to 33s (~31.5" tall), your actual mileage wouldn't change much, but it would APPEAR to drop ~9% to around 13.5 MPG only because your odometer is now ~9% wrong. By calibrating the PSOM, it would appear to go back up to ~15 MPG, but it was really ~15 MPG the whole time.
On trucks with a PSOM, gearing has no effect on odometer accuracy. On all vehicles, gearing has very little effect on MPG.