I had an issue that caused me to have to get a new PSOM. I could not look at old PSOM calibration (if there was one) because the buttons on the board were stuck so i couldn't go into calibration mode.

I got my new-to-me PSOM and sent it off to get mileage copied over. Full 6 calibrations available.

I did NOT check my speed with stock calibration of 984.

Old tires on old PSOM were worn Yokahama Geolandar 315/75/16's (when new, 34.5" and 602 revs/mile). Speedo was about 5 mph off at 60 (55 speedo, 60 actual).

I got new tires while PSOM was mailed off. BFG KO2 315/70/17's. 34.4" and 604 revs/mile.

I did the calculation of 604 × .0150 = 9.06. I set to CAL to 906. My speedo is WAY off. At 30 speedo it's saying 41 GPS and at 55 speedo it's showing 63 GPS (Which makes no sense to me, as differences get bigger as you go UP when %'s are steady).

I used the .0150 because i read that's for 120 tooth ring in 10.25 diff, whereas .0135 is for 108 tooth ring in Ford 8.8 diffs. Btw i read that you use .0135 for 15" wheels and .0150 for 16" wheels. Which means i should be using .0165 as a multiplier for 17's. This can't be right because tire diameter, not wheel diameter, makes the difference in the speed calculation. I changed wheel sizes, but tire sizes stayed essentially the same.

The .0150 is derived from 10.25 diff tone ring teeth (120) ÷ 8000
The .0135 is derived from 8.8 diff tone ring teeth (108) ÷ 8000

The following page says a lot. It has a couple equations.

https://www.f150forum.com/f10/adjust...re-size-18295/
ConvConst = RevsperMile x ToneRingTeeth ÷ 8000 = Revspermile x .015
8.800 = 586.7019 * 120 / 8000
RevsperMile = 20168.11439 ÷ TireHeight
586.7019 = 20168.11439 ÷ 34.3754
Tire Height = 2 × Width × (AspRatio÷100) × .0394 plus RimHeight
34.3754 = 2 × 315 × (70 ÷ 100) × .0394 + 17

604 revs, given by all websites, equates to a 33.4" tire. That's science. 34.4" tires will rotate 586.9 times in a mile. That's science.
My conversion constant should be 8.80. I don't understand how a -.26 change will equate to +8 mph on the speedo, when -.78 (9.84 to 9.06) equated to -3 change.
What gives? Anyone know where i went wrong in calculations?
To top it all off, speedo is f'in bouncing like before. Which indicates bad VSS.

 

I also have 4.10 gears which shouldn't make a difference.

 

315/70/17 = 587 rev per mile
587 * .015 = 8.80 Cal

Why would say bad VSS?
Bouncing needle most of the time is either bad bearings in rear end or bad PSOM.

 

Because I've changed PSOM now and speedo is still bouncing at higher RPM's.

 

So I initially set calibration to 9.06 due to websites saying 604 revs/mile. I've since changed the calibration to 8.79 after calculating revs/mile to 587 based off of a 34.4" tire.

Here's the deal: speedo vs gps doesn't change. AND I'm off 10-11 mph at 30 (on speedo) and 7-8 off at 50 on the speedo. Which makes no sense. The actual difference should go UP as speed goes up because speedo/gps bring off is a constant %.

I'm pretty sure my VSS is screwed. Teeth on ring gear look good, no symptoms of excessive play in ring/pinion...

 

TTT

Any advice?