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My actual tire height (leveled flat bar across middle of tire) at 35 psi is 33.5” off ground on rear tires. Just did rear diff service and verified clean, no chips, 120 tooth ring gear. New motor craft VSS.
My computations put me at 904 cal. Previous frustrations had me at 8.xx. My speedo was showing 30 when gps said 40, showing 40 when going 49’ish, getting better as I go faster. Showing 60, when going 66-67. Etc.
My calculated revs/mile are 603 for 33.5” of tire height.
I know now these formulas back and forth. I know I’m not miscalculating for a 108 tooth gear.
Since I set to 904, it’s off 15mph at 30mph (30 on speedo, 45 actual) and 13mph off at 45 (45 on speedo, 58 actual).
what the actual f&$k is going on? I’d just wire-in a speedohealer but those are based off of percentage, and the percentage discrepancy changes as you go up in speed so that won’t work...
I’ve seen that video. Multiplying revs/mile by .0135 is for 108 tooth ring gear. For 120, it’s .015 (ie. 120/8000).
For my 35” tires, 592’ish is the advertised revs. However, my tires are 33.4-33.5” diameter at the center point of tire, which calculates to 604 revs. The tire sites seem horribly inaccurate if you do the math.
I’m gonna have to get a used PSOM and play around.
I’ve used that calculator. I have 4.10 gears, 33.5 of tire diameter, 120 tooth gear (visually verified with an open and drained diff). Calc says 903, I had set at 904 (calculations by hand). When I set to that, I’m off significantly (10-15mph off depending on speed). I’m extremely proficient at math, and I understand these calculations completely.
Something else is wrong in the signaling of my speed. I may get a new VSS. Although I got a new Motorcraft one because I was trying to fix a bouncing speedo. It fixed a little bit. Perhaps it’s signaling capabilities are not correct.
I have had my speedo needle off to change out PSOM’s before. Perhaps the needle is set incorrectly. I figured if all the gauges, the speedo was not absolute because of how it always falls (other needles don’t move when power is not applied).
Originally Posted by lonewolf_
it's stored in the EEPROM and you only have like 6 attempts ... IF you don't get it right the PSOM will no longer allow you to make new changes.....
the best thing to do is make a backup of the EEPROM before messing with programming then put a socket on the PSOM so you can change the EEPROM easier.
The 8 Pin IC on the image marked PSOM.01 is the EEPROM.... there is plenty of posts about it, so I won't get into a bunch of detail in this Thread
903 is correct for what you have posted ==>> 33.5 tire diameter, 4.10 rear and a 120 tooth Tone Ring
do you get a different cal number when you use the actual Tire values on the Calculator ??
IC1 (not shown on the Pic I posted) on that PSOM board is what receives the signal from the VSS then from IC1 it goes to IC4 (the large IC in the pic) that is where all the magic happens.
IF that board has bad caps or other components then it could cause it to read improper.
IF you had a Function Generator you could bench test the PSOM... the last one I messed with was reading about 30Mph with around 600Hz signal applied but I don't recall the Cal Factor at the time ... too many years ago.
my 93 has the 10.25 rear and it is either 3.55 or 3.25 it's type 69 ... are you sure yours is a 4.10 .... BTW I get 864 using tire diameter of 35 inch.. but I don't know what tires you actually have.
I'm not doubting your axle is a 4.10 but not knowing the axle code I have to ask.
EDIT: From the sounds of things you may be real close to "End of allowable changes" too
I can double check my axle code tag tomorrow. Gear ratio shouldn’t matter, as the teeth on the exciter ring are 120 per full rotation of the ring gear. The calculator PSOM constant does not change. And the difference between 9.03/9.04 does not explain away the large discrepancy.
Tires are 35”, but I have used flat bar across the top/middle of tire to measure diameter, I have done mark on tire and ground + one full rotation. Both arrive at 104.5 circumference (33.4 or 33.5 actual height). I run about 35psi. I arrived at 9.04 through my own calculations.
p.s. on tire rack, etc al, their revs per mile are so
purely hypothetical.
Honestly, I may be looking at bad parts of the board. That’s a good point you make. I can always inspect for burned components. I was thinking of getting a dash with an RPM gauge anyway. I can just swap out PSOM boards. Same price as buying them and using their PSOM’s to swap into a socket.
that is true that the Ratio don't matter as the tone ring has 120 teeth.
and Kudos to you for knowing the math as most don't.... on the Calculator site I posted you can Right clic and View Source and you will also see their math formulas are correct too,
Rear Axle ratio really only comes into play when the Speedometer is driven off the tail end of the transmission.... which is not the case on our specific trucks
'course you have to consider tire stretch too when going highway speeds, so how bad was it off using 864 for a 35" tire... if it was still 10 or 15mph off then I have to agree something has to be wrong with the PSOM .... the VSS has a resistance of between 1300 and 1550 it is also connected to the RABS controller so if something in the RABS was loading the signal down I could see it causing a PSOM error too, BUT I would expect to see an ABS warning light if that was the case.
I would think IF one was loosing pulses from the VSS the speedometer would always be low and never High.
that is true that the Ratio don't matter as the tone ring has 120 teeth.
and Kudos to you for knowing the math as most don't.... on the Calculator site I posted you can Right clic and View Source and you will also see their math formulas are correct too,
Rear Axle ratio really only comes into play when the Speedometer is driven off the tail end of the transmission.... which is not the case on our specific trucks
'course you have to consider tire stretch too when going highway speeds, so how bad was it off using 864 for a 35" tire... if it was still 10 or 15mph off then I have to agree something has to be wrong with the PSOM .... the VSS has a resistance of between 1300 and 1550 it is also connected to the RABS controller so if something in the RABS was loading the signal down I could see it causing a PSOM error too, BUT I would expect to see an ABS warning light if that was the case.
I would think IF one was loosing pulses from the VSS the speedometer would always be low and never High.
I have a Jeep where you would change a gear in the transfer case. It actually has a shortened tail shaft now, with a tone ring/hall sensor added on. That’s how I know about Blue Moneky speedo healer. On newer jeeps, they sell flashcal programmers. Idiot proof.
Anyway, no ABS light. As for the bolded statement, my speed is always low. I’d love for that to be true because a simple PSOM swap should fix things.
When I was set to 8.64 (or 8.79 for assumed 34.4” standard assumption for 35” tire), it was still showing 5-7 fast, depending on speed. It’s always off more at a lower speed. I tried to wrap my head around how that result relates mathematically to pulses and calibration constants. Still haven’t gotten there lol. Also, tire swelling, like at CAL difference between 903 and 904, can’t describe the vastness of discrepancy. Tire swell could have something to do with the discrepancy going down at high speed, but again, the percentage difference at 30mph actual is 40-50%. At 60, it’s closer to 20%.
I never even thought to test the VSS for resistance btw. I’ll do that. Assuming this is RABS module?
