I think I've found a simpler way to accurately calibrate the PSOM for different tire sizes than consulting a table. To calibrate tire size on a PSOM the basic technique is to ground the PSOM terminal (located behind the glove box) using alligator clips to ground (passenger seat rail is a convenient option) then turn the ignition to on while holding the reset button top-left on the speedometer. The speedo will swipe and a code will appear. Hit reset again and CALXXX will appear (XXX being some constant -- mine was 972). Hit Reset again and at that point punching the 'select' button will change the Calibration number. When you get it to the right number hit Reset again to set it. There are a ton of youtubes on how to do this part.

The challenge is to get the CAL number right (and the PSOM only gives you eight tries for some reason). This CAL(calibration) number is a constant that represents the ratio of axle turns to miles. There are tables out there and equations on how to set this number but for my method just ignore the value of this number for now. It's just a number that tells the PSOM what to multiply it's input by to give the correct MPH. A larger number indicates a smaller tire (as a smaller tire has to go around more times to cover a mile). A smaller number indicates a larger tire (as it takes fewer turns to cover a mile). It's a linear ratio -- so one just needs to figure out by what percentage it is off by and adjust accordingly.

To adjust this number drive your truck on a nice long straightaway where you can observe the measured speed from the speedometer. Use your phone with a GPS app (like google maps) to observe the GPS measured speed. If for instance -- your speedometer reads 50 but the GPS reads 60 your ratio is 50 / 60 or 0.833.

Take your current CALXXX number (e.g. CAL972) and multiply it by the fraction you got when you divided speedometer speed/GPS speed. So in my example CAL972 x 50(speedo) / 60 (GPS) = 972 * 50/60, = 809. 50/60=0.833. So 972*.833 = 809. Set CAL to 809 and you should be right on. (809 is just my result -- use the equation (CAL number*speedometer/GPS speed) to set the CALXXX number using the procedure in the 1st paragraph

 

I should add - my truck is 1995 F-150 V-8 5.0 XLT. OBD-1 EEC-IV if that make a difference.

 

Very interesting. That sounds like an easier way to figure out the calibration constant.

 

Getting the conversion constant dialed in is a good thing, but in my experience the speedometer will still not be accurate at higher speeds.

 

My speedo is dead on up to 80 mph. Above that I'll leave it to the CHP for my calibration

 

Just an FYI, most of the major tire manufacturers publish a revolutions per mile number in their model-size-specs.

Its what I used & then verified with an app & was spot on.

Nice to have an alternate way though.

 

I have a couple suggestions for OP to make this guide a little more idiot proof:

1) clarify that there is no way to increment the number. This screwed me up in the past as the PSOM will dutifully decrement the # of changes count even if you aren't changing the value.
2) If the CAL number isn't flashing, you have a bad ground to the terminal and you're basically in read-only mode.

cheers!