Hi I have a 96 F150 with the I6. I recently got some new 31" tires, 2" larger than stock. I was wondering how to recalibrate the speedometer or if that is even necessary. I think i heard somewhere that it might adjust itself. Thanks.

 

From Steve83: What's a PSOM & how do I "P" it??? Click the photo in the first post

 

I copied this from an earlier post. You can do a search also.


Just one word of caution the module can only be programmed 6 times

1. Locate the dealer mode programming enable single-terminal connector. The connector is located at the bottom of the instrument panel below the center of the glove box. On all vehicles, the dealer mode enable wire is Circuit 567(LB/Y).

2. Insert a jumper wire or alligator clip in service wire and clip other end to vehicle ground with the key in the OFF position.

3. Turn key to RUN while holding down the trip odometer RESET button on the speedometer.

4. Release RESET button The Speedometer will prove out(sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display id the dealer mode lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a revisionis required a service instrument cluster must be ordered.

5. Press and release RESET button again and PSOM conversion constant without the decimal point will be displayed followed by the word CAL.

6. The new conversion constant necessary to correct the speedometer can be found by determining the current percentage error. One way to do this is by comparing the indicated miles versus mile markers on an interstate trip. The longer the trip, the better indication of the error since interstate mile markers are not always exactly 1 mile but the average is very close to 1 mile. Going to taller tires (indicated miles less than true miles) will mean the conversion constant must be reduced by the same percentage as the speedometer error and going to smaller tires (or a numerically higher rear end ratio) means the conversion constant must be increased.

7. Press and release the odometer SELECT button as many times as necessary to change the conversion constant to the correct value. The constant will only increase (not decrease) when the SELECT button is pressed so reducing the constant means counting up until the constant jumps to the bottom of the scale and begins counting up from there.

8. Pressing and releasing the RESET button to lock in the new conversion constant reduces the number of times the PSOM can be reprogrammed by one count. If you change your mind and want to go back to the old conversion constant, turn the key OFF BEFORE you lock in the new constant.

9. Turn the key OFF and disconnect the ground wire and check to see that the speedo works correctly.

 

I'm assuming this will have to be done if I'm changing rear ratios?

 

No changing ratios doesn`t change the speedo on `92 and up trucks.