I finally got around to reprogramming the PSOM on my 1993 F-150 4x4 after having run 33's for over 4 years. I have posted a couple of times on this board asking whether or not running oversize tires would affect the EFI and economy and the overall consesus was that it would not.

Well...I don't know what to make of the results...My old conversion constant was 9.72 and for my 33's the constant is 8.59. the difference is just over 13 percent. In the past 5 years I have been using this 13% difference to caculate my fuel mileage as well, having driven with a GPS, I saw that my vehicle speedo showed 100kms when the GPS showed 113. My fuel mileage has also been hovering between 12.5 and 14.5 with an average in the 13's.

I went on an extended drive with the PSOM reprogrammed and my mileage has jumped significantly. With both tanks full at the start and after topping up both tanks following 450kms of mostly highway driving I saw over 17mpg. I did have a bit of tailwind but their were lots of hills with an elevation climb from sea level to about 3000ft.

The only way I can explain it is that the EFI in my '93 does indeed use vehicle speed to caculate fuel/timing etc. So, as was suggested to me by other members of the board here, you have nothing to lose and perhaps, like in my case, something to gain....

WHaynes
-93 F150-