Thump, I did the roll and measure method and came up with 2692mm for my 35x12.50x18 Nitto Mud Grapplers inflated to about 41PSI.

I also checked this with a handheld GPS and it was within a mph of what the edge said.. (my speedo shows about 2 mph faster than the edge though)

 

Yeah, I'm freakin baffled -

This morning my RPM's were running around 2200ish @ 74 MPH as previously stated.... On my way home I GPS'd myself with the custom level 3 tune I've been running a for a few days. Edge was off by about 2-3 MPH. Pulled over and reset the Edge to just run on regular level 3 program - GPS was dead on and my RPM's back to 2090's - 2100ish @ 74 as they have been in the past. Only thing that I can guess would be wrong is the what the rest of the guys stated about incorrect tire size.....I have also lost a couple MPG and the shuddering occurred this morning again around 50 MPH, RPM's were bogging, etc.....Looks like its back to Edge level 2 until this fiasco is taken care of.

Thanks for everyones input thus far

 

Just a little up date on my little tire/speedo problem. I went through and redid my lvl3 programming, then went out with the gps again. According to Edge my 325/60/18's are supposed to be something like 2660mm. I set it to this and at 60 mph I was again off 5 mph. So after doing a bunch of playing with different sizes, I finally have it set on 2520mm and the Edge is right on with my garmin gps. And the speedo must be right this time, because this morning I was going to town, I had the cruise set at 60 mph and a state trooper was going the other direction. He went past me and the car that was behind me (about 1/4 mile), but the 3rd car (about 1/2 mile behind) got pulled over. So I figure that was a pretty good test....

Paul

 

Thats the tire size I have and 2478.2 is right on the money. Mine was way off and now that I changed it the speedo is perfect with the Edge. It also drives totally different. My tire size is 255-70-17

 

I changed mine to that but the unit reverts back to 2480 for some reason, will take it out of the truck and do the custom tune tonight. Its running 2-3 mph lower than the speedo.

 

Thats not to far off is your truck 255-70-17 or 255-65-17?

 

Bill explains that Ford speedo is off by 1-2 mph (Edge being correct):

Okay, one quick word about the speedometer. Well, maybe not so quick. The speedometer and the Evolution get their signal from the exact same place... the PCM. On practically every truck we've tested (regardless of tire size / gear ratio setting) the speedometer reads 1-2 MPH higher than the value coming from the PCM (and subsequently the Evolution). Given the analog nature of the speedometer, we just accept this as normal. We have speed tested with both our Dyno and a GPS and have been dead on with the Evolution MPH while the speedometer is 1-2 MPH high (always high for some strange reason). Even the factory scan tool indicates a discrepancy. However, the odometer being digital does read out correctly despite the variance in the speedometer. Again, it all falls back to setting tire size and gear ratio correctly.

Copy of link to quote above:
https://www.f150online.com/forums/sh...97#post2008697

 

Mines 255-70-17 ( General Ameritrac TR). Will keep the setting that I have and see how it goes.

 

2478 is the setting then I have the same exact tires.

 

Ok thanks alot, when I tried tio set it at 2477 while still in the truck it reverted back to 2480, hopefully when I take it out and program it from the computer it will set.