Since increasing my tiresize, I have found I am showing 15 mph faster than actual speed that I verified with my GPS. Do they still use cables in the 04 F150 and would another correction gear be needed or is there a digital solution?

 

I have the same problem with my 35's. I had my dealership adjust my truck's PCM but they said it "maxed out" just shy of actual. Now, at 70MPH I show ~68MPH. Much better than before, but not close enough for the $150 I paid to have it done. I'm wondering if the new programmer's (Superchip/Hypertech) coming out will have the same problem? I talked to the lead mechanic and he doesn't believe that an aftermarket programmer could do something that Ford's computer can't.

Any thoughts from those who have done this with earlier models?

 

Perhaps I'm misinterpreting your statement, but it appears to say that since increasing your tire size your speedometer indicates (incorrectly) that you're traveling 15 mph faster than your actual (correct) speed.

If that is your statement, something is basically wrong. With an increase in tire size, your speedometer will read SLOWER than actual speed, not FASTER.

For example, if OEM is 255/70/17 and you increase size to 265/70/17 (1) your revs/mile decrease (2) your speedo will be 1.8% SLOW (3) your speedo will read 60, but your actual speed will be 61.1 mph.

Maybe it's a matter of semantics. Did you really increase tire size, i.e. height/diameter/rolling circumference or did you go fatter/wider but less height. If your new tire is taller, then the example will apply...the speedo will show slower than actual speed.

 

Grimalkin-

I'm assuming you're talking to jdmccl, cuz I said I am showing slower than actual. Although I'm sure that is jdmccl meant to say too. For some reason this is always confused. But I'm sure he know if his tire is bigger than his stock size.... his math can't be that bad.

 

Right on, Schmidty...

I'm still not sure what jdm had in mind...I'll await his/her rationale before I eat crow.

 

The speedo on my '04 Screw is actually 1-2mph off and everything on it is stock. I totally trust my GPS when it comes to this calibration and when my speedo says 60, my GPS hovers around 58-59mph. Strange!

 

What is the fidelity of your GPS, a lot of those units carry a pretty big margin of error (i.e. +/3-5 mph). Although I am sure Ford calibrates all their trucks with the middle tire size and the small XL tires and large Lariat tires are a little off.

 

Sorry for the confusion but my spedo reads 100mph when I am travelling 85 according to my GPS. (My truck max is 90 mph, 95 with a tailwind downhill). I went from 19 inch rims to 17 inch rim with the 35 inch tires. I also put 410 gears in. I will verify, but my gps is an 12 year old garmin (still works ok, I thought). I get the same reading when passing the chp "your speed is" contraption from the police. I am going to try another reading with maybe a better gps because, as a pilot, mine doesn't too well at altitude so an upgraded model is overdue.