Hi again,

I have a 2003 F150 XL and the previous owner changed the wheels tires on it. Factory it has 16" wheels and he upgraded to 17s that were another OEM option on the truck according to tirerack. He said the speedometer was off a bit due to the larger tires so today I decided to see how much. I used an app on my iphone that is called speedometer and set it in the cardock and drove around at different speeds to see how much it is off. This is where I get a bit confused. When I'm driving at slower speeds (20, 30, even 40) it seems to be dead on according to the GPS in my phone, but once I start getting over 45MPH a discrepancy starts to show. By the time I hit 60, its about 5 or 6 MPH off according to my phone. When I'm going 60 the truck is actually moving at around 65-66 according to the GPS. I figured if it was off it would be off the same amount all the way up. Can someone please explain this to me? Is my odometer off then too? Can the speedo be adjusted and corrected to match the bigger wheels at the dealership?

Thanks!
-Justin

 

yes mine does the same thing i havta go about 52 53 to be going the speed limit u an ether get a programmer and adjust it back to rite or a dealership

 

Does it screw up the mileage on the odo? I figured it might and therefor my mileage will be off :-/

 

no the odometer isnt effected

 

Now I've gotta see how much its gonna cost me at the dealer to have it adjusted (PCM program?) I could probably live with it, but I like my truck so much I want it perfect :-) lol

 

Most dealers charge around $80 for the PCM reflash, a programmer will run in the $400 range but you get better performance and a better driving experience with the programmer.

https://www.f150forum.com/f2/tuners-pcm-ugrades-124931/

 

What about the guys with the mechanical odometer and speedo any way to correct that?

 

https://www.f150forum.com/f6/speedo-...estion-140179/

Figure out what gear you need, then the dealer and several places online sell gears to change then out.

 

How do you figure? The odometer gets its info from wheel rotation.