Does anyone know how to figure the difference in speedometer readings relevant to the size of the tires? I don't know how much a FORscan costs and I would probably screw up my computer messing with it.
I have a 2018 F150 XLT with after market tires: 275/65R20-34", the factory tires were 275/55R20-31". I just bought this truck used, to replace my totaled 2018 F150XLT. Any comments would be appreciated.

 

https://tiresize.com/comparison/

 

Thank you very much!

 

You can also skip all the inaccuracies of the truck speedometer by using GPS, either in the navigation if equipped, or a stand alone unit like a Garmin Drive-smart, even most phones will give speed via GPS. KM

 

Our F150s with navigation display speed limits but not GPS speed (why not?). Almost any stand-along GPS unit or nav app displays both. Speedometers are usually set just a little fast from the factory, on purpose, so any error will cause the driver to drive a little slower instead of a little faster, and someone trying to drive the speed limit will be just under it, not over. The error will increase a little over the life of a set of tires as they wear and their diameter decreases. Displayed speed on the speedometers of most of my vehicles in the age of GPS have been about 1 mph faster than actual at highway speeds.

 

Forscan is free, you only have to pay for the cord. It’s pretty easy to use.

they sell tire size calibrators but they run $200 or more.

you can buy a used tuner usually for a little cheaper or get a custom tune.

i just got done redoing mine on forscan, dead on compared to Waze, Garmin, and some other random app.

 

Thank you for the useful information!

 

FORScan is really easy. There are calculators and spreadsheets with values you can use. Plug in a value that’s close enough for you, or be **** about it and use hexadecimal values until you’re spot on.

If you go the FORScan route (cheapest solution), you’ll be able to modify a ton of other settings that I’m sure you’ll enjoy.

 

My F150 does not have built in nav, my wife's Edge does and has a GPS speed readout. That is also why I listed the many other common GPS sources including smartphones. KM

 

Weird that an Edge shows actual MPH on the nav tab while my ‘19 F150 (sync 3.4, 8” screen) and ‘22 Bronco (Sync 4, 12” screen) do not. Can you post up your APIM and IPC as-built info?