I have a 3.55 gear ration in my truck and I am installing 275/65R20 KO2'S. Yes I finally bit the bullet and realized if I wanted bigger tires the a lift would need to happen so I have ordered Bilstein 5100 struts set to a 2 inch lift. But that is getting off topic.

My simple mind thought if I have a gear ratio of 3.55 and I want to correct my speedometer then the dealer should be able to recalibrate my truck with another gear ratio (3.31). That should put my speedo almost right on.

So can the dealer do this type of recalibration thinking that it is another original equipment gear ratio?

 

The dealer can't calibrate the speedometer. You have to use the Hypertech speedometer calibrator that costs around $229. Item number
742502.

 

Not completely true, they can use other parameters within factory settings. Going from a factory gear ratio (3.55) to another trucks factory setting (3.31) is possible. Where I'm not clear on is if they (the dealer) will need to use another trucks VIN to change it.

Thought I'd through this out there for the guys who have Forscan techies...

 

The speedometer can be corrected in Forscan, there is an address which has the value of your tire circumference. However, when you change it from your factory value you get a DTC in another module. I had changed mine when it was first discovered in Forscan, but restored factory value when I got the DTC and could not clear it. Some here have changed their values and have not had any problems as a result of the DTC. I am considering changing it again. The Forscan thread has a spreadsheet on the first page that shows where this value is stored. It is the circumference of your tire in millimeters converted to hex. There is another spreadsheet where you put in your tire size and it does the conversion for you and returns the value to enter in Forscan.

 

That is incorrect. The dealer can't change tire size or gear ratio settings on 2015+ F150s even if it was available within that model year as a factory option

 

I installed the 275/65r20s on my XLT Sport 3.55 gear/stock rims. I only noticed about a 4.5km/h difference at 100km/h. I don't know the tolerances in the Nav system but I believe this is really close (if not with in it) as I haven't had any issues. To me, it's acceptable till I find a fix that's more economical than the $300 hypertech unit I'll only use once. Please post your findings
Cheers,
iJet

 

do you know if a forscan spreadsheet for tire size accepts values like 35x12.50 or does it have to be converted into millimeters?