2012 F350 DRW...changing from 31.5" 17's to 35.5" 20's

I'm soon to change my 2012 F350 DRW with 3.73 rear end from stock LT245/75R17E tires (31.5" tall) on 17" x 6.5" wheels to LT295/65R20E tires (35.5" tall) on 20" x 8.25" wheels. This is a fairly significant tire size increase so my speedo and odometer will be a good bit off. What is the best way to correct this and to ensure no error codes? I am running stock program (no tuner).

It's my understanding that the Ford computer (even at dealers) can only change to what Ford offered in F350's in 2012 which the largest would be an approx. 34" tall tire on 20" wheels with a 3.55 rear end offered in the single rear wheel F350s. So even the dealer cant completely solve the problem.

There are so many on this site with larger tires (than stock) so looking for everyone's advise.

pro comp offers a little handheld device that corrects speedo for bigger tires. accupro speedo calibrator. local 4 wheel parts store tried selling me one but i went with a programmer instead.

I bet the dealer can get you programed pretty close. when you measure the tire with the truck sitting on them how tall are they actually. Probably pretty close to 34" tire.

I agree that they could get close to the 35.5" by using the 34" (maybe close enough for no error codes I hope) but the problem is they can only input 34" tires by changing to the 3.55 rear end and I'm a 3.73. While changing from 31.5 to 34" takes me in the right direction, the rear end change takes me backwards, if they could do 34" tires with a 3.73 gear then I would agree with you but I understand they can only program what Ford offered as stock...they only offered the 31.5" (17" wheel) with the 3.73 gears.

So if above is true, I'm interested in hearing from guys with 35" tires or larger while the programmed for a 3.73 rear end or what is best suggested for me?

Why do they have to program the gears at all? They should just be able to change the tire size. Like I said even the the tire says its 35.5" tall I bet its not with the weight of the truck on it. My 37x12.50s measure about 35.8" tall with the truck sitting on them.

it's my understanding to get to the 34" tall tires which were offered on the 2012 f350 single rear wheel, if you don't also enter 3.55 gear ratio, you will get an error.

Do you know anyone with a shop and an IDS that could experiment with it?
Maybe plug in the 34's and 3.55's and see what happens?

Well I went with Fuel Offroad Dually Cleavers (20") with Toyo MT 295/65R20E and I really like how it turned out (see one before and remainder after photos). I drove this way with spedo about 11% off for 3 weeks with no issue but today went to Ford Dealer for service and they were able to reflash computer for 275/65r20E tires (an option of the single rear wheel F350s) and now spedo is only about 1% off.

That tire is only 35.1" and at 34" programming my speedometer is dead accurate.

Glad they got you all figured out.

2012f350drw said:

Well I went with Fuel Offroad Dually Cleavers (20") with Toyo MT 295/65R20E and I really like how it turned out (see one before and remainder after photos). I drove this way with spedo about 11% off for 3 weeks with no issue but today went to Ford Dealer for service and they were able to reflash computer for 275/65r20E tires (an option of the single rear wheel F350s) and now spedo is only about 1% off.

Click to expand...

Truck looks great! Now it just needs PTM front and rear bumpers to finish the look.