I just got a 2012 F250 4X4 with the 3.73 rear end. I measured the height of the tires that came on it and they're basically 31's. I'm considering replacing them with 32's or even 33's and would like some advice from the more experienced here.


Using the formula old tire diameter / new tire diameter X original ratio =
I get that the effective ratio with 32's would be 3.61 and for 33's would be 3.50.


Using the formula mph X rear end ratio (3.73) X 336 / tire diameter =
at 60 mph I get 2,425 rpm for 31's, 2,399 rpm for 32's, and 2,278 rpm for 33's.


Question 1: Will 33's even fit without doing any lift?


Question 2: Will I take any kind of REAL performance hit by changing to either of these bigger tires?


(The truck will be used for highway driving, backcountry exploring - NOT serious rock crawling or mudding - and will sometimes be pulling a 3,000 lb. trailer.)


Finally, what's involved in correcting the speedometer? I'm guessing it's electronic and not mechanical. (T or F?) Can my ford dealer reprogram for the bigger tires, or what?


I'm sure people here have done this kind of tire swap, and would appreciate any real world experience you can give me before I lay out $$s. Thanks.


Regards
John

 

I have 3:73's on my truck and am running 35x12.5x20's. Don't know the math, but I'm around 2K rpm at 70mph.

The dealer can flash the speedo, but I think only for the different stock tire sizes. The largest stock tire/wheel is 275-65-20, which is about 34". I haven't had it done yet, so I guess my warrantee will last longer. My gps can tell me my speed though.

I noticed a little sluggishness in the truck when I first did it, but I don't notice it anymore and it doesn't stop me from towing anything.

 

Hmm. #5's would get you down to 3.30. You probably get much better gas mileage with them. The 35's fit without any lift?


Regards
John

 

Yes. No lift, but I do have the plow prep package so front sits kind of level.

Stock wheels/tires:



35's: