Got my X4 from 5star last week and went to adjust for my 295 70 18s but noticed that the rotations per mile did not match the diameter listed with it. My toyo at2s are 34.3" and 606 rotations per mile. On the tuner 605 rotations is 33.3". Would you choose the closest to diameter or rotations per mile? I went with rotations for now. Thanks!

 

Start with whatever number you think is right, verify it with a GPS and adjust accordingly.

I adjusted my speedometer with the Hypertech speedometer calibrator and set it to the actual measurement of my tire (32.75"). This caused everything to be pretty far off at highway speeds. Using a handheld GPS and my cell phone, I was able to dial it in to be exact. This took 3 tries.

With the speedometer being exact, the odometer and MPG calculations are dead on as well.

 

Cool never thought about using a gps to verify

 

I'm new this forum, but from my short reading, it appears that there a lot of experienced people here.

I just purchased a used 2014 FX4. It's still at the dealer.
It came with winter tires P275/55R20, and also the stock all season P275/65R18.
I have no idea why the previous owner would not stick to the stock 18" tires. Maybe the wrong salesman.

If I use the 20" tires, do I have to get the speedometer recalibrated to ensure the speed/mileage are reading correctly?

 

When I just went from 18's to 20's, and 305/55, I looked up the revs/mile for the tire I went with. That got my speedo spot on.

 

Those two sizes are std for the F150 and are almost identical in size (well except for the sidewall). There won't be any noticeable changes in your speedo when you swap them out. Just google tire size comparison and fill in the sizes if you want to see the diff.
As to why they ran two sizes, you can only guess. Maybe a good deal on the wheel/tire combo? Maybe the dealer uses takeoff wheels and sells winter rubber as a sales bonus?

 

We see this all the time where I live (or at least we did in the oilfield haydays), guys would buy the FX4 package with 20" rims, and right off the lot go get a lift/wheels/tires and sell the takeoffs. So lots of other more 'frugal' guys would end up picking up the 20's for cheap and use their stock 18's for winter.

Then when guys went to trade in (on something the rims wouldn't work for) they just left both sets for the dealership to throw in to the next guy as they got them cheap in the first place.