I am buying a 2012 FX4 and I want to run 34" tires (20 inch rims) on it in the summer and 33" tires in the winter (don't 34's in same sectional width for an 18 inch rim).

I now that the speedo will be off but I was told that the shift points will be affected and that I need to get a programmer. Is this true?

Can Ford recalibrate the speedo in-house?

 

Or you can use a programmer and fix both at same time yourself. :-) But your different tire size isn't going to affect your shift points all that much.

A SCT unit lets you adjust for different tire sizes and shift points yourself.