i know when you build a truck on the ford website when you opt for the 20's it gives a different gear ratio then the 18's...do i need to change this on my kr or no?

 

No, you probably won't even notice a difference unless you get 33" or bigger tires.

 

If the overall diameter of wheel/tire is the same, then the final drive ratio will be the same regardless of the wheel size. I went from 18s to 22s and the overall diameter remained the same so the speedometer is still accurate also.

 

Like they said, it won't change a thing. I went from the stock 17's to the OEM 20's and didn't alter a thing. Overall wheel and tire combo has near exactly the same height.

 

I don't think a gear change is in order as they come with only 2-3 gear options from the factory, for example if your truck has the 3.73 gears with 18's and mine has the same with 20's you should be fine.

As far as a speedo being off you can get a programmer, Troyer, Edge or Superchips and reprogram your computer to the correct tire size, plus get more HP and fun out of the truck.

 

I agree with everyone, no need for a gear change and your speedo should be pretty acurate still as well.

 

I went from my stock 18's to 20's with 33's. No need for gearing. Ive been wheel'en and all and havent noticed any sluggish issues. Im running 3.73's.