Going from 17's to 18's question
 

My question is this...I have a 06 supercrew XLT that has the 17in wheels on it. I plan on buying the 18in rims that come on the new FX2's. Will one inch throw my speed off and by how much. Any one else just go up an inch? What was your input? Thanks!

It goes by the tire diameter, not the wheel.

Sorry for the poor info. Tire wise I'm going from 255/65/17 to 265/60/18. Thanks!

Are you sure of the old tire size. I thought the size on OE tires was 255/70R17.

Using the tire sizes you list, your actual speed when the speedometer read 65 will be 66.02mph.

You can get differences by going here.

Yes, my stock tires are 255/65/17 but thank you so much for the link. Now I can maybe throw other sizes around as well.