wheel size
 

if i got 17's in the front and 18's in the back would i have to adjust anything? my friend said i would need to adjust the speedo, is that true? and is there anything else?

As long as both the front tire/wheel and rear tire/wheel is same diameter it will be fine. The speedo/computer whatever it is won't be able to tell the difference. EX: 17" rim in 265/40/17 might be like 26" in diamter and a 18" rim in 295/35/18 might be like 26.4", that is what you need to look at.

is adjusting it hard? could i do it myself or would i have to take it somewhere?

and how do i get the diameter? i have an idea but i don't want to do it wrong and i'm sure i'm making an ass of myself by asking something that seems so silly

I think you're confused, or maybe I am. You look up the diameter on the tire specs. Such as tirerack.com, and look under specs for a specific tire.

What are you talking about adjusting?????

i was talking about adjusting the speedometer, or doing whatever you said to the ecu. and i think i'm just going to do the all 18's thing. seems a lot easier.

you don't need to do anything to the speedo if the diameter's of the tire's are consistent

Yes you will have to adjust your speedometer with a new 18 inch tire. Its gonna be real close to stock but would be off a little bit. It could be fixed with a speedcal, or any hand held tuner such as XCAL, XCAL 2, diablow preditor, sct 4 bank, ect...

A ford dealership would also be able to fix it with a tech handheld.

This would hold true if I wanted to go to a 18x9 up front and a 19x10 in the rear?I'm currently running 18x9 all around but think 19x10 in the rear would be nice.I found a 285/30/19 that is almost dead on(even same tire rev per mile) w/ the tires I'm running now.As for weight,the rims are considered "lightweight ",and my street racer days are WAY behind me
(no I could not find an 18x10 but did find a 19x10.)