Ive just put a set of 18's on my car and have been told by a mechanic that anything over 16's and you will need to get your speedo recalibrated. Is this totally true or would the difference be so small that it wouldnt be worth it

You can use the tire calculator on this site to see the difference.

-DavidCPA

if the readin is off a lot and it bothers u,,, ur have some problem cus the tires/wheel overall diameter is off,way off

whether you have 14 or 20s your going to have the same problem. thats why you get the tires that are closest to stock so that it really isn't necessary to recalibrate it... you probably dont have to tho, what size tires do you have? i have 215-40-18, slightly bigger then stock but who cares, not a big difference...