If I replace my stock wheels on my v6 with some 18" chrome from mustang tuning or wherever, will this slow my acceleration down or what will happen?

 

Usually.... Yes. Bigger wheel & bigger tire equals more mass; vehicle weighs more & the heavier wheels are more difficult to rotate smartly.

 

Well maybe.,, But if the 18" rims are lighter than the rims they're replacing, and you go to a lower profile tire, then the whole combo might result in about the same total height as the original wheel/tire. In that case you won't notice any performance difference except the car will corner a LOT better but maybe ride a bit rougher too.

This is what's called "plus sizing"

 

Therefore my reason for using the key word "Usually' at the start of my sentence....

I am a drag racer, think only of straightline accelleration , & I was speaking\ posting in that context. With less sidewall at the dragstrip; it is more difficult to launch. This can kinda be compensated for with a wider tire; but then theres that darn weight & rotational weight factor comming into the picture once again....