'04 6s hatch...are the specs that much different compared to the speed6?
Honestly? I don't know.... I do know that my GF's 05 6s hatch has some aftermarket wheels (not sure of the offset but they're not that low because they don't stick out) and she rubbed with her 225 tires, I had to roll the rear fenders.
With an offset like that you need to worry about the calipers as well, you don't want your rims hitting that while you're driving. What kind of car? I know I've heard of SOME running a +38 offset on the Speed6.
what wheels does she have on it?
You can go as wide as 235 but only if the offset on your wheels is close to stock. To be on the safe side, stick with 225s. You can do 225/45R18 or 225/40R18. Each manufacturer is a little different so check the dimensions carefully.
thanks i went with Eagle GT's 225/40/18.. Ill be dropping with H and R's soon, hopefully wont have to roll
With the 225s you'll be fine and shouldn't have to roll unless you've made any modifications to the wheels like spacers etc.
Also keep in mind that since you went from 45 to 40 your speedometer is going to be SLIGHTLY off, so when you're doing 60 you'll actually be doing closer to 58.2 (dunno)
Here's another great one:
http://www.wheelsmaster.com/rt_specs.jsp
You can input rim size in it as well.