Did you keep the overall diameter of the wheel and tire consistent with what was there with the original equipment? Having gone up a size you'll end up with tires with shorter sidewalls if you're keeping dimensions consistent.
This has probably been seen/used before by some people, but here's a little item that'll give you an idea of what I mean.
http://www.miata.net/garage/tirecalc.html
Put in your original tires' diemnsions and those of your new tires to see not only visually and numerically the difference but also how much % off your speedometer would be.
I'm just thinking that, if you're feeling physically higher up, as opposed to the car just looking kinda high due to the wheel change, that maybe you might not have gone with the right tire size. If you did go with what's supposed to be on the 18s from the factory, though (235/40-18 Front and 265/40-18 Rear), then the difference should be absolutely minimal (the new size is actually very slightly shorter in height).
I'm also assuming you went from a 17 to an 18, and not the other way around...
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