try this tool out:
http://www.fatboyraceworks.com/gears/
you'll have a ball with it..
But to answer your question... when plugging in your numbers, it would suggest that you should be going 88mph (Back to the future anyone?) if you really are in 5th gear.. By the looks of it, you have an Si transmission... Or... you're not actually in 5th gear.. BTW do you have lean burn on that car? Also if you really do have HX rims, then they're not 15" but 14"... VX rims are 13".
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