Gz250 total speed challenge

[jambo6984]

I've heard people say there gz250's can go 75 or more but my stock gz250 only goes about 55 mph pegged out, it really feels like I should have another gear to go into, my question is how many teeth should I go up or down in the back to achieve 75 mph, because as it stands I don't even want to take it on the highway were the speed limit is 55 because everyone travels at 60 atleast, I'm not trying to turn my gz250 into a racer or anything doing any tweak I can , but would love to be able to reach a top speed of 75, so I can go on the highway or even interstate if I wanted, I feel more like I have a moped, only being able to do town riding a back roads were the speed limit isn't over 45, if anyone has a specific way to find this out some kind of calculator or something that would be great , I love my gz250 but it needs to have a faster top speed, thanks for any help