Getting the axle gearing to accomplish your speed requirements involves a number of considerations that really need to be worked out before jumping off. Re-gearing is not cheap and getting it wrong can be even more costly.
You need to take into account your engines preferred highway RPM (also known as its' "sweet spot")...your tire size...your final drive ratio coming off the tranny...and knowing if your motor will indeed pull higher gears.
Good news is there are a number of online calculators that will do most of the work once you have the correct numbers.
There are several below. I used the "Transmission Final Gear Ratio Manual Entry "
on the one below to sort out mine...
Engine RPM Calculator