There are 2-3 sites that have the rear end gear ratio calculaters. I am at work and can't post them at the moment, but I am sure you can Google them. I can say this thou, if your happy with your 60 ft. times, and your running short on RPM at the big end something has to give....LOL
If you real close on the big end, maybe consider useing a taller tire or even a stickier compound. Of course there are a lot of variables. Are you spinning a lot out of the box ? If so how bad ? You may find out a tire change is all thats needed?