You need to look up the following things your sproket size, tire size, and your gearing. Sprocket size and Gearing should be in a manual. Tire size if for the rear tires, just look at the tire it should be the first number in a sequence like 22x10x9. Plug those numbers in and hit calc. This will then tell you your top speed for each gear. Then you can play with your sprocket sizes to see what going up or down a gear will do. These are not real top speeds this is varied by rider weight and other factors like tires and ground conditions.
Hope this helps, I hope to have a calc with differnt gears pre-set for each bike soon.
Thanks
Jason