I have cobbled together (actually, heavily modified an existing tool) an Excel spreadsheet that allows you to see what RPM your engine is running at a certain speed in an specific gear.
You can also use it to view the impact of changing tire sizes and/or number of teeth on your sprockets.
It is an excel spreadsheet that REQUIRES You to Install the Excel Analysis Toolpak. This comes with Excel, but is not installed by default. To install, go to Tools, Addins, Check "AnalysisToolpak".
It is currently set up for the stock GS500 configuration. Simply change any of the YELLOW fields to see the impact... it should auto-update. You can change front and rear sprocket ratios, rear tire width, rear tire depth, rear wheel diameter. It displays RPMs and MPH for each gear. You can also enter a specific RPM that you want to know exact numbers for... by default, it is set to 7500. (this field is also yellow)
I hope that makes sense.
If you don't have excel, follow the links below for screenshots of a stock setup and a 15-tooth front sprocket setup.
Here is the zipped excel file (please use right-click, "save target as"):
http://users3.ev1.net/~krpoun/public_html/albums/gs500/speedo_gs500.zip (only 9kb)
Here is the screenshot of the stock setup:
http://tinyurl.com/8lse5 (64kb)
Here is the screenshot of the 15-tooth setup:
http://tinyurl.com/ap58s (64kb)
Please note the the GS500 has peak horsepower at 9500 RPM and peak torque at 7500 RPM.
Not all tires of the same size have the same diameter.
Here is another one: http://evilpumpkinracing.com/gearing.xls