I've had something similar for years - I made it in Excel with two "groups" of data to compare speeds in gears when changing internal ratios, final drives, or tires.
One thing that programs such as these don't take into account is that the tire isn't perfect - it compresses under load, and at very high speed may compress less (I didn't want to say "expand"). This only matters for tire comparisions - if you want to do a gear comparo on the same tires, then it all washes out.
To be truly accurate, you'd want to use rev/mile for whatever tire you're using on your car, which can often be found at Tire Rack, etc., rather than computing the mathematical diameter of the tire.
Then if you really want to get crazy, you use a plot of your torque curve to calculate thrust over the RPM range for a given car in each gear, and you can see where the curves intersect. This can help pick your shift points, etc.
Then when you really realize you're a complete geek, you could factor in wind resistance by utilizing drag coefficient and frontal area, and using that to more accuartely approximate top speed, etc.