As seen in my previous post, Wolfram Alpha has an awesome widget creation UI. This time I dug a little deeper, though.
I've made a pair of widgets that sort of work hand in hand. The first calculates the speed of a vehicle after you input the tire size, gear ratio, differential (final drive) ratio and engine speed. Check it out here.
The second one is geared more towards people who do not have a tachometer (myself). It will calculates the engine speed in RPM when given the tire size, gear ratio, differential ratio and vehicle speed. Check that one out here.
These ones seem to be working a bit better than my first one. Hopefully they work for you, and are as fun for you as they've been for me (and hopefully Gawker lets me embed these soon!).