Hey,
I was surfing the internet when i stumbled upon this little page. It will give you your definate top speed of your car. It's a little too-bad its in imperial but its a page worth posting none the less.
http://www.bgsoflex.com/rpmmph.html
Type in your diff ratio, gearbox ratio and tire diameter (in inches) and you'll be set up with your cars top speed.
(e.g if my diff ratio was 4.11 and using a K40 gearbox in 4th with a tire diameter of 30.49 inches my final speed would be 284.793 km/h :) )
Anyway just a little knowledge.