Shomeyrwit -
If I were you I would fiddle around with this website :
Fatboy Raceworks Graphical Transmission Gear Ratio Calculator
You can input gears and tire sizes. By changing the value of the RPM Redline to the RPM you want to drive at, the program will output the "Max MPH" for that RPM.
In terms of finding your drivetrain's sweet spot, I would read this thread on BSFC (Brake Specific Fuel Consumption) and start searching the interweb :
http://ecomodder.com/forum/showthrea...-got-1466.html
CarloSW2