First we need to know how many cells you will be running. Then, the next thing to do is enter all the info into the gearing calculator, experiment with the numbers a bit (aiming for around 35-40mph is a good starting point) and go from there. If you have any questions as to what you need to enter into each field there are little question marks you can click on to help explain, otherwise ask here in this thread and we'll be happy to help.
Look forward to seeing your setup!
http://www.scriptasylum.com/rc_speed/_top_speed.html