A 13 tooth pinion puts you at 42mph on 6s and 28mph on 4s. That sounds perfect to me as a starting point. Temperarure of the electrical components is what you need to know to determine ideal gearing for any brushless setup.
Gearing a monster truck for about 40mph is an excellent starting point for determining proper gearing.
I got the mph figures by using BrianG's speed calculator. He is a moderator here at RCM and has developed an invaluable tool for the r/c hobbyist. It is an easy to use tool with drop down menus for inputting your parameters. It is found in a sticky titled "speed calculator" in the brushless section.
Using it you will never have to wonder about gearing again, here's a link to it:
http://www.scriptasylum.com/rc_speed/_top_speed.html