how to work out what gearing to use

[Gee]

Here you go.

http://scriptasylum.com/rc_speed/index.html

That is the link to the calculators and lots of great tools and info. Play around with the caclulators section to get an idea of what it can do for you. THere is also one to solver a parameter. You can put all you info like spur gear, batteries, motor, and how fast you want to go but leave the pinion blank and it will tell you which one you need. The options in the calculators have drop down list that have differential ratios already inputted, just click on yours. You post doesn't say what you are running so that's some info people would need to answer what you diff ratio is.