Is there a rule of thumb for working out motor amp draw

[pinkpanda3310]

Set up is-
e-maxx 3905
mmmv3
medusa 36-80-1600
neuenergy 4300 25c 6s
Integy 40 series sq pattern

Gearing is subject to change if im out front of my house then it bearly does 30mph on 4s (and bearly warms up) but I like to bash it in sand and occasionally do hill climbs and I prefer a bit more speed, 23/58 is the tallest gearing i have but am reluctant to use that anywhere but on the flat. I probably have a bit up my sleive but don't want to push it in the hot dry dirt of oz.

Thanks for your reply Arkt1c, I'll have to start jotting down points like that to make up a bible for myself.