adjust gear ratio according to top speed or to cell count?

[mistercrash]

I'm running the MMM combo that came in my E Revo Brushless Edition. You're right about a certain speed. I'm aiming for 38 - 42 no more. The more I think about it, the more I tend to want to race with my 4S packs. Controllable power if you know what I mean. I'm also running LST diffs in this Revo. Is 54 spur/23pinion on 4S acceptable or will it make my ESC and lipos over heat?