Gearing/Battery choices; gear up or increase voltage

[silentbob343]

So I have settled on the ESC/Motor combos to use on my vehicles. Thanks to those that posted in my other thread.

Now I need help on gearing and battery selection.

1. SC10 with Sidewinder SCT system
-was thinking 2s initially, but playing with the speed calc it won't be fast enough on stock gearing. Castle's gearing guide does recommend anything higher than 18t pinion. Should I jump to 3s to stay within Castle's recommendation or can I gear up?

2. Mini STe with MMPro SCT system
-was going to run on 3s, but playing with the speed calc it would not be fast enough for me. To get the speed I'm looking for it would take an 19t pinion which is way out of Castle's recommendation of 10-13. Should I jump to 4s to stay within Castle's recommendation or can I gear up?

I know the line of thinking is higher voltage is better for the ESC in terms of reduced amps. The only concerns I have are battery dimensions and capacity/runtime. For example I can get a 3s 5300mah battery that will fit in the STe. If I want to go 4s I need to go down in capacity, from 5300mah to 4000mah. What kind of reduction in run time am I looking at?

In the SC10 I need to go down in capacity as well if I want to keep the stock battery retention system, although some slight mods and I should be able to use the 5300 3s pack I had planned to use in my STe.

Both will be used for bashing in fairly open areas so the trucks will be able to stretch their legs. I was wanting 35-40 for the STe and on 4s a 14t pinion will get me 37mph and 16t yields 42mph. To hit 37mph on 3s I need to use a 19t pinion.

The SC10 RTR brushless stock should be good for 23mph, well if what I saw was 23mph I need something a good bit quicker. I thought the bump in kv from the castle motor would satisfy this, but it seems that extra 500kv only equates to another 3mph for 26mph, not enough. Going up to 3s will get me at 39mph on stock gearing, perfect.