I’ve got 215kv at 8s with wheel diameter 80mm, I’m aiming for a top speed of 45km/h with 14 teeth on the small cog, how many teeth would I need on the large one? And would this be to little torque? Thanks 
You would need a 30T spur (wheel) pulley. Assuming 60mm center to center your belt pitch lenght would be 240mm 48T and yeild 6 teeth in mesh.
Using android app CalcRc
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14/60 on 8S and a 245KV motor. I use VESC in FOC mode and I’m very happy with the build. Top speed is higher than I can handle, and hill-climbing is awesome, I can rip up and over any hill I’ve tried. The only problem with this kind of aggressive gear ratio is that if any VESC-related problems occur at speed, the reduction ratio can be enough to throw me off the board. Also kick-pushing this build sucks because that’s a lot of power lost to the motor if it’s not powered.
@LucidLunacy thanks! would this be to much for the motor to handle and aim for a lower top speed? (I’m 70kg)
90mm wheels, 149kv, 1:1 ratio 
RPM is a little low. Most brushless motors optimal RPM range is around 8000-8500. If you were to increase your voltage to 10s, youd end up with 7955rpm. You could then change your wheel pulley to a 36T. This would give you a 2.5:1 reduction instead of 2.1:1 which will incrersase your torque. So 10s with 14:36T pulleyes, and 80mm wheels will yeild you a top end of 47 km/h @ 7955rpm and a reduction rate of 2.5:1. = Better efficiency, higher reduction rate and relitive same top speed.
what size wheels are you riding?
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cool what you think in-wheel motor drive
I Went 6s 16/36 T. Gets 25MPH easy, still great torque
What motor?
149Kv SK3. A bit on the expensive side, but it rides like a dream
That is really surprising… I pluged those numbers into http://toddy616.blogspot.com/2013/07/electric-skateboard-calculator.html?m=1 and got 15MPH. Is that calc complete BS or am I missing something?
No way can you get that speed. Did you mean km/h? I am riding 6S 15/36 192kv and I get 25kmh.
Idk what to tell you, but I haven’t ever used that calculator thing for speed… my GPS app (thanks for that thing, it really works @fraannk) registered 26 today on a newly paved road, its completely flat slop-wise.
EDIT: Maybe I’m missing something? I do usually overcharge my batteries slightly to get higher output voltage…
EDIT 2: Double checked my numbers, my motor is 190Kv. Sorry for the mistake.
What size wheels?
on my 2 builds I got: 90mm wheel, 190KV motor, 15/36T, 46kph real world topspeed with 10S 90mm wheel, 168KV motor, 16/36T, 44kph real world topspeed with 10S
both work really well and both ride very similar. I would always go for the largest possible motor pulley - only advantages and no downside! @Haimindo 1:1 looks cool 
I dont really believe in that superefficiency around magic motor numbers like 8500rpm. pretty sure in daily life its all marginal effects that might play a role at max speed, but then youll have the shortest range anyway, because air drag burns your battery. most setups I know run most of the time on partial throttle, which is always in any case far off the 8500rpm, so I really stopped caring about these numbers. just want a pleasant sound tbh and the lower rpm the motor runs, the less audible & annoying it is. 
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don’t do that!!!
(What voltage are we talking? Don’t go higher than 4.2V per cell!)
My build: 149kv 83mm wheels 6S 20/28 gearing results in ~40km/h top speed (measured 38 but with slightly decreased battery voltage)
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Hehe the highest I’ve ever gone was 4.05, lowest I’ve ever gone was 3.6. I keep my batteries in good condition