Hey, I want to build a street e skateboard.
I bought 2 6355 motor Link 2 3lipo with 8000 mAh Link 2 vesc Link 83mm flywheels
So my questions are: What is a good gear Roation for this kind of setup? It is better to use 12mm belts instead of 9mm belts? Or it doesn’t matter? Which awg wire should I use 12 or 10?
I used the esk8 calculator to see what kind of speed this setup will give you. See the picture below. I would say that your setup will work but I think it will suffer from too little voltage using vesc and with 190kv motors. My first suggestion would be to change the motor to 260kv, but I see DIY is out of stock. So in that case I would suggest getting a third matching 3s battery to boost the voltage. 12 gauge should be fine, but I use minimum 10 gauge as a personal preference. 12mm belt will give better stopping power and torque transfer. 15 or 16/36 gearing is somewhat of a standard. It depends also on your weight and where you will be riding. Is there lots of hill? Are they steep?
you might not be able to fit 12mm belt with two motors on one truck 
Two of Dexters 6355 motors do fit on a caliber II truck with 12mm belts using either the DIY V4 mount or Enertion mounts. I am using dual 6355’s with both those mounts and 12m belts. My guess is most mounts will work in this configuration, there may be some that won’t work. It’s worth investigating and asking questions though to be sure if using other mounts.
Im not that Heavy. Only 70 kg. Yeah it is kind of hillish. So we not talk about Mountains only small long hills. Do you think it is okay to use 6s Lipo? I don’t need a lot of speed. Maybe 25 km/h or 14mph.
What size does my motor have, 63mm diameter and 55mm Length? Our hills are 7/8% and only few 12%
Well…for that you deffinitely do not need 2 Motors 
Yes, it is 63 dia and 55 length…
I also weight 70kg and I conquered 20% hills with ONE 6s 235kv Motor (15/36 ratio) and still I had a max speed of 34kmph 
