most road car tanks seem to be approx 12/13 mm up to 19mm
but i suspect that there is no right or wrong here
the bigger the vent the less likely of blowback when filling
but you could get away with one the same size as the fuel outlet in reality
as air wont need to get in any faster than fuel gets out
its the filling side that can be a pain as the gun at the pump keeps clicking off with any backpressure
cheers les g
|