Suedehead:
I know ime going off on a tangent but is there any truth, that if you hold the trigger at half speed there is less vapour going into the tank . . thus more petrol?
There is no vapour flows through the pump under normal operation, the discharge point on the storage tank is obviously at the bottom under the liquid, the vapour is above the liquid and is vented to atmosphere
via the vent pipe which you can observe at any filling station usually next to the tank fill point.
If the storage tank runs dry then the vapours may pass the pump but will not turn the meter in the pump