[Jeff H]

gasoline or ethanol? If they don't have proper vapor recovery systems on the vehicles I can definitely see it evaporating slowly over time. Just think of the gas can in your shed that builds pressure as it warms up in the shed. If you release the pressure on the cap, those vapors are gone and won't be recovered. But if the vehicle is driven on a regular basis I doubt the evaporation would be too big of a deal. I'm not sure if ethanol evaporates even faster than gas, but it might.