This talk of the "ideal" ratio for combustion reminds me of a program I watched on one of the satellite chanels (TLC?, Discovery?,) that showed a group of oil company emergency responders learning how to deal with the natural gas or propane pipeline rupture. They were standing about 100 yards away from a pipeline spewing propane into the air. Each person had a flare gun and took turns trying to hit the sweet spot in the plume to set it on fire. The very point being discussed here was discussed there, about the air to fuel ratio needing to be correct for combustion to take place. There were several burning flares that went directly through the plume without lighting it on fire. I don't recall them giving an exact air to fuel ratio required for ignition. Undoubtedly there are sparks inside the tank, especially when using an in-tank electric pump, but the level of oxygen is too low to support combustion. I suppose if your point is there is a chance an explosion could occur you're right. There's also a chance you could win the lottery and get hit by a meteor too. Not worth fretting over IMHO.
