This doesn't make a lot of sense to me.
You are saying that fuel is sprayed on the filler nozzle when the tank still has room for almost 4 more gallons of fuel. The tank is not pressurized so please explain how fuel gets lifted two feet from tank level in order to spray out near the filler orifice without a pump to do the lifting. Yes, there is a vent line but how is it pressurized?
Mike,
good question.
With my very limited understanding of the inner working of EVAP systems I can't come up with a fully satisfactory answer, but there is definitely some pressure in the tank as it gets filled and fuel definitely gets pushed up thorough the vent pipe.
check out this ford patent from 1993.
http://www.google.com/patents/US5327934
a relevant quote from there:
As fuel continues to enter tank 10, tank pressure rises until it causes fuel to back up into the filler pipe 12 consequently shutting off the fuel nozzle (not shown). Thus, containing fuel vapor within the fuel tank during refueling contributes to prevention of tank overfill.
There is lots of fuel vapor still in the tank when the filling stops. If one tops off the tank, or if something is wrong with the system (which is what I suspect happened to OP), it's possible to displace that vapor with fuel. if that's done, the filler neck can also be used to store extra fuel.
The vapor area combined with the filler neck is likely where the extra 4 gallons go.
obviously those 4 gallons should not be used for storing fuel, but the volume is there.
I fully agree with you that squeezing 18 gallons into a CX-5 is at the very least, harmful to the car, bad for the environment and at worst a serious fire hazard. I would also never attempt to overfill my tank in that manner.
I think that if OP's CX-5 really does take 18 gallons (which IMO is very possible) he should get his EVAP system checked by the dealer ASAP.