I dont know what it is. but I JUST hit half a tank and I have 430 miles on it so far. I do know that the last few tanks I have been filling up really slow, one click on the nozzle, so maybe I have been robbing myself of a full tank every time I filled up in the past?

 

Now you see why I always set the pump handle to fill as slow as it will go so as to have less foam getting in the way. The less foam in the way allows more fuel to be in the tank, that is unless you have so much time you can sit for 1/2 hour or more while clicking the pump to get the tank full.