I get lousy mileage too. I've been afraid to check but I'm filling up at 250 so I guess around 15mpg. 33's and 4.10s for me. Lately my commute is only 7min to work so that doesn't help either.

My gas gauge has gone off the deep end and I'm going to have to take it to the dealer. It reads half full when the tank is empty (guess how I found that out) and 1/4 full when there are a couple of gallons left. Sometimes after filling up it really gets confused, goes to E and the fuel light starts flashing. I replaced the sender with the TSB version a while back.

Here in CA the tanks are underground. How would the fuel temp/density really change much morning to evening? In the winter, mileage gets worse because of the additives (ethanol?) and the additional startup temp difference. Per your theory, shouldn't winter mileage be better? The vapor pressure of gas DOES really vary on temperature now that you mention it. Those Jerry cans really get some pressure in the heat.