A few months ago an engineer friend mentioned to me that he'd heard that cars tend to get better mileage with a fuller gas tank. In other words, that the mileage obtained with a 1/2-full to full tank will be better than that experienced when the tank is less than 1/2 full. He believes this, and gave as a reason something relating to pressure, the full tank creates more pressure of some sort (vapor? I don't remember), which improves efficiency.

I thought this was probably a bunch of, shall we say, "hooey" (to be polite). But interestingly, now that I've been paying attention, it seems that for my last 3 or 4 tanks, I've experienced exactly that result! I have averaged between 36 and 40 (yeah, 40!) mpg for the first 100-200 miles of each of my last tanks, with the average FE going down slowly but steadily over the remainder of the tank. It almost doesn't seem to matter where I'm driving, and I do try to use the same kinds of techniques (DWL, easy acceleration, anticipating stops, no A/C, windows closed when driving over 45 miles per hour, etc.) all the time.

Anyone else experience this? Anyone know if/why my friend's theory is true?

I think after this tank (avg. FE = 40 mpg at 100 miles, 37 at 150, downhill from there, currently at 34.4 after about 200 miles), I'm going to try doing a few "small" tank fillups, running about 150 miles and topping it off, to see if I can actually average over 37 mpg for a full 4-500 miles that way.