Because I have run several tanks of each of the various blends thru my vehicles and have a good working relationship of the fuel economy I get from each fuel. I then just use that info in relation to the pump prices and use what would offer the best value on a cost per mile basis. I don't even consider the dweeb sheets on what energy loss should be. There is more to the picture than just energy loss. For instance, the engine designer Ricardo has taken a 3.2L GM V6, modified the guts and boosted it up, and it can get the same fuel economy and power as the 6.6L Dmax diesel, but using E85 fuel. Cummins has done the same thing with a 2.8L E85 motor that is getting ready for actual production use in some commercial vans. There are characteristics to ethanol that allows for some interesting things when the engines are designed primarily for it and not an afterthought like the current crop of flex fuel engines. Even on my 6.0L Vortec, the ECM reads the sensor for ethanol content and can advance timing to take advantage of the ethanol. It is not a straight across the board deal regarding lower energy content of ethanol vs gasoline and subsequent lower fuel economy. There is a fuel economy loss, in general, but not always. I have not experienced any appreciable difference in fuel economy between E10, E15, and E20. But, prices can vary quite a bit making one more cost effective on a per mile basis. Right now, it is E15. E0, I can pull off 1 mpg better, but the cost is so prohibitive to make it worth it. E85, I do take a hit, and the price spread has to be pretty big to make it worthwhile.