Fuel BTU variance?
 

Anyone have resources or personal experience with controlled experiments on this?

I drove about 600 miles this week and I tend to have the instant mpg gauge on all the time, and I noticed my gas mileage just totally took a dump after I added some fuel. The confounding factor is that I had changed the ECU mapping not long before, but the fuel in the tank was from the summer and I assume I just added winter fuel. Another confounding factor is I poured a bottle of Techron in the tank just to see if the fuel trims would come closer. With fresh Raley's fuel comprising most of the tank and the Techron in there, it's getting 10% lower mpg than the highest I've seen, all cruise control at 60mph.

The EPA says winter fuel has 1.7% less energy but somehow I feel like every time I have ever gotten that first tank of winter fuel it's been a much bigger drop.

I seem to recall seeing some study that found a 4% difference in heating value between Shell and off-brand fuel, but I can't seem to find it.

So let's say the Raley's fuel has 4% less energy and the extra butane from winter blend drops it another 2%, I'm still missing 4%...and my long term fuel trims have only increased 2% going from Costco to Raley's gasoline. I'm going to test reverting the latest ECU mapping changes but I feel it's not going to help as the changes were very small.