Fuel Addetive Question

With all the current talk of MTBE vs E10 across the country, it leaves me wondering how to tell what's going in my tank. Does my current fuel contain MTBE? Or did it, and it's now E10? Or is it just straight gasoline? How does one know?

And along similar lines, is it documented somewhere which areas switch formulations for summer/winter, and when? One document I read said all US sources switch for 'winter', and another said only cities in high elevations, or those prone to pollution use a different blend during 'winter'. And of course, none of them define 'winter'.

One of the reasons I ask, is because my MPG has been at the low end of what other have been reporting, and if there are fuel formulation reasons, I'd like to know. Now, this is mostly Short Trips, in 30 degree winter weather, but even so, 35 seems low. Long trips (same weather) averaged only 40.

Now - I have no complaints, none at all. I just seek understanding of what's flowing through my car's veins, and how much that's effecting my results. I'd love to see if there's a significantly change caused by the current warm weather (70s - 80s), on possibly summer gas (here yet?), without MTBE (gone yet, was it ever there?), and know if I'm taking a small E10 hit.

I'd try it myself, but my poor Prius has been in the body shop for 3 weeks, so I'm missing out on driving in warm weather, so all I can do is talk about it. And right before the accident, I filled it up and topped it off with (presumably) winter gas. So even when I get it back, it'll be a month before I could even try 'summer gas', if they switch in this area.

The area is German town MD, and I have no idea if these things vary state by state, city by city, county by county, etc....

As someone else said here - I want my Prius back, and I want it NOW! Letting me have one for two months, then taking it away are replacing it with a below average rental is about the cruelest thing the universe could do to me.

-Ken