Well, technically you should see about a 3% drop in mpg with ethanol over regular. I don't know about regular with MTBE added. 3% of 43 mpg is a little over 1 mpg.

I've been running regular instead of E10 the last couple of tanks to see if I can notice the mileage difference. So far, I can't tell the difference, but there are so many variables involved that I doubt I'll be able to correlate a difference.