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Gas mileage

My primary vehicle is a 2005 Dakota. I use 3 primary gas stations in

Parker, Colorado. I keep close records on gas mileage. I decided thru this summer to mix the stations I use to 3 each over 3 months. I found significant differences in gas mileage, up to 10% to 15%. At station #1 mileage is 18.2 MPG, station #2 16.8 MPG, and Station #3 16.0 MPG. Station #1 is always greater than the others. I suspect the difference is either the lower heat content in enthanol fuel or poor measurement. What do you think?

MTBE was banned in CO in 2002. Any other chemical that raises the octane rating should be allowed. TAME, ETBE or ethanol are usually the only ones used. Check the 3 stations to see if they all use ethanol and how much is used. Differing levels of ethanol or the use of ETBE could account for the difference in mileage.

Seems a reasonable explanation. How many measurements (tank fills) did you do at each station? That info is needed to establish a standard deviation and thus a significance level.