MPG observation

While entering my gasoline purchases into my budget, I noticed something strange.  At one point last month, the Shell gas station was less expensive than my usual grocery store generic gas, so I bought two consecutive tanks of the Shell gas.  Then the prices changed, and I went back to grocery store generic.  However, during those tanks of Shell gas, my miles per gallon went up.  One tank even had 39 miles per gallon.  That makes me wonder if I can save money by buying the more expensive Shell gas because I get better gas milage.  So, how to figure that out.  If the Shell gas is $3.97 per gallon and I get 35 miles per gallon, that comes to 11.37¢ per mile.  The generic gas is $3.88 per gallon and I get 32 miles per gallon, that comes to 12.15¢ per mile.  I would save 0.78¢ per mile by buying the more expensive gas.  I drive about 330 miles per week to work, so I would save $2.57 per week on gas.  This is, of course, as long as the 35 miles per gallon using Shell was truly cause-and-effect.  I may give this another try to verify the hypothesis.  I will also need to find a way to calculate the greatest price variance that would still save me money.