[Jasen]

My previous tank of fuel I switched from Chevron @ $3.95 a gal. to Costco @ $3.54
The 1st. tank of Costco my UG showed 15.8 avg. mpg the same as the previous Chevron tanks. I had an exhaust leak that I fixed at my 2nd. tank of Costco fuel. My 1/2 tank avg. is 15.1 and no matter what I do I can't get over that, driving the same route on my commute. The avg temps here have dropped about 10* as we head into Autumn.
My cost per mile was .25 cent's, now it's .21 cent's, which is better on the wallet.
I'm wondering, is it

A. weather change

B. fuel brand

C. the exhaust leak was just right to increase FE

D. all of the above

I had hit 15.9 mpg a couple times on the previous tank and 16 was within reach <sigh>