A one time drop isn't necessarily something to worry about because, often as not, it's some error or a temporary change in situation that caused it. There are many things that can cause a drop in MPG, up to an including fuel that is more foamy than normal and you don't get as full a tank. This is definitely the foamy fuel season. Yesterday, I was able to eat an entire ice cream cone while topping/tricking off to my normal level (14.5 mpg, btw). Five minutes at least. I check my mileage every tank and have since 1987. I've seen drops like that and usually, it's either a bad load of fuel or me being lazy at the pump and not taking the extra time to top the tank to the same level each time. When it foams over the first time, I can usually get 2+ more gallons in. If it's bad fuel, it will bounce back. Ditto for fueler errorIf you see the low number consecutively from now on, you then can officially start tearing hair out.