As my commute is 75 miles/day, I buy gas quite frequently. I always buy 91 octane from my neighborhood Chevron station, usually at the same pump, and I always average 25 MPG per tank. On the last tank, though, I averaged 21 MPG, and on my current tank (about 150 miles in, so far), I'm averaging 25 again. Would it make sense that I had a bad batch of gas, since the weather, tire pressure, driving style, etc. have all remained the same? What else might cause this?

 

It could have been something else. It would have to be a really bad batch to lower your mileage that much, and you probably would have noticed the performance difference and maybe even felt it run a little rough.

Remember, when you fill up you usually have 3-4 gallons left in the tank, which means about 25% of the new tank is the old gas, which lessens the effect of a bad batch.

 

It's definitely not a bad batch of gas! That is a myth. You will not get the same gas mileage week in and week out. There will be some variation no matter how repetitious your driving habits. Maybe up to this point everything has been consistent but now you are seeing one of those variations.