I just got what may be the best mileage I've ever had with my Maxima and I can't figure out why. The car is a '97 with 186,000 miles on it. It's due for an oil change and I haven't checked tire pressure in maybe three months. I spent over an hour in stop-and-go traffic on Sunday getting to the SF Bay Bridge toll plaza, running air conditioning in 100º heat, then drove around downtown with traffic and hills. Later I went to Santa Rosa, then the next day to Sacramento, all in 100º+ heat with air conditioning running, averaging between 70-80 mph. When I fueled up this evening, I filled up with 11.095 gallons after travelling 295.8 miles. That's 26.66 mpg when I usually average around 21!

 

26.6 sounds just normal for some city and highway driving.
I get upwards of around 28-30 on a trip with the A/C blowing.

 

I get about the same, usually 24-27 mpg a tank for mixed driving