Why Would Gas Mileage Suddenly drop to below 12mpg, city driving?

Thank You for responding. The truck has 87,000 miles. no missing, no hestitation, no power loss, no codes. have had diagnostic check of all systems nothing found.

Maybe just the difference in the gas itself!... I have noticed different mpg on my old Corolla from one tank full to the next...while nothing has changed on the vehicle..... There is a big deference in petrol from one tanker load to the next one.... At the same filling station/pump!

Thanks I Had used fuel from a different station

That's most likely what's happening since it's still running ok!... IF it were anything drastic, should see a decline in performance...