What type of driving are you doing. If this was all around town then this is not really bad. If it is freeway driving then it is a bit low. 16 to 17 is pretty normal for combined city and highway. Over 20 on the highway and around 14 to 15 around town.

 

Speeds, tires, acceleration, length of trips, extended warm ups etc. I get worse MPG in the winter ( some places also change the blend for the cold weather and it has less BTU also)

 

Right on money...

Higher the cetane the lower the BTU's...


Engine Coolant temps and IAT temp play a huge role in MPG's.

Engine coolant is optimal between 190-200*F Just dropping 10*F down to 180*F can have effect.
IAT temp are optimal at 100-140*F for MPG's. The warmer the manifold temp the better the MPG's.

Length of trips... This varies from person to person. Like me anywhere I got I've got a 50-100 mile drive so warm up period typically are short and constant drive speeds. Compared to stop and go travel and driving a short distance and never getting fully warmed up.

Tires... The bigger the tire the more rolling resistance and rotational mass. This why racing slick are smooth face (least amount of rolling resistance) and light weight (reduced rotational mass) to get the maximum amount of power to the ground. So just going from 265's to 285's will reduce MPG by at least 1-2 MPG easy if not more depending on tread face design. (Rolling resistance).

Speed. Wind drag doubles from 55 to 65 MPH so like me 55 MPH is the best of all worlds but 65 MPH is a 5 MPG drop for me. I lose roughly 1 MPG for every 2 MPH over 55 MPH.