Fuel consumption

There's numerous reasons for a loss of MPG to give a single answer. You can start by giving your car a tune up, new plugs, wires, rotor & cap, clean or replace every filter, check timing, if motor is high mileage may want to check compression, check rings with a leak down test, check back pressure on exhaust (namely restricted cat), check any vacuum leaks, check PCV and EGR as both of those get dirty and can rob an engine of fuel economy. A tranny shop can run a computer diagnostic and see if there is any slipping in any gear, a lot of times a TC clutch starts to go and you lose 100% lockup at highway speeds in OD which again robs you of fuel economy. You can also check for any sticking brakes, as brake lines corrode or brakes hoses decay, can cause brake fluid to be retained in the caliper and brakes drag all the time. There are other mechanical things you can check as well, like leaking injectors or running rich that cripples fuel economy. A bad O2 sensor can be throwing off the computer.