You can start by looking at the fuel trim numbers on a scanner that displays live data. If they're high negative the Engine Computer sees too much fuel going into the engine and it is trying to subtract some from the factory pre-programmed values. If the numbers are high positive it is adding fuel in response to seeing a lean condition. Usually that is the result of a vacuum leak allowing in too much air, a spark-related misfire allowing unburned oxygen to go into the exhaust, (along with the unburned fuel which doesn't get detected), or an exhaust leak ahead of the front oxygen sensor that lets outside air get drawn in between the pulses of exhaust gas flow.

If the fuel trim numbers are all close to "0", look for dragging brakes, excessive fuel pressure, typically caused by a leaking vacuum hose going to the fuel pressure regulator, or a torque converter that isn't locking up in third gear above 45 mph. Beyond that you need to look at the rolling resistance of the tires, wind resistance, and friction to see if anything has changed compared to how it came from the factory.