Well first check the obvious: tire pressure, alignment, dragging brakes, clean air filters. Has it always gotten bad mpg or is this recent ?
Then you wil need a scanner (PC or Tablet based is under $50 and permits logging/comparison) and this is the boring part: need to make a number of passes with each car while monitoring the paameters.
At the same speed in cruise on a long flat stretch look at throttle position, fuel trim (short and long term), ignition advance, injector pulse width, manifold vaccuum (IMAP), IAT, coolant temp and if any trouble codes are set as a start
Something is going to be different. The most common is a partly clogged O2 sensor that has not failed but is making the engine run rich (short and long term trims).
Anything that can make a 33% difference (18 vs 24) is going to be pretty obvious.
Trouble is that the takes time by someone who is skilled at reading the information. The dealer maynot have anyone like that. However with two vehicles you do not need to understand the values, you just need to look for differences and post here.
As mentioned if you have a Android phone, tablet, or a PC the cost is under $50 for everything from Amazon, can be moved between cars, & most apps can e-mail the results (for iPad or Mac it costs more).
For more than you ever wanted to know see here.