First grab a calculator.
You haven't said how much you drive, so I've made some assumptions:
If you used to fill up once a week, now you have to fill up more often, 56 times per year under the 'new' lessor mileage. That 4 extra tanks per year, and at $3.5 per gallon and 20 gallons per fill, that about $300 for an entire year.
Having the labor done for free, $300 can buy a few things, but nothing really serious.
All that's IF there is a mechanical problem, with more than 40 years of experience with cars, and like many others, I have my doubts.
Do a little OT at work, or sit in front of the computer and generate a few posts instead. Financially, and mentally, you'll be ahead.
Remember, if there is nothing wrong, the BEST you can do is simply waste all the money and time you put into the project. Oftentimes you'll not only waste the money, you'll end up with even more problems or lower mileage.
I’m just trying to figure out what is going on.
The government does so many stupid things with the fuel blends and mandated computerized controls, these days, that it is difficult to determine if a change of a variable like milage is the result of a mechanical problem or some unintended consequence of a government policy.
You can’t look in the newspaper to tell you what is going on. The fuel companies will never tell you when the energy content of their fuels changes. The Octane Rating, which people have always used as a proxy for BTUs is useless.
So I turn to my friends at FR to see what is going on. It is a sad commentary on society that FR is the only place to go to get useful information (after a little filtering, of course...)