Getting Bad MPGs in my 2008 Mitsubishi Outlander XLS V6 ?

Did you replace the spark plugs and air filter yet?

Even if the vehicle came with Iridium spark plugs from the factory thy need changing between 90,000 and 100,000 miles.

Only NGK or Denso brand spark plugs can be used in a Mitsubishi.

Preventative Maintenance.

Have a tune up, coolant flush, and oil change done.

When was the last time you had a complete tune up done on your car? New spark plugs and cables, new fuel injectors, replace all filters, etc. Your 11-year-old car probably won't EVER get the same fuel mileage and performance as when it was brand new, but with proper care and maintenance, it can come close.

First thing you do is make sure all maintenance is up to date. Plugs, ignition wires, air filter, all will have detrimental affects on MPG is not changed on the correct intervals. Tire pressure, change in tire size (bigger tires will make it seem like mpg is down because going farther on every revolution)

When was the vehicle last given a full service? When did you last check for correct tyre pressures as stated by the vehicle manufacturer? When did you last clear out any unnecessary weighty clutter you’ve been hauling around inside the car? Has your driving style changed to be far more “welly and brake” than it was?

Sort all of those out first. But you can also expect some fall-off in mpg as the car ages, although most diesels with just 125k miles should show no real drop at all if maintained correctly.

Try the universal fuel savers of removing unnecessary weight and making sure the tire pressures are correct, as well as driving carefully. Other than that a good service will doubtless help. I'd be looking at replacing air filter, spark plugs etc. if that isn't covered in the service.

It’s getting on a bit. The engine is probably not quite as efficient having lost some power and maybe you’re stepping on the fuel to get the equivalent performance ? Things wear out. Probably needs a full service, replacement parts etc if you want to keep investing in it?