Larger Gas Tank?

Another curiosity post here, but has anyone with a GX550 averaged more than 17MPG? I know that my larger tires aren’t helping my cause, but I feel like I’m barely scraping by with 300 miles per tank and an average Mpg of 16.2-16.5. I didn’t buy this thinking it would be a gas sipper, but has anyone else out there getting “decent” gas mileage? Any suggestions?