F73 Low MPG

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Hello drivers,

My problem today is with my MPG.

I'm getting very low mpg on my 1998 F73 and am determined to improve it. Has anybody had same problem? What would you recommend? I have read the other Posts on that subject but have not found the solution.
I have done regular maintenance on it since I purchased it in Sept 08. Oil/Filter, Fuel Filter, Air Filter. Every 2 to 3 months. I have put new tires on it (255/70/15 / Bigger than the OEM specs, I know). I have maintenanced the brakes. But I am still not getting more that 19 MPG on it. And have never gotten more, even with the previous 235/75/15 tires. Wether town or highway MPG. No more than 19. And I know I should be getting at least 25 to even 30 MPG. According to what I have read here.
What would you recommend I check? Getting desperate and was really counting on the Trak's Fuel economy.
Let me know if someone has any idea. I'd really appreciate it.
Thank you very much.