2003 Suburban MPG

Has anyone else experienced a fairly drastic drop in gas mileage? I mainly have hwy miles and my Suburban has always gotten 17.8+ mpg per tank of gas, on occasion it would be over 18 mpg. The fuel "average" indicator has steadily climbed since I purchased this vehicle last February, the last it attained was 16.8 back in June or July, 2003 and has remained there.

But suddenly, at 15,000 miles, the mpg dropped to 15.8 per tank of gas. The fuel "average" indicator dropped a point to 16.7 in just 2 weeks. I had the fuel filter and the air filter changed and it only helped by .18.

To me, this is a drastic drop since I was getting 550+ miles per tank of gas - now I am getting only 480 miles or so and if I'm lucky, 500 per tank. I'm spending about $75 (depending on price per gallon) more to go the same distance I was going before the odometer hit 15,000. It's like someone flipped a switch and stepped up the fuel flow. It runs great as it always has, the tires have good pressure as always. I can't find a reason to lose 2+ mpg and would be most grateful for any advice as to the cause.

I said I didn't like the cramped leg room in the new Suburban but would live with it since the gas mileage was so much better than my '94. Well now the mileage is not better than my '94.....