1999 Avolon only getting 18-20 mpg

I'm having a hard time finding out why this 99 model Avalon gets such poor gas mileage, and most of the driving is done on the road (55-60 mph).

-I've checked the air filter and it's clean.

-The tires have plenty of air (35 psi).

-I don't hotrod the car (no going from 0 to 60 in 5 seconds).

-I'm pretty sure I'm measuring the gas mileage correctly; I fill up, keep track on how many miles I put on it in between fill-ups, and then divide the number of miles by the number of gallons it takes to fill it up (example: I drive 190 miles and fill up again, and it takes 10 gallons to fill it up, 190 divided by 10 is 19, so it'd be 19 mpg).

-The engine runs fine; it does not run hot and has plenty of power other than the fact the gas mileage is so low.

-The transmission shifts fine (engine probably never gets over 2,500 rpm as I accelerate).

Now I do live on a dirtroad, and the car has 218k miles on it, so this could be a factor as well. But any ideas on why the gas mileage is so poor?

Don't judge the mileage by one fill-up. Total and average 4 or 5 fill-ups. What about on highway trips where you are only driving 55-60 mph for extended periods on flat terrain with the a/c off and the windows closed? This is where you get your best mileage and the best judge of what your car is capable of.

Are you filling the tank with the recommended octane gas as stated in the owner's manual? Lower octane gas can decrease your ignition advance to prevent knocking and lower your gas mileage. I believe your recommended octane is 91 or better for best engine performance.

Spark plugs?
Fuel injectors need cleaning?
How's the mileage with the a/c off as described above?
Don't idle for long periods.
Check parking brake adjustment.
Changed transmission fluid?
Alignment?
Check your intake duct for cracks and loose connections.
PCV valve

I agree with all of the above. Check your air filter and do you have custom tires on the car (not the original size that the car calls for)? Sure its not missing or a coil on plug is out?

I have a 2003 Avalon and had the same problem. One way to tell is if the idle is high. Our car had 140k miles and started idling high (14-1800 rpm) and getting only 20-21 mpg. We changed the spark plugs; it is possible that one cylinder wasn't firing. It helped a lot.