Why is my gas mileage so bad?

I have a 2000 Honda CRV and I'm almost just giving up on this car, I love the SUV but I no joke barley get 200 miles to the tank, with this mileage I could be driving a truck or a Toyota 4Runner (which is what I really want), I changed and did the following: air filter, fuel filter, inflated tires, O2 sensor, rebuilt transmission and catalytic converter. The spark plugs and timing belt are fairly new and in good condition. I have invested enough money in this car to last another life time, I even took this car into firestone to check what was the deal and they said the car was in great condition, I understand this car is 17 years old but its only a 4 cylinder and in good condition

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