Hello,

If you were driving a full size pickup that would be normal.
You ask,
"So, if that is low mileage what might be some not so obvious reasons for it?"

Bad maintenance or a worn out engine. Just how rough is this car mechanically?
A tune up might help. You have to be running rich somewhere besides on the highway.

If it starts up when cold and just chugs along you might be looking at a bad fuel pressure regulator.
Then again, the coolant thermostat is stuck open. Does the temperature gauge go up to about half way?
A engine temperature sensor for the computer, not the instrument gauge, can be bad as well.
All three affect mixture.
A tune up in the ignition area will have to be considered or least factored in since you are apparently dumping fuel and the O2 sensor is probably contaminated.

So here is a question.... Does the Check Engine Light bulb work in the dash when you turn the key.
This type of mileage should have that light glowing!

Get back to us with more details on the cars condition.

Phil