This is why I have been questioning the focus of this thread almost entirely on an ECU incompatibility.

 

yea me too
that's why i said this earlier:

"Really though, I think the previous owner probably has no idea what they're talking about and/or was lying to you to sell the car and make up a fake explanation for the bad gas mileage."

I think the previous owner made up the ECU thing completely.

 

No, but I can think of some engine/ecu combos that would explain it, but I won't get into those right now since that's highly unlikely. Most likely the car needs a full tune-up and a compression check.

 

No way its an ecu problem to get mileage that poor more likely mechanical im sure a tune up could help a ton homefully u still have good compression but it could also be an issue. Are there any aftermarket parts like fpr or "piggybacks etc" on the car if those are set incorrectly could be part of the problem.

 

Does the car smoke when you shift? Ask a friend to drive behind you. Do you smell gasoline at idle? Does the car sputter? Try revving the car to the red line, where does it actually stop? Do the RPM drop when you step on the brake pedal?