It might be third. Let's say you have a ton of KR and a flashing check engine light etc that you didn't mention. Car can't make the power and if you are in third at 90ish.. the kr etc is probably holding things back.

Something as simple as a dirty MAF, bad TPS can mess with shifts badly. Also KR etc can.

Fuel mileage..your testing wide open runs with no maintenance and saying mileage is bad? Not saying this to be mean, but to open your eyes as to what I'm reading into when you say that.