Low power, poor gas mileage in 740 sedan, 1991

[Tore]

I have been the proud owner of two 1991 volvo 740 sedans. The engines are identical, standard 8 valve. One of these gets good acceleration and 25 mpg. The other is noticably slower and gets 23 mpg. It went to a shop, and they replaced the harmonic balancer and retimed. No change. I replaced the oxygen sensor. No change. Replaced spark plugs, wires, rotor and cap. No change. When the power wire to the fuel injectors are taken off one by one with the engine running, the power drop is about the same for each. The compression tested 180,181, 180, 179 on the four cylinders. Quite respectable! It started running rough, and the volvo house replaced the engine speed sensor. Smooth running, but same low power and gas mileage. Could there be a sensor malfunctioning? Air temperature, air flow, etc??? Any ideas? Thanks, Tore