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Any ideas? loss gas mileage & higher RPMs at 60mph

07G35

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Texas
2007 G35 Sport Sedan w/ 105K miles

I do mostly highway driving and monitor my gas mileage regularly. Recently I noticed a sudden 3-4mpg drop in gas mileage and the RPMs seem to be slightly higher than normal at highway speeds of 60+MPH. To me it felt like the car wasn't going into overdrive. All the gears still shift smoothly but it feels like there is resistance at highway speeds.

I took the car in and they couldn't find any codes or diagnose the problem. While it was in the shop I had the spark plugs changed and they reset the computer. The throttle body was also cleaned as well. 30 miles after pickup the Service Engine Soon light came on. Took it back in and they said it was the catalytic converter. I made sure they checked the O2 sensors first and they said they tested the voltage and all sensors were fine. So they replaced the catalytic converter (I'm broke now) and I was hoping these issues might have resolved my problem.

Car runs slightly quieter and smoother but the gas mileage issue and higher RPM issue at highway speeds still seems to be there.

Does anyone have any idea what the problem could be?
 
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