1985 380SE Gas Mileage Decrease

All,
My 1985 380SE would get 21 miles per gallon on the highway. It then started running like a dog and I was getting 8 miles per gallon. I have 173,000
miles on the car and replaced the O2 sensor. The car now runs great and sitting in the car you barely can tell the car is running - - it runs that smooth. The gas mileage has improved But, the car is not getting the mileage it use to and is now getting 14 miles per gallon. I do notice that when I drive the economy gauge on the dash seems to have the needle sitting in the middle while I drive. When the car is at a stop, the needle on the economy gauge is to the left. Any suggestions of diagnosis I can perform to figure out the reason for low gas mileage?
Thanks in advance.
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Did you replace the plugs? If the car was running wildly over rich due to the O2 sensor problem, the plugs are fouled, and should be replaced...
Marty
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I checked the spark plugs and they were fine, but I replaced them anyway. The car is running great and none of the cylinders are missing.
said:

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I wonder if your catalytic converter may have gotten clogged. When mine went, I first noticed a sudden loss of power going up steep hill at high speed upon downshift. Soon it got so bad I could barely drive 50 MPH.
-Kent
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