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Is this a std our automatic transmission?
What state and brand of gasoline are you getting? How are you filling your fuel tank? Are you fueling until it clicks off the first time?

using a calculator, does your car calculate the same mileage you calculate by hand? I always log the mileage / odometer and use the trip meters and manually divide miles per gal and verify the car is calculating the same as I do…within a reasonable error.

i ask because if your transmission is an automatic, it could be it’s not going into lockup. That is, the torque converter is not locking up into direct drive. This could cause a 5%-15% reduction in fuel mileage, depending on how and where you drive. Also, different parts of the country change their fuel specifications in winter. This to can decrease fuel mileage.
Some of the dumber things receintly proposed by the idiots in charge is to increase the amount of alcohol allowed to be sold for street use. This too can cause a 5%-15% reduction in fuel mileage.
 

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Best way is take it to a transmission shop and ask them. One way is watch for a 200-300 rpm increase under light load or a light tap on the brakes while in cruise control…if you have one. While in cruise, with the torque converter in lockup, a slight increase in throttle will cause the torque converter to come out of lockup and start to slip ( hydraulic couple) and should show a slight increase in RPM until the load is reduced and the torque converter goes back into lockup.

The other way is with a good diagnostic reader watching the transmission signals from the Transmission Control Unit.

what city is the car in?
 

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I take it your watching the up stream O2 sensor change…..normal…..the rear sensor, or down stream sensor does not move as much. At 75k miles, I would not expect they are out of order.
 

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If you watch both at the same time, you will see the upstream changing rather regularly while the rear or downstream changes much less frequently…….to a steady throttle / RPM setting as the ECU changes the fuel mixture.
 
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