i don't know how you drive or how much is highway but missing overdrive gear will take about 4-6 mpg off highway. if you were getting 22 highway that is horrible to start with. it should be 28 mpg highway and 18-20ish city.
to make sure its working, drive the car at 70 mph or so and look at the rpm's. they should be at 2000. if they are at 3000 you have lost overdrive.
the driver center, the thing that tells you your fuel mileage, is an average reading. so if you are driving on the highway your average mpg reading will go down if you don't have overdrive.
