WOW .. if you are not getting overdrive then this will up the fuel consumption to the point
where you are now ,the OVERDRIVE is a big fuel saver .
Linda i suggest you check out another repair shop as it looks like they are not checking your car
This could well be a transmission issue that they failed to check if the shop you was using did not have the equipment to check all system then this maybe why they failed to find the problem .
Sorry you been though $1000.00 and not come to a fix ,
If high mileage (100.000 + )then the cats and sensors will be a good change and no loss .
Here are 12 points to help on a better MPG.
12 ways to improve your gas mileage.
Keep your engine tuned up. Incorrect fuel ratio, bad spark plugs and incorrect spark timing can have a big effect on gas mileage.
Don't use your air conditioner (A/C) unless you really need it. The A/C compressor puts an extra strain on the engine.
Make sure your tires are properly inflated. Under inflated tires cause more resistance to travel.
Avoid stop and go traffic by taking alternative routes or less congested travel times.
Ask other people with the same type of car what kind of gas mileage they get, it may alert you to a problem with your car.
Use a good engine oil, preferably a synthetic or standard oil treated with a friction reducing oil additive. These have been known to improve fuel economy by as much as 12%.
Reduce the weight in your vehicle as much as possible. (Good excuse to get the mother-in-law out of the car....)
Use your cruise control whenever possible.
Keeping your highway speed at 55 mph can improve you gas mileage by as much as 25%, compared to 75 mph. (this one's tough for us too...)
Avoid trying to increase speed while climbing a hill, your engine is already working hard trying to overcome gravity.
Keep your windows closed when possible. Open windows actually reduce gas mileage compared to using the A/C.
Remove accessories, such as luggage rack, etc. that lower the aerodynamics of your vehicle.
Hope the cats and O2 sensors where original Mazda parts or Denso.NGK if any other cheaper sensor used then this may cause bad MPG ..
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Kind regards Greg.