Possible reasons for increased fuel consumption with automatic transmission:
Automatic transmission slipping:
* Worn or dirty friction discs and bands
*Low level or poor quality transmission fluid
* Dirty or clogged oil filter
Engine problems:
* Faulty spark plugs or injectors
* Problems with ignition or fuel injection
* Loss of compression in cylinders
* Sensor malfunction
Transmission problems:
* Transmission fluid leaks
* Worn bearings or gears
* Damaged differential
* Incorrect adjustment of shift cable or vacuum line
Other factors:
* Transmission fluid that is too viscous or dirty
* Worn or flat tires
* Installed rack or roof rails
* Aggressive driving style (sharp acceleration and braking)
* Clogged air filter
* Use of additional equipment such as winches or air conditioning
Diagnostics and troubleshooting:
* Check the level and condition of the transmission fluid.
* Scan your vehicle’s computer for trouble codes.
* Inspect the automatic transmission for leaks or damage.
* Check spark plugs and injectors.
* Measure the compression in the cylinders.
* Check vacuum hoses and sensors.
* Replace worn or damaged parts.
* Drain and replace transmission fluid and filter.
*Adjust the shift cable or vacuum line.
*Consult a qualified mechanic for further diagnosis and repair.