Q: My Toyota Avensis 2006 has had a dramatic Fall in MPG - pre and post-service

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I have a 2006 Toyota Avensis. It had a new clutch 6 months ago. Six weeks ago there was a sudden reduction in mpg from 43 to 37 for no apparent reason (no change in driving, conditions, route). Two weeks ago I had part of the gearbox replaced along with a wheel bearing - as well as a FULL service. The garage said that the mpg would rise after the service. In the week since I have picked it up the mpg has fallen further - to 33 mpg. I am driving no differently and using no different route to what I have over the last six years. It has always reliably done about 43 mpg. I have regularly tested this by filling up the tank, noting the mileage and noting the mileage again when I next filled up. Yesterday it had fallen to 33 mpg. The garage have now given me an additive to put in the tank when it is 1/4 full to clean it out - but surely that would not account for 12mpg difference in fuel consumption? Please help!

My car has 149000 miles.
My car has a manual transmission.

Hi there:

There are multiple reasons why recent service might negatively impact fuel economy on newer vehicles. As the vehicle continues to operate, the ECU (engine control unit or module) continues to learn and make adjustments to ignition timing, fuel and air mixtures and fuel flow. When the service is completed, sometimes it takes more time for the ECU to update this information; and in some cases, if any error codes have been stored in the computer and have not been removed; it can impact fuel efficiency.

Another item to consider is your location. Fuel’s are changed prior to winter and summer seasons. Summer blended fuel tends to be a bit more refined; while winter blends are not as clean. As such, fuel economy can be reduced as much as 15% just on seasons.

The additives can sometimes quickly reduce fuel efficiency as they literally "burn" carbon deposits from fuel injectors. The best advice I can give is to keep running your Toyota for a while with normal fill ups (if possible from the same service station) and see if the MPG begins to slowly improve.

If not, contact a mobile mechanic, such as one from YourMechanic, who can come to your location to complete a diagnostic on your decreasing gas mileage. Best of luck.

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