If you are using the same fuel, and your tune-up was done properly then check the brakes for drag, make sure your tire pressure is up, check to see if the air filter was replaced in the tune-up. If all that is good then disconnect the battery and let it sit overnight. Reconnect it the next morning and see if that helps. The number one effect on fuel economy is the driver. Drive like a little old lady and the fuel economy will go back up. If there is an engine problem it is brobably bad injectors - the cleaner you put in the gas tank will clean the lines, tank and pump and deposit anything small enough to get past the filters into the injectors. You have to disasseble the injectors to clean, check for wear, and adjust them. The mechanic in a can works cheap but he doesn't always do the best job.
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Retired Mechanic
35 years of building performance C4s
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