Always a good precaution to never "assume" that everything was done during a service. There is also a broad scope interpretation of the term service; it can mean as little as just an oil change.

Check your air filter etc. as mentioned previously.

Also remember there are factors other than the engine that cause low mpg: Binding brakes, poor chain condition, chain too tight, low tyre pressures, poor fuel etc. Even the weather can have an effect!

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