Sounds like your lambda sensor is knackered. When they fail they read lean. The Ecu will be throwing in fuel thinking it is running lean so doing little for your mpg. The afr gauge after about 5 mins running should oscillate back and forth between rich and lean at idle and light cruise. Then switch to reading rich when above approx 3000rpm or when generating positive boost.
I would suggest throwing a £30 universal lambda on it. Should sort it and also increase your mpg.
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