Ford Focus Fuel consumption?

I have a 2012 (62) 1.6 Ti-VCT (petrol) Ford Focus and the trip computer is saying that I am only getting 25 miles per gallon around the town etc (urban) The owner's manual says 35mpg! The car has had a full service including spark plugs.

Also when considering how much petrol is left in the tank would you use the trip computer or the pointer on the fuel gauge to judge how much petrol is left?

Thanks for answers.

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