How did you obtain your TAS? Whizz-wheel? Or does your ASI have a sub-scale for temperature and altitude so you can read it of directly?
Did you use the exact power AND MIXTURE setting indicated by the manual? For one aircraft I fly regularly, the airspeed and fuel flow data is for leaning peak EGT, there is a second data set for leaning 100F rich EGT, which gives you 10% higher fuel flow and 7 kts higer airspeed...
And of course the flaps were fully retracted after take-off...
C.