Is it possible tyres are causing my car to do 27mpg driven locally and steady?

A4 TDI 130 BHP,originally fitted with audi alloys with 195 65 15 (6.5 j)

now i have upgraded to different alloys (genuine audi) fitted with 235 45 17 (7.5 j) ET 35

i get 38 mpg on a run and 27 local...thats driving steady,checked all else cant find any faults.

its a 2001 (b6) tdi standard model.