ur car is doing 10cm for every rotation extra with big tyres
if ur car tire is doing 219 rotations in 1km, on 145 70 12 tyres
its doing 206 rotations in 1km on 155 70 12 tyres
there is no loss on fuel efficiecnt, its just that your meter will show you wrong readings because its not stock size...
it mean tire is having more rotations on short tire size....