It goes without saying that city MPG will depend on each driver and driving cycle and may vary greatly from the EPA estimate. One should first examine one's highway MPG as conditions on a long highway cruise at a steady speed are generally more stable than city driving.
Remember that city driving usually involves lots of idling. Idling equals zero (0) MPG and this must be factored into the MPG calculation whether one works it out by hand or by simply spying the instrument cluster's average MPG display which does this automagically.
So, in essence, lots of idling will show up as less than spectacular city MPG.