City driving is very tough on the engine. No driver is like another, and
that would make a big difference and as such no two cars will give you
same mpg even if they are brand new, and that is why even the
manufacturer uses "Average". Now where you live you might have
some hills, where your friends live there might not be any, that would
be a huge factor for lowering your mpg. If you run into many stop and
go lights/signals or live near a school area, where speeds are reduced
and eventually you accelerate when you pass school area, you will get
lower mpg. If you wait until you reach the red light/stop sign to hit the
brakes whereas your friends coast the car from far when they see red
light, that would make a difference too.
Do this as a test. Zero both trips, and drive in the city where there is
at least couple of miles long highway, not freeway with speed limit of
at least 45 mph. See what your average mpg you get. And let us
know, I would be very interested to know and might be able to better
diagnose. Oh and check the tires pressure too, take it to Discount
Tires they will check your tire pressure for free, no affiliation with them
at all.
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