I had a 2014 I Sport with about 60k on it. Didn't keep it long and decided to trade it on a new 2016 I touring. Didn't have any problems. I was averaging about 44 MPG according to the monitor. Probably in reality around 42.
We have a 2015 Mazda 3 i with 84000 miles on it (I drive between 75 - 125 miles a day), I have done oil changes and a brake change, also filters. Nothing else has been done and the car is as good as the day I bought it. My Wife has a 2014 CX5 with about 87000 miles on it, same thing with that car. I did replace plugs in the CX 5. My 3 gets about 35-37 mpg on average.