The average city dog lives three years longer than the average country dog.

thats because most country dogs are let to run around, get attacked, hit by cars, shot, eat bad things, and dont see the vet

and they say city life is unhealthy

Yes, there are more dangers out in the country bit it's usually not the dogs getting attacked.
More dogs are hit in a city then in the country.
Shot? You're talking of the old days when there were no laws against it.
Dogs see the vet depending on the owner not the area they live.
You should also realize that most of the dogs out in the country are lager and have shorter life spans while people in big cities usually have smaller dogs due to a lack of space.
City life is very unhealthy. It's the reason why we now have a bed bug problem.

Some dogs are mistaked for coyotes or wolves in the country. Therefore, shot.

This could be because most country dogs are big dogs (German sheperd, Rotwiler, etc.) which typical don't live as long as smaller dogs (chihuhua Frence poodoles etc).