> NYC cabs average 100,000 miles per year and use 1976 cars.

I don't think that's true anymore.

http://www.nyc.gov/html/tlc/html/industry/taxicab_vehicles_i...

http://nypost.com/2013/10/28/citys-final-fleet-of-crown-vict...

Anyway, If the cabs were getting 100k miles per year, how often were they getting replaced?

Some of these cabs are lasting 30+ years, though they often use old cop cars which also have high mileage. The limitation is offen accidents not direct mileage.