I agree with Kat Ballou. Residents who choose to live in Centro (downtown) and the adjacent neighborhoods in San Miguel de Allende, really do not need a car to get around town. It is so easy to catch a cab and rates are very affordable. Many rent a car or hire a driver to make once a month shopping trips to nearby cities.
I know quite a few residents, however, who do end up purchasing a vehicle after living in San Miguel for awhile. Having a vehicle allows them to make trips to stores like Costco in Celaya or Querétaro (40 minutes and 1 hour away); Walmart in Querétaro; and to see doctor specialists in Querétaro as well. A vehicle also provides mobility when it comes to taking trips to fun places like Puerto Vallarta, Troncones, Mineral de Pozos, and Guadalajara.
For residents who choose to live in gated communities located 10 to 20 minutes from Centro such as Club de Golf Malanquin, Rancho Los Labradores, and Otomí; having a car is essential to get into town and to and from the grocery store.
(The beautiful abandoned buildings of Mineral de Pozos, Mexico, pictured.)