My garage worked great for lagers, it was like 29-40 out and my garage stayed a constant 48-52 when the door was closed. Which put my beer at around 50 most of the time according to my thermowell, it drifted up and down a degree or two here and there but it was very very slow drift..like i'd maybe lose 1/2 a degree overnight when it got really cold.
It takes either a very large temperature differential between the liquid and surroundings, or a very long time for the temperature of 5 gallons of anything to change. The garage routinely got down to 30 when i opened it to brew, when i'd go upstairs to look at my temperatures they maybe dropped 1 degree over the 4 hours...
It really depends how well insulated your garage is i suppose..between my gas furnace, and a PC server running out there it maintains temps pretty well...that said i do want a ferm chamber...once i move my current keg freezer inside and turn it into a keezer, ill have room for another temp controlled freezer in its current spot.