Old structure: 2X4 studs at 24'' on center
I live in a tract house built 1970 which is framed with 2x4 studs in all exterior walls and interior load bearing wall on 24 inch centers. It is structurally sound, but the insulation is not good. I think 2x4s on 24" centers is fine for a garage.

Houses of this vintage and quality didn't use finger jointed studs. This may be stronger than the finger jointed studs but of course it meant that some studs were curved as installed or curved in place. What type of studs are you going to use?