GAR or German Auto Repair on Hwy 9 just south of McFarland and Dunkin Donuts is a good place, They have done basic work/repairs on 3 of my cars (MB, BMW, Porsche) over the last 5 years. They have a Porsche specialist who is suppose to be top notch. All the mechanics there do good work at a reasonable rate and the owner Alan is a good guy too. They were in Roswell, but recently moved between Cumming and Alpharetta to a nice new facility.
