Might check the condensate puddle from the A/C and see if that's it. Evaporator coils get mildew and mold, but then you should also smell it inside the car.
I guess you already changed gas brands. There's some problems with sulfur smell with some gasolines, but that should smell around the exhaust and not the front wheels.
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I don't smell it at all inside my car. I do run the A/C alot, but I would assume that it would be smelled in the car as well if it was a mold issue. As far as the gasoline, I use chevron and shell premium gas only.
Walking around my car, you can smell the oder only in the front of the car.
I could give a little history of the car to see if it helps. The car was in a minor collision where the front right suspension was replaced along with some body work to front right side as well. The car was sitting for about a year before the work was fully completed (saved up). I had the car inspected at a Lexus dealership, where it was said to be in perfect shape (besides needing an alignment).