Most effective way is to put them in the right folder (there's one for every track) and load them into the garage in Heat.
So if I donwload a setup for the STCC mod on the Aardvark track, I put the setup (the .gar file) in:
c:\program files\hasbro interactive\nascar heat\stcc\setups\aardvark_t\
Then I start the STCC mod and go to the garage and click load, select the setup and are on my way.
To the best of my knowledge, there isn't any standalone program to read .gar files.
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Sports Car Online Racing Experience!
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I'm sorry, but the arcade.gar is packed into the tracks file (.trk). There is a way to change those files but really isn't worth the time and the effort. You're better of switching to Expert Mode since that isn't that much harder and then you're able to change the garage settings.
_________________ S.C.O.R.E.
Sports Car Online Racing Experience!
Plain and simple fun!