NASCAR 14

The quickest way to do this is to complete private races at Watkins Glen. You can do these by yourself too.

Watkins Glen
The 2% races are only 1 lap. It will take about 1:15 per race. Anything else, will take longer than this.

Hope this helps!!

For all the online achievements, you can select a private race by yourself and earn them. Select some of the shortest tracks such as Bristol, Dover, and Richmond to mix it up a bit. If you would like to boost with others (trust me, this gets boring and quick) don't be afraid to make a session!

Credit to wright3034 over on my home forum (XBA/x360a) and VELOSCILLATOR here on TA for discovering that private races work, and that they can be earned solo!