I would go with a 16 tooth sprocket in the front and start with the stock 40 at the rear if you're using a stock wheel size. When you are consistently hitting all your shift points in the right places then change to a 39 in the rear and try to hit the same shift points, once you can do that take another tooth out of the rear and so on as you get faster.

To get proper info you'll need to know what your laptimes are at the circuit and see what the locals are running at the same laptimes. Don't try copying the fast guys' sprocket ratios because they won't work for you if you are slower.
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