how do you determine what gears to run at different tracks?
Best is Ask the Track Owner / Promoter if he does not know he'll find out, he wants your attendance.
ask other racers then pay attention on the grid, if they tell ya say 15-66,67 look at there's if indeed rear is 66 67 chain will be closer flush with gear guard, if there not running a gear guard you can see the number, then you just need to verify there tell n the truth about the front. Of course ask the faster guy's.
Also visit other tracks when you can even if you do not wish to race, then ask pay attention and keep notes.
Or make sure you know exact track size and just ask on here, there's a lot of guy's with lot's of experience that will get ya started out.
Good Luck !!