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Are you bringing an extra fuel can or is fuel available nearby? How much track time will you actually have? These things may have a bearing on how much you want to carry on-board. Fuel is weight but IMO how much fuel you carry depends on how serious you are about your lap times. If you're just learning I'd concentrate more on other things rather than worrying about shaving every pound you can. You can worry about record times later.
When I go road racing for the weekend, even though I bring an extra 10 gallons with me and there's fuel available at the track, I generally start off with a pretty full tank. Averaging 5-6 mpg I can burn through a tank of gas very quickly on a road course. When I run time trials I like to have a half tank or preferably less.