Create a weight penalty for every 1kg of fuel added above 100kg. If your team estimates you need 105kg to finish the grand prix at full throttle apply a 4kg/1kg of fuel weight penalty. Adding an extra 20kg of weight to the car that you can't burn off as you use fuel is a significant penalty in F1. It'll motivate teams to use or get to the target number of 100kg without turning the teams loose to burn as much fuel as they want.
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