A two-week rule can help here: anything that hasn’t been used in the last two weeks can be deleted. For example empty crates or boots from the last hike.
The spare wheel and on-board tools are also useless if you call the breakdown service anyway in the event of a breakdown. Alternatively, a lighter tire repair kit can be packed into the car. Mandatory items such as first aid kit, high visibility vest and warning triangle must of course remain in the car.
The AvD estimates that average consumption can be reduced by a few tenths of a liter with such measures alone. The ADAC calculates that if you drive around 100 extra kilograms in the car, you can expect an additional consumption of up to 0.3 liters per 100 kilometers.