The 2026 cars have to use 30 kgs of fuel only to generate electric power, according to the latest calculations. Wheelbase set to be shortened by 30 cm, new gearbox with only 6 instead of 8 gears. More about the 2026 regs in our AMuS story (in German): https://www.auto-motor-und-sport.de/formel-1/f1-reglement-2026-details-motor-technik-radstand-getriebe/
I guess the move to 6-speed comes from the extra torque the electric motors provide making 8 a bit unnecessary.