To slow down a car while speeding up a flywheel requires an infinitely variable gearbox. Ie. When you first start deceleration and the flywheel is stationary

To slow down a car while speeding up a flywheel requires an infinitely variable gearbox. Ie. When you first start deceleration and the flywheel is stationary, you'd ideally have infinity:1 ratio. When the car is pretty much stopped, and the flywheel going fast, you'd use 1: infinity. And in between you'd use every other ratio. Such gearboxes aren't really a thing (at least not without massive caveats). The best you can do is fixed gear ratios and a clutch, or a CVT and a clutch to handle the ends of the range. Both are mechanically complex and not very efficient (a typical shift under load with a clutch wastes half the energy into heat in the clutch material). Compare to electric, where a 'gearbox' consists of software/inductors/capacitors/switches, and you can effectively have an infinite ratio (software) gearbox.