Extra electric load will result in extra fuel consumption. Approach you are suggesting wouldn't recoup lost energy, as it actually would require to spend more energy.

The real wasted energy from cars is heat. Only 20-40% of burnt fuels energy is converted in motion, the rest is pure heat. That's why cars need radiators, to get rid of this heat.

If you want energy savings, look for a way to capture this heat and convert that to electricity or something.

Also braking is a huge waste of energy. As brakes work by converting kinetic energy, which was gained burning fuel, into yet again - heat. To accelerate again you have to burn more fuel.

That's why electric cars have regenerative breaking, where it's literally doing acceleration but backwards. Instead of spending electricity to make car go faster, you slow down car to generate electricity.

And why electric cars are more efficient, as they converter almost 100% of their "fuel" into motion. There's very little waste heat.