The R1T weighs in by spec at 7250 lb (3288 kg). At 45 mph (72 kph, 20 m/s), there is 0.5 * 3288 * 20^2 = 657 kJ (1J = 1 kg m^2/s^2) or about 182 Watt-hours (1 WH = 3.6 kJ) of kinetic energy in the moving truck. We approximate energy consumption of the truck at 450 WH per mile at 45 mph, Consuming 27kWh over a one hour trip. To increase efficiency by 20%, regen would need to recover 5.4 kWh over that hour. Assuming 50% efficiency in recovery, this equates to 2 * 5000 / 182 = 54 times over that hour, or about once a minute, slowing down from 45 mph to zero, then speeding up again. That’s an ugly hour of driving! I suppose its possible, but I don’t recall ever being in that kind of traffic in my 35 years of driving.

Hilly driving will recover more. E= mgh, g=9.8, so it takes 32 kJ (8 watt hour) to lift the Rivian up 1 meter, so we need to see an altitude change of just 1,350 m ( 4000 ft) to recover 20% in an hour. 4000 feet in 1/2 hour at 45 mph corresponds to a grade of around 1% for 30 minutes. That’s something I’ve seen. And regen works nicely in this situation to improve economy.

However, in most situations, regen adds roughly 5% to fuel economy in my experience with hybrids. I don’t imaging its too different with EVs. There has to be data somewhere though.

I did this too fast... anyone want to check my math?