Powerboost defeated after changing tires

[Suns_PSD]

From a weight standpoint, you are mostly correct, but missing a couple of practical points:

1) Much of the tire weight gain is at the tread, the furthest point possible from the axis,
2) Moving from a street tire to an AT introduces additional rolling resistance, not just weight.

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[amschind]

Assuming that the wheel/tire functions as a relatively efficient flywheel to and from which energy can saved and extracted would be a far more valid assumption with a full EV. The very limited size of the electric motor and battery in the PB (which is the only conduit for the wheel/tire to function as an energy storage flywheel) limits that process in two ways:
1) extraction of the flywheel energy is capped at 0.3*1.5kWh=0.45 kWh by battery size and SoC limits
2) even if the battery were infinitely large and perfectly efficient, the electric motor size is so small that during most acceleration you're just turning the ICE back on in a process that is independent of the flywheel action of the wheels/battery/motor.

That said, even in an EV, efficiency stacking means that a rough round trip from the wheel to the battery is maybe 72% efficient (0.98*0.98 for the CV shaft, 0.98 for the ring/pinion gear, 0.95 for the dynamo and 0.95 for the battery=85%, then back again via the same path gets you to 72% round trip). Based upon that back of the envelope calculation, you're probably still better off with lighter wheels and tires even in a best case scenario.

It gets even more interesting in terms of hub motors, where you lose the CV shaft losses but gain unsprung weight, more mass at the center of the rotational axis and most importantly durability concerns from bouncing your wheel/motor bearings around without the benefit of the suspension. You may also lose the ability to easily use the 8:1 and 10:1 gearing that is commonly integrated into the differential for EV drive units, which you could possibly overcome by reinvesting some of the weight savings into larger motors optimized for lower RPM. Electric motors DO have power bands, except that in their graphs Y is load and X is RPM with values showing markedly differing efficiency based upon the combination thereof. There is also the question of inegrating braking into the drive motor, which offers weight savings, computer integration, system elimination and maybe even simplicity at the cost of requiring electric power for the brakes to work.