Roads? Where we’re going, we don’t need roads. (Photo stolen shamelessly from DAZER.)

...you’re gonna see the horsepower peak.

So, I should explain something about how the Prius transmission (in any generation) works. It’s fundamentally based around a power split device, which uses planetary gearing to split engine torque into electrical and mechanical paths, and give most of the benefit of a serial hybrid, with higher efficiency. The engine’s connected to the planet carrier, the first motor is connected to the sun gear, and then the ring gear goes to the wheels (the second motor also goes to the wheels, but it has some of its own gearing).

Now, what ends up happening is, the ring gear ends up subtracting rpm from the sun gear, whereas the planet carrier adds it. So, you get this formula:

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MG1 rpm = (3.6 * engine rpm) - (2.6 * ring gear rpm)

The MG1 in the Gen 4 Prius is limited to 9750 rpm, as far as I can tell. This means that at a complete stop, your maximum engine rpm is 2708, and it climbs linearly until the 5200 rpm power peak. However, that doesn’t happen until 87.7 mph, at least with my tires - close enough to 88 mph.

It’s also worth noting that if the engine is off, MG1 hits -9750 rpm (that’s spinning backwards) at 95.4 mph, and the engine is forced to start to avoid overspeeding MG1. In practice, Toyota claims 68 mph max for that, and I’ve seen as high as 73 mph.

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The graph of mechanically possible vehicle speeds vs. engine speeds looks like this (with the caveat that it won’t actually spin the engine at anything below ~1000 RPM or so):

The funny thing is, I’d actually count behavior like that in a true CVT (like the one in the Honda Fit) as a mild variety of bullshift, albeit not as bad as one that pretends to shift between discrete gear ratios. But, because there’s an actual underlying mechanical limitation involved, I’m OK with this. (That said, increasing the maximum speed of MG1 would reduce the mechanical limitation, and improve acceleration.)

That is all.