Quote:
Originally Posted by Ecky
I'm not highly educated on the older Toyota drivetrain behavior, but this went away in the newer ones. I suspect the reason for high RPM was not actually battery charge, but a gearing / electric motor RPM limitation. The final gearing is a ratio of gasoline engine and electric motors spinning together or in opposition. The Gen2's were able to get up to around 41-42mph on the electric motor only before maxing out in RPM. Gen3 raised that (I believe) to 46, Gen4 could drop into EV-only in the 70's even though it preferred not to, so there was clearly a mechanical change there.
It's worth noting that Atkinson cycle engines move their peak thermal efficiency point to a higher RPM and lower load point in their maps, though it is still ultimately a limitation due to the parasitic losses of just spinning the engine faster.
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This was an A.I. response : Good question — that person’s explanation is actually pretty close to what’s going on.
On the 2008 Prius (Gen 2), the limitations on MPG improvement from aero mods aren’t just drag reduction — they’re tied to how Toyota’s Hybrid Synergy Drive (HSD) is designed. The key factors are:
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1. Planetary Gearset / Motor RPM limits
The Gen 2 has MG1 (the smaller motor-generator) acting as both starter and a “virtual CVT” control.
MG1’s RPM is mechanically linked to both the gasoline engine and the wheels.
If the gas engine is completely stopped at higher speeds, MG1 would overspeed (spin too fast).
To prevent this, the ECU forces the ICE (engine) to spin above ~42 mph, even if it’s not providing power, just to keep MG1 within safe RPM.
�� This is the “gearing limitation” your friend mentioned.
Gen 2 → ~42 mph EV max.
Gen 3 → ~46 mph EV max.
Gen 4 → ~70+ mph EV possible because Toyota added another planetary gearset and clutch system that allows MG1 to decouple and avoid overspeed.
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2. Atkinson-cycle efficiency sweet spot
The Prius’ Atkinson-cycle 1NZ-FXE engine has its best thermal efficiency at mid-load, moderate RPM (~2000–2400).
If your aero mods reduce load too much at highway speed, the ECU may choose a less efficient operating point (e.g. engine idling + higher MG use).
So unlike a conventional car where less drag = always better mpg, in a hybrid the controller’s “sweet spot” matters.
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3. Battery state of charge (SOC) constraints
The car tries to keep the HV battery around 60% SOC.
If aero mods reduce load and the battery starts overcharging, the ECU will spin up the engine to burn off energy — even when you don’t need it.
That limits your ability to take full advantage of drag reduction unless the control system is reprogrammed (which most people can’t do on Gen 2).
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4. Fixed final drive ratio
Unlike a manual or conventional CVT car, you can’t “regear” the Prius easily.
The hybrid transaxle has a fixed ratio; all speed/load balancing is done electronically via the motors + ICE.
This makes Toyota hybrids less flexible in how they take advantage of extremely low drag or rolling resistance.
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✅ Bottom line:
On a 2008 Prius, aero mods absolutely help (people have seen 5–15% gains depending on how extreme), but you’ll run into diminishing returns because:
MG1 overspeed forces the ICE on >42 mph.
The engine’s efficiency island may not match the reduced load from aero mods.
SOC management sometimes “wastes” extra efficiency.
That’s why Gen 3 and Gen 4 Prius models show bigger MPG gains from the same aero mods — their drivetrain design is more flexible. " ( ENDQUOTE )
Thanks for your feedback !
So if I understand correctly, " freewheeling " in 'N' wouldn't mean the engine shuts off and lets me coast.
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