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This keeps coming up that a series hybrid will get amazing mileage.
If you're going to use an engine to keep a battery topped up then it just ends up being an inefficient electric car and get the same or worse mileage than a parallel hybrid or even a small engined conventional drivetrain because of the additional conversion losses of converting fuel energy to electricity, storing it in a battery and then using that to drive an electric motor instead of having the engine drive the wheels directly. And unless you have a large battery and start every trip with a fully charged battery you could end up with an empty battery (for example spirited driving around town and then start heading into the mountains). Then you'd only have a fraction of that 37hp engine's output available at the wheels because of the conversion efficiencies in that complex drivetrain.
Take the 2017 eGolf, an engine with an optimistic 50% efficiency and a 90% efficient generator.
City energy usage is 126mpge or 26.75kWh/100 mile.
City energy usage. 267.5 Wh/mile
Generator to battery efficiency 0.9 none
Engine output power required 297.2 Wh/mile
Engine thermal efficiency 0.5 none
Engine input power 594.4 Wh/mile
Fuel energy 33560 Wh/gallon
Mileage 56.46 mile/gallon
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