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My impression is that all the mass produced hybrid vehicles on the market are parallel hybrids, namely both the ICE and the electric motor are both connected to the drivetrain to drive the wheels.

But it seems to me like series hybrids would be more efficient since they eliminate the transmission ( I assume ) and could also eliminate the entire drivetrain by using in wheel electric motors.

Am I mistaken, and they're being mass produced, or it not, why?

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I don't entirely agree with the upvoted answer. I have read somewhere (but cannot locate the source right now) that the electrical power transfer pathway is 70% efficient in Toyota Prius, whereas the mechanical power transfer pathway is probably over 90% efficient. Consider this: the electrical components (inverters) have water cooling, but the mechanical power transfer components do not. This illustrates that the electrical components produce more heat than the mechanical components, and are thus less efficient.

It is true that the electrical components would need to be much bigger to have series hybrid operation. In a series-parallel hybrid, part of the power is transferred mechanically, and thus only part needs to be transferred electrically. Therefore the electrical components can be small. But this is not the only reason: efficiency is another major reason.

In fact, if you're building a plug-in hybrid electric vehicle (PHEV), you need to have big electrical components. Then you could in theory have series hybrid operation. But it would not make sense, because PHEVs are occasionally driven long distances, and then the power transfer from the gasoline engine to the wheels needs to be as efficient as possible. Efficient power transfer means mechanical power transfer.

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The short answer is power requirements. Most of the vehicles are parallel, or mild hybrids, some are series/parallel (Prius and Tahoe come to mind), and there are a few series ones (Chev Volt).

A series hybrid has to have enough electrical power to get the vehicle to top speed. This means bigger motors, even bigger batteries, and a decent size genset or fuel cell.

They will become more prevalent as the technology gets better. And as Hydrogen stations become more wide spread.

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The BMW i3 REX is basically a series hybrid. It is mostly electric. The gas engine is used to generate electricity when the main battery is below a set point. I suspect they were not common because the technology wasn't good enough yet. That will probably change with more development of electric drive.

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