Why The 2013 Toyota Prius C EPA Combined Ratings Are No Higher Than The Prius III

Having immersed myself in nineteen fifties-tech frames, I need a breath of fresh air: Like the new 2013 Prius C. OK, not exactly a CC (only half-way there), but the problem is, I’ve yet to read something that puts it fully in its proper context. Way too many articles and comments bitching about why its EPA numbers 53/46/50 (city/highway/combined) end up with the same combined rating as the larger, more powerful and heavier regular Prius (51/50/50), and not enough explanation.

First of all, the Prius C wasn’t designed specifically to beat the Prius III in fuel economy. It’s part of Toyota’s oft-stated goal to offer hybrids in every class of vehicle they sell. The HSD system in this Prius C is also going into the European Yaris hybrid. The design goals for this system were to most of all be cheaper to build, and weigh less. All the key components have been slimmed down, including the electric motor/generators (MG 1 & MG2). And the battery has been substantially reduced in size, weight, and voltage (144 V vs. 273.6 V). That alone may account for the single biggest part of the equation.

The battery plays a critical role in the Prius, because it both stores converted kinetic energy from braking as well as buffering the small 74hp 1.5 L Atkinson cycle gas engine. Toyota has only revealed the voltage, not the actual capacity. But given Toyota’s tendency to use certain size cells in their NiMh batteries, this downscaled battery may possibly only have as little as half the capacity of the Prius III battery.

A smaller battery means more frequent use of the gas engine, and possibly in rpm/load ranges that make it less efficient. This is Prius Lite, or HSD Lite. Undoubtedly, the Prius C’s almost 500 lb weight advantage is what gives it the slightly higher city EPA number (53) than the Prius III (51).

The Prius C’s lower highway mileage is undoubtedly a combination of aerodynamics, as well as the reduced battery buffering (to a smaller extent).  The C’s Cd is 0.28; the Prius II’s is 0.25. We don’t know yet what the frontal area of the C is, which will somewhat mitigate that higher Cd number (total aerodynamic drag is the Cd x FA (frontal area)).

The Prius C is almost twenty inches shorter than the the Prius III, and as we know from these two stubby Falcons trying to turn themselves into one long Foord (which added 500 rpm to their top speed), a longer body almost invariably has better aerodynamics.

The Prius C’s role is to expand the Prius brand downward, given its sub $19k price, just like the Prius V extends it into the realm of a station wagon/MPV. And the plug-in Prius will be the banner-wearer for ultimate mileage numbers.

Toyota’s greatest achievement with the Prius C is in its continued cost reduction of its HSD components, not in shoehorning in the battery under the seat. A decently equipped 50 mpg Prius starting at $19k is the breakthrough.

Anyway, when one gets in the questioning a couple of mpgs one way or another at around the 50 mpg point, it really becomes largely irrelevant. It’s really an artifact of our system of using miles per gallon, which makes it seem that every additional mpg is the same degree of improvement as the one before it. Not so. The European convention of Liters per 100km much more accurately shows the actual degree of improvement in fuel use. Improving mileage from 40 mpg (5.88L) to 50 mpg (4.70L) results in a saving of 1.15 liters; going from 50 to 60 mpg (3.92 L) results in savings of .82 liters. Especially with American fuel prices, these differences are mostly useful for forum debating points. But that’s ok too.