Hi folks,
Recently, a Honda advocate suggested that Toyota engineering designed the Prius just for good EPA mileage but in fact it delivers worse performance than the equivalent Honda Civic. As we were discussing the inflamatory nature of a "cheating the EPA" claim, I realized there was some information about the Prius that the Honda advocates might not be aware of. By discussing this from the Prius side, there is a chance for enlightenment not only for our Honda advocates but also Prius pilots who may be disappointed in their MPG performance.
Perception: Honda Civics get EPA numbers and Prius Dont
The Honda advocate believes that Honda Civic owners have a higher ratio of those getting EPA numbers than the Prius owners. His source, is the GreenHybrid.com database. So let's check the numbers.
We have two generations of Honda Civic Hybrids and two generations of Prius with CVT transmissions. Starting with the gross numbers:
Median mpg (US) Mean mpg (US) Middle 50% mpg (US) Standard Deviation Standard Error Cars in Sample
Honda Civic Hybrid I 2WD CVT (48/47 EPA)
46.0 45.4 42.4–48.9 5.2 0.3 424
Toyota Prius I (THS) 2WD eCVT (52/45 EPA)
45.0 45.5 43.1–47.6 3.5 0.5 55
Clearly the HCH I out performs the Prius I, 46 to 45 MPG. Yet the larger standard deviation suggests HCH I drivers are getting a larger variation or spread in performance. This may be due to the smaller sample set of Prius I. But the situation changes with the HCH II and Prius II.
Honda Civic Hybrid II 2WD CVT (49/51 EPA)
46.9 46.4 43.7–50.0 4.8 0.3 226
Toyota Prius II (HSD) 2WD eCVT (60/51 EPA)
47.7 47.7 45.3–50.4 4.4 0.2 683
The Prius II has a clear advantage, 47.7 to 46.9. With both vehicles having respectable sample sets, the Prius II has a somewhat smaller standard deviation suggesting slightly more consistent performance. But these numbers only apply if there is a normal distribution.
So this is the HCH II distribution:
Here is the Prius II distribution:
These charts suggest a lot of HCH II drivers are achieving the median and lower values and a few stars are achieving excellent results. The situation is reversed with the Prius II, many are at and above the median with a faster fall-off on the low side. Simular distribution curves exists for the HCH I and the Prius I.
So how do they compare to the EPA numbers?
Prius I - median and mean equals the lowest EPA number, 45 vs 45 mpg
HCH I - median and mean are below lowest EPA numbers, 46 vs 47 mpg
Prius II - median and mean are below lowest EPA numbers, 47.7 vs 51 mpg
HCH II - median and mean are below lowest EPA numbers, 46.9 vs 49 mpg
So what can we now assert:
- Prius I - the only hybrid to reach even the lowest EPA numbers
- Prius II - has the largest deviation, ~3 mpg, from lowest EPA numbers
- HCH I, HCH II and Prius II - all achieved lower than the lowest EPA numbers
- HCH I outperformed the Prius I
- Prius II outperformed the HCH II
Does this fairly state the facts? Later today, I'll try to edit the EPA expected values and differences. I believe this is calculated 40/60 highway vs. city, unless someone elses posts the values.
Bob Wilson
ps. Later, I'll offer a description about how the Prius architecture works versus the EPA and normal driving. Hopefully this will show there was no "cheating" or "designing for the test" but reveal insights that can be used to improve Prius MPG and meet the EPA numbers.