Most hybrid drivers know there is no single fuel economy number for their vehicle. How’s the weather? Have you been commuting in city traffic or taking a long trip? Who’s behind the wheel? Are you looking at the dashboard fuel economy or calculating it yourself? Answers to these questions can send the resulting mpg figures way up or way down.
Car and Driver’s Patrick Bedard states it beautifully in his recent article entitled, "Lousy mileage? Is it your car, or your foot?" He writes, "Your gas mileage is you…the driver is the wild card in any discussion of car performance." Bedard ran a LexusRX400h through a battery of tests, changing driving speeds and accel/decel rates to achieve mileage ranging from 25 mpg to 49 mpg.
All of this doesn’t stop the quest for "real" mpg. HybridCars.com is no exception.
Put to the Test
Eight months ago we recruited over 300 hybrid drivers to participate in our own hybrid mileage study. We devised the test to root out as much false information as possible. To discourage boasting—of course you wouldn’t do that—the participants couldn’t see each other’s numbers. To eliminate bad math, we relied strictly on total lifetime odometer readings. We even tried to eliminate computer glitches by having some participants send in paper records to back up their data entries.
HybridCars.com worked with Claudette Juska and Charles Griffith of the Ecology Center of Ann Arbor, Mich., with some guidance from Walter McManus, auto economist (and the king of mutlivariate regression analysis) to edit out any entries that were incomplete, and to crunch the data. Our decision to be very cautious, and to choose quality instead of quantity, reduced our sample size to 111 total hybrid drivers. In some cases, that reduced the sample size for a particular type of vehicle to below five, certainly not a big enough group to provide a conclusive figure on fuel economy. But that wasn’t the point. We simply wanted to do our own study to cross-reference against other mileage data and the EPA’s own figures.
Here’s some of what we learned:
The average fuel economy for 47 drivers of 2004/2005 Priuses was 49.95 mpg, or 9.2 percent less than EPA’s combined estimates.
The average fuel economy of 34 Civic Hybrid drivers was 47.36 mpg, or 0.8 percent more than EPA’s combined estimates.
The average fuel economy of 7 Accord Hybrid drivers was 31.81mpg, or 0.6 percent less than EPA’s combined estimates.
The average fuel economy of 5 Escape Hybrid (4wd) drivers was 28.57 mpg, or 7.8% less than EPA’s combined average.
The average fuel economy of 5 Lexus RX400h drivers was 25.15, or 13.3 percent less than EPA combined average.
By asking the participants a few more questions about their driving conditions and odometer numbers, we reached a critical conclusion that hybrid drivers learn how to maximize their fuel economy over time:
Adding 10,000 miles to the odometer could increase the ratio by 3.0%
The data also indicates:
Using the AC or heat could decrease the ratio of actual to expected fuel economy by nearly 3.5%
Driving on hilly terrain could increase the ratio by 3.2% or more.
It should be noted that ratio of actual to expected fuel economy translates into a different number of miles per gallon for each vehicle type, i.e. a ratio change of 3% translates into 1 mile per gallon on the Escape Hybrid 2WD and 2 miles per gallon on the 2004 manual Insight.
We recognize that this analysis only provides a first look at the relationship between driving conditions and fuel economy, and that the large and unavoidable variability in driving behaviors and driving conditions presents a huge challenge in determining actual fuel economy figures.
Our next step is to open the mileage tracking tools to a much wider sample of hybrid drivers. Mileage tracking is merely one facet of HybridCars.com’s big push to provide a full range of tools for drivers to document their experiences and share it with others in the hybrid community. Soon, we will provide more details and instructions for how you can create your own hybrid blog, upload photos, track mileage, and participate more fully in an online community of hybrid drivers.
These tools will be fun to use, provide an opportunity to let others know about what you drive and why, and will allow HybridCars.com to expand its efforts to come closer to answering the unanswerable question: How many miles to the gallon do you really get?