Automotive

Why doesn't my car get the mileage that it's claimed to get??

This is a constant refrain which I have heard from friends and neighbors. Last night a car buddy who's being "forced to buy a new car" (because Audi is taking back his diesel) complained that he couldn't understand how fuel economy is calculated on hybrids versus conventional internal combustion engines (ICEs). So I had to go off to do some research. I thought it might interest a few people here.

NOTE - this is specific to the US market, but similar procedures have been developed for most regions. Note that we are talking ENERGY CONSUMPTION here, not CO2 or emissions or anything else.

In the US market the manufacturer does the testing and the EPA does follow-up checking. All these runs are done indoors, on a dynamometer. Before a vehicle hits the dyno, it is run up to 80 mph on a level roadway and allowed to coast to a stop. The distance traveled and the decay of its speed indicate the wind and tire and driveline drag. This is done a few times, indoors and out, on road and dyno. Then the dyno is set using a formula to reflect the car's drag.

Here's a photo my wife took last week on her way back from LA. This guy is well-known as having driven his Honda Insight more miles than anyone else. He does radical drafting behind trucks and drives coast-to-coast multiple times each year.

CITY

The first run is for 30 minutes at an average of 21 mph and is designed to simulate city driving. It involves cold and hot starts but no 3-minute-long stop lights like I experience on my way to the highway. As mentioned earlier, the dyno is adjusted for the weight and drag of each vehicle.

HIGHWAY

The highway test runs for 12 minutes at an average of 48 mph and top speed of 60 mph (that's roughly 100 klicks for you metric guys).

HIGH SPEED

Because no one drives like the old highway test, the EPA added this additional 10 minute test also averaging 48 mph but hitting 80 (130 k) for a few seconds. This still does not replicate the typical PuristS driving mode!

There are two additional tests which reflect A/C off and on, and extreme cold weather operation. They don't have to be performed unless they make a notable difference in the vehicle's operation.

A Note on SAE & ISO

SAE (Society of Automotive Engineers) is an international association of engineers from all companies who come together to hash out the best ways to design or test components and systems. They work together with ISO (International Standards Organization) on certain projects involving transportation engineering. The EPA endorses one of their specs, and all manufacturers then can follow the same process and the industry itself keeps things up-to-date. I was intimately involved in the creation of SAE J2008 (service information), and worked on an ISO committee to design the standard symbols on your dashboard (somebody has to do this boring stuff!).

Here are some well-known SAE standards - they all begin with the letter J (you can search on the web using this number):

J1962 - Diagnostic Connector. Standardizes the shape and number of pins in a vehicle's Diagnostic Link Connector (DLC), and the function of seven of the 16 pins. Manufacturers may use the remaining pins as they wish.

J1978 - OBD II Scan Tool. This standard establishes specifications for standard generic OBD II-compliant scan tools. Manufacturers may create their own enhanced tools for dealers, but must support the generic one.

J1979 - E/E Diagnostic Test Modes. Standardizes data messages for accessing Diagnostic Trouble Codes (DTCs), freeze frames and data stream.

J2008 - Organization of Vehicle Service Information. A handbook for manufacturers to develop service & diagnostic information and products. It is frequently revised to keep up with changes in technology.

J2012 - Diagnostic Trouble Code Definitions. Defines and standardizes the OBD II alphanumeric Diagnostic Trouble Code numbering system.

END OF NOTE

The emergence of alternative fuels and hybrids have complicated things greatly. This table shows you the type of electric and hybrids we now see, and in the right column is the SAE standard procedure used for its testing.

So once you figure out which type of car you have, then you have to calculate a realistic test procedure. A different software strategy will be used by each manufacturer for operating their vehicle and thus a single test really doesn't equate across brands and technologies. You can see below that there are different ways that the engine and electric power can be shared under different conditions.

WARNING - EPA STUFF FOLLOWS

Electric Vehicle Test Procedure - In general, EPA testing follows SAE J1634 from 1993, "Electric Vehicle Energy Consumption and Range Test Procedure" or SAE J1634 of 2012, "Battery Electric Vehicle Energy Consumption and Range Test Procedure." These test procedures are dynamometer runs.

Electric Vehicle - Single-Cycle City Test Procedure Summary - The battery is fully charged, the vehicle is parked overnight, and the next day the vehicle is driven successive city cycles until the battery is discharged. The battery is recharged from a normal AC source using the manufacturer’s charger. The energy consumption of the vehicle is determined (in kW-hr/mile or kW-hr/100 miles) by dividing the kilowatt-hours of energy to recharge the battery by the miles traveled. To calculate energy consumption in units of mpge (miles/gallon equivalent) EPA uses a conversion factor of 33.705 kilowatt-hours of electricity per gallon of gasoline (basically a measure of the energy in gasoline (in BTUs) converted to electricity).

Electric Vehicle – Single-Cycle Highway Test Procedure Summary - The same test procedure is used to determine the highway energy consumption and the highway driving range, except the vehicle is operated over highway cycles.

Electric Vehicle – Multi-Cycle City/Highway Test Procedure Summary – The battery is fully charged, the vehicle is parked overnight, and the next day the vehicle driven over successive city, highway and steady-state cycles until the battery becomes discharged. DC discharge energy and DC discharge amp-hrs are measured for the entire test. The battery is then recharged from a normal AC source using the manufacturer’s charger. The energy consumption of the city and highway cycles (in kW-hr/mile or kW-hr/100 miles) are determined mathematically from the recharging energy, the DC discharge data and distance for each cycle.

Electric Vehicle - Adjustment Procedure used to Derive Fuel Equivalent Estimates - EPA regulations require fuel economy, energy consumption, CO2 and driving range values listed on the Fuel Economy window sticker) to be adjusted to more accurately reflect the values that customers can expect to achieve in the real world. EPA currently allows fuel economy, energy consumption, CO2 values, and range values for electric vehicles to be adjusted using one of the following methods:

• a. multiplying city/highway fuel economy and range values by 0.7 and dividing city/highway energy consumption and CO2 values by 0.7;
• b. using a derived 5-cycle method;

• c. using a method equivalent to the vehicle specific 5-cycle method;

• d. using adjustment factors which are based on in-use data (with prior EPA approval). Most EVs use the first (the 0.7 factor).

Plug-in Hybrid Vehicle (PHEV) Test Procedure - EPA testing follows SAE J1711, June 2010, "Measuring the Exhaust Emissions and Fuel Economy of Hybrid-Electric Vehicles, Including Plug-in Hybrid Vehicles," with some exceptions.

PHEV - Charge-Depleting Operation - For the electric portion of PHEV operation (charge-depleting mode of operation), EPA testing follows SAE J1711, which is essentially the same single-cycle test procedure as used for electric vehicles. The charge depleting test for PHEVs starts with a fully charged battery and ends when the battery is discharged. If the gasoline engine operates during the charge depleting mode, both the electric energy consumption and the gasoline consumption are used to calculate the mpge values for the charge-depleting (mostly electric) operation.

PHEV - Charge-Sustaining Operation - For the gasoline portion of PHEV operation (charge- sustaining mode of operation), EPA tests the vehicle similar to any other conventional hybrid vehicle---using either the derived 5-cycle (city/highway) method or the vehicle specific 5-cycle method. The charge-sustaining test for PHEVs starts with a discharged battery.

PHEV - Adjustment Procedure used to Derive Fuel Equivalent Estimates - EPA requires city and highway energy consumption and driving range values listed on the FE Label (window sticker) to be adjusted to more accurately reflect the energy consumption and driving range that customers can expect to achieve in the real world. For PHEVs operating in the charge-depleting mode, one of the adjustment procedures is used as described (above) for electric vehicles. For PHEVs operating in the charge-sustaining mode, the adjustment procedure is similar to any conventional vehicle, and dynamometer test values are adjusted in the same way.

END OF EPA NONSENSE

My wife ran away from these guys as she couldn't stand their slow pace. I guess that's why we only get an average of 53.5 mpg with our Honda Insight (that's cumulative over 105,000 miles).

Cazalea

Why doesn't my car get the mileage that it's claimed to get??

Why doesn't my car get the mileage that it's claimed to get??

Been looking for a more "efficient" way of travel...

My 2012 E250 did its 3 year lease and 25k mi p.a. like clockwork [nt]

Good to know!

There are too many low-cost-per-mile choices!

That's a good list! [nt]

I get better than claimed, Mike . . .

Running the A/C is hardly considered “a load” even for a Honda