Aero Drag and EPA mpg

[mort]

Originally Posted by jakobnev

Not even a hint of suspicion that the aerodynamic properties of the car are used to determine the resistance of the dyno rollers? (among other things like mass)

I think there is some misunderstanding coming from where this thread started. Gino Sovran of General Motors wrote a report showing how reducing aerodynamic drag would affect mpg figures obtained from the EPA testing procedures for a given car. The report makes the point that these changes would not be applicable to "real word" mpg. YMMV is all he meant.

The EPA tests are done on a programmable dynamometer. In order to have a dynamometer simulate the total road load of a vehicle, the dynamometer power absorber must be adjusted to reflect the road load characteristics of the vehicle. Most certification vehicles are tested using dynamometer power absorption values obtained according to the methods in the "EPA Recommended Practice for Determination of Vehicle Road Load." In this method the basic concept is to perform a series of road or track coastdowns with the vehicle. The coastdown measures the rolling resistance and aerodynamic drag in terms of varying deceleration. Coastdowns are then performed on the dynamometer at different power absorber settings, and the dynamometer power absorber adjustment is determined when the vehicle dynamometer coastdown time matches that of the road coastdown in a speed range of 55 to 45 mph. Since 1986 manufactures are allowed to use alternative methods to calculate road load, like using wind tunnel results to estimate aerodynamic drag (see here).

Aerodynamic drag is included in the dynamometer load. The procedure is far from ideal, and modern equipment could give more accurate results, yet the EPA mpg figures are very close to real world numbers for most cars.

-mort

[ps2fixer]

Originally Posted by mort

I think there is some misunderstanding coming from where this thread started. Gino Sovran of General Motors wrote a report showing how reducing aerodynamic drag would affect mpg figures obtained from the EPA testing procedures for a given car. The report makes the point that these changes would not be applicable to "real word" mpg. YMMV is all he meant.

The EPA tests are done on a programmable dynamometer. In order to have a dynamometer simulate the total road load of a vehicle, the dynamometer power absorber must be adjusted to reflect the road load characteristics of the vehicle. Most certification vehicles are tested using dynamometer power absorption values obtained according to the methods in the "EPA Recommended Practice for Determination of Vehicle Road Load." In this method the basic concept is to perform a series of road or track coastdowns with the vehicle. The coastdown measures the rolling resistance and aerodynamic drag in terms of varying deceleration. Coastdowns are then performed on the dynamometer at different power absorber settings, and the dynamometer power absorber adjustment is determined when the vehicle dynamometer coastdown time matches that of the road coastdown in a speed range of 55 to 45 mph. Since 1986 manufactures are allowed to use alternative methods to calculate road load, like using wind tunnel results to estimate aerodynamic drag (see here).

Aerodynamic drag is included in the dynamometer load. The procedure is far from ideal, and modern equipment could give more accurate results, yet the EPA mpg figures are very close to real world numbers for most cars.

-mort

Seems they are not so good with Toyotas, my Camry is 22mpg highway, back road country I averaged 27mph driving "normal" at 60mph before ecomodder. My corolla is something like 26mpg highway, and I pulled 33mpg on the highway going 75mph, alignment WAY off, bad tires/struts, etc. I get 40-45mpg with very little mods and slowing down now.

I can't seem to understand why the numbers are so way off. For example, the Camry, I went 70-75mph with a trailer behind it and got 22mpg and the car has 300k miles. Acording to the EPA, i should have gotten under 19mpg and 22 with out the trailer at all. Back then I was just a typical driver, didn't even think about saving fuel besides getting a smaller car for 30mpg+.

[Vman455]

Originally Posted by ps2fixer

Seems they are not so good with Toyotas, my Camry is 22mpg highway, back road country I averaged 27mph driving "normal" at 60mph before ecomodder. My corolla is something like 26mpg highway, and I pulled 33mpg on the highway going 75mph, alignment WAY off, bad tires/struts, etc. I get 40-45mpg with very little mods and slowing down now.

I can't seem to understand why the numbers are so way off. For example, the Camry, I went 70-75mph with a trailer behind it and got 22mpg and the car has 300k miles. Acording to the EPA, i should have gotten under 19mpg and 22 with out the trailer at all. Back then I was just a typical driver, didn't even think about saving fuel besides getting a smaller car for 30mpg+.

The EPA rating for your Corolla is 23 city/31 highway, and the Camry 18 city/25 highway. And these are the revised numbers; when the cars were new the Camry was rated 21/27 and the Corolla 27/34. Your observed economy is well within those ranges, and similar to what I got out of my Civic before I started ecomodding (revised EPA 34 highway, original 37, and I got 36-37 driving normally i.e. wastefully).

[ps2fixer]

Your correct on the numbers, I didn't know they were revised, the old figures make sense since I counted myself an "easy driver" as in easy take offs and coasting. Now I coast at least 4-5 times longer of a distance lol.

[aerohead]

Originally Posted by Sven7

Didn't they significantly change test procedures since 1972?

Early dynamometers could not withstand acceleration rates in excess of 3.3 mph/second without tire slippage on the rollers.
Improvements with modern dynos have allowed the EPA Mobile Sources folks to test at higher acceleration rates and velocities.
In the September,2009,CAR and DRIVER's "THE TRUTH A8OUT EPA NUMB3RS (sic),by Dave Vanderwerp,they go into the US06 high-speed (80 mph max)cycle;the SC03,or "A/C," cycle,and cold FTP test,all of which are attempts to generate mpg estimates which better reflect the 'real world.'