Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Where do the MPGs go? in town/highway consumption breakdown

pfunk678 · 05-08-2012, 06:33 PM

This info taken from a Transportation Research Board study found here:
onlinepubs.trb.org/onlinepubs/sr/sr286.pdf (page 40) Thanks, tasdrouille for posting the link!

In Town driving results in a 68% loss of efficiency due to engine and drive loss. The remaining 31% of fuel goes to:
idling (17%)
braking (6%)
rolling resistance (4%)
aero resistance (3%
accessories (2%)

This 32% is the fuel consumption we have control over.* Of this 32%:

Idling (53%)
Braking (19%)
RR (13%)
Aero (9%)
Accessories (6%)

As I'm sure everyone here is well aware that significant gains in mpg can be made by reducing idling and braking in town, but what was eyepopping to me was the highway numbers from this study.

Highway driving results in 74% engine and driveline loss. The remaining 26% is broken down into:

Aero resistance (11%)
Rolling resistance (7%)
Idling (4%)
Accessories (2%)
Braking (2%)

Of the 26% we can readily control: *

Aero (42%) !
RR (27%)
Idling (15%)
Accessories (8%)
Braking (8%)

After reading this information I arrived at several conclusions:

1. Hypermilers drive in such a way that there is great focus on reducing idling & braking. In short, we get more highway-like results from in-town driving.
2. The lowest hanging fruit to harvest in highway driving (or hypermiling) is to reduce aero (it is clearly the biggest chunk)
3. Idling & Braking are driving behaviors. They are decisions you make while driving
4. RR, Accessory usage, and Aero are mostly determined by how you configure your car in your driveway. (air dams, low RR tires, removed electrical fixture, etc) but some of this is determined by driving behavior (drafting, choosing to drive in dry/wet conditions, using a/c).
5. The * earlier in the post are due to my ignorance/inability to grasp BSFC concepts. I'm sure that loss due to engine efficiency can be influenced by understanding and exploiting it. This would change how big a piece of the pie all the numbers listed above are.

My intent in posting this was to help provide a context and framework for hypermiling and to understand where the biggest gains can come from and how much gain can come from mods and how much can come from adjusting the "nut behind the wheel". I welcome your feedback/corrections on my calculations and thoughts. Thanks!

05-08-2012, 06:33 PM	#1 (permalink)
pfunk678 EcoModding Lurker Join Date: Apr 2012 Location: Georgia Posts: 14 Red Chicken - '95 Honda Civic LX 90 day: 42.61 mpg (US) Thanks: 13 Thanked 7 Times in 3 Posts	Where do the MPGs go? in town/highway consumption breakdown This info taken from a Transportation Research Board study found here: onlinepubs.trb.org/onlinepubs/sr/sr286.pdf (page 40) Thanks, tasdrouille for posting the link! In Town driving results in a 68% loss of efficiency due to engine and drive loss. The remaining 31% of fuel goes to: idling (17%) braking (6%) rolling resistance (4%) aero resistance (3% accessories (2%) This 32% is the fuel consumption we have control over.* Of this 32%: Idling (53%) Braking (19%) RR (13%) Aero (9%) Accessories (6%) As I'm sure everyone here is well aware that significant gains in mpg can be made by reducing idling and braking in town, but what was eyepopping to me was the highway numbers from this study. Highway driving results in 74% engine and driveline loss. The remaining 26% is broken down into: Aero resistance (11%) Rolling resistance (7%) Idling (4%) Accessories (2%) Braking (2%) Of the 26% we can readily control: * Aero (42%) ! RR (27%) Idling (15%) Accessories (8%) Braking (8%) After reading this information I arrived at several conclusions: 1. Hypermilers drive in such a way that there is great focus on reducing idling & braking. In short, we get more highway-like results from in-town driving. 2. The lowest hanging fruit to harvest in highway driving (or hypermiling) is to reduce aero (it is clearly the biggest chunk) 3. Idling & Braking are driving behaviors. They are decisions you make while driving 4. RR, Accessory usage, and Aero are mostly determined by how you configure your car in your driveway. (air dams, low RR tires, removed electrical fixture, etc) but some of this is determined by driving behavior (drafting, choosing to drive in dry/wet conditions, using a/c). 5. The * earlier in the post are due to my ignorance/inability to grasp BSFC concepts. I'm sure that loss due to engine efficiency can be influenced by understanding and exploiting it. This would change how big a piece of the pie all the numbers listed above are. My intent in posting this was to help provide a context and framework for hypermiling and to understand where the biggest gains can come from and how much gain can come from mods and how much can come from adjusting the "nut behind the wheel". I welcome your feedback/corrections on my calculations and thoughts. Thanks!