Speed. What is it Good for?
"...Good God, y'all!" (with apologies to Norman Whitfield, Barrett Strong and Edwin Starr - "War, What is it Good For")
This Christmas (December 23) my wife Tricia, daughter Margaret and I headed from Hingham, MA to join our son and his family in Towson, MD.
Sharing the driving as we were, we each ran the Waze application on our iPhones — a profligate overuse of cell data, I know!
At about the East Windsor, NJ point on the New Jersey Turnpike we felt a bit pressured to get to Towson, MD (150 miles further) before it got too late and we checked Waze. Tricia’s Waze said we’d arrive at 8:30 PM. My Waze said we’d arrive at 8:55 PM and Margaret’s, incredibly, said we’d arrive at 8:08 PM! Tricia quickly checked Margaret’s route; It turned out Margaret’s Waze route duplicated her own, and mine. What up?
We figured out that Waze calibrates travel times based on the user’s history of driving speeds. Tricia always drives faster (~70 mph) then I do (~55 mph) and Margaret is always bumping up against 80 mph. No wonder Waze says she arrives 20 minutes before Tricia and 40 minutes before I do.
Here’s a view of the flat, straight, wide and open New Jersey Turnpike traveling south from East Windsor, NJ:
That got me wondering how much more petroleum and carbon emissions would Margaret’s 40 minutes of “saved” time actually cost over my so-called ‘slow’ driving?
I found that contributors to the Ecomodder Forum had developed and refined an “Aerodynamic & rolling resistance, power & MPG calculator” that could help me come up with an answer. In their words describing this model, “[the] online tool .. asks you to enter or accept default values for:
- Vehicle weight
- Crr (coefficient of rolling resistance - .008 represents a low rolling resistance tire on a smooth surface - see Wikipedia for other sample values)
- Cd [drag coefficient]
- Frontal Area
- Fuel energy density (in Watt hours / US gallon)
- Engine efficiency
- Air density (rho)
... and it spits out a table showing resistance force values, estimated power requirements and fuel consumption across a range of speeds.”1
Great. That could work for my calculations. I looked up the particular values for our 2009 Prius e.g., drag coefficient equals 0.26 -- very low! a 2014 Corvette's drag is worse at 0.29 --, frontal area equals 23.9 square feet, low rolling resistance tires rate a coefficient of .008, etc.2
With these variables entered the model calculated efficiency and wind resistance and more for speeds from 5 mph and up. Note that the model assumes flat terrain and no slowing for traffic and all other such real world effects, but it works well to understand one’s basic efficiency at speed. Here’s what the results looked like, with a special callout for my speed efficiency at 55 mph and Margaret’s at 80 mph:
This chart shows that both Margaret, at 80 mph, and I, at 55 mph, spent the same energy, 115 watts3, to overcome rolling resistance. But at 80 mph Margaret needed to spend over double (452 watts) the force I used (214 watts) to overcome the increased wind resistance. This brought her miles per gallon down to 28 mpg, 42 % below mine at 48 mpg.
Margaret, at 80 mph, would cover the 150 miles to Towson in about an hour and 50 minutes while it would take me, at 55 mph, 2 hours and 40 minutes. However, to save those 50 minutes Margaret, speeding at 80 mph, burns 2.28 more gallons and adds 44 more pounds of carbon to the atmosphere than I would have at 55 mph.4
And the car burned all of that extra energy just to push air! faster. As Will Rogers once said, “...and FOR WHAT? Why, just to get somewhere a little quicker, that is if you get there at all.”5 Kind of a waste. But going further, if I had made the drive at 40 or even 25 mph I would have saved that much more. In fact if I had driven that 150 miles at 25 mph, I would have used only 1.5 gallons, and cruised at 98 mpg. It would have taken me 6 hours to get there, just 4 more than Margaret's 2 hour / 80 mph trip. Maybe we just need to "slow down to the speed of life."
Morals of the Story
Slow down and save life on the planet
Slower speeds reduce noise, pollution, crash fatalities, road wear, fine particulates from tires and components and just make life more possible
___________________________________
Notes:
3 ways to calculate power output. (2010, July 30). wikiHow. Retrieved December 30, 2022, from https://www.wikihow.com/Calculate-Power-Output
Beginner's guide to aerodynamics: Drag equation - Level -1 - Answers. (n.d.). NASA Glenn Research Center. https://www.grc.nasa.gov/www/k-12/BGP/Sue/drag_equation_ans.htm
Drag coefficient. (2022, December 4). Wikipedia, the free encyclopedia. Retrieved December 25, 2022, from https://en.wikipedia.org/wiki/Drag_coefficient#Automobile
Online tool: Shows aerodynamic drag, rolling resistance, power required & est. MPG. (n.d.). Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com. Retrieved January 1, 2023, from https://ecomodder.com/forum/showthread.php/online-tool-shows-aerodynamic-drag-rolling-resistance-power-6341.html
Wallis, B. (2022, July 18). Seven reasons Einstein would support 20mph. Bristol Health Partners. Retrieved January 1, 2023, from https://www.bristolhealthpartners.org.uk/news/seven-reasons-einstein-would-support-20mph/
Will Rogers 0n Auto Fatalities & Progress - Syndicated Column, April 4, 1926. (n.d.). America in Class: Resources for history & literature teachers. Retrieved January 1, 2023, from https://americainclass.org/sources/becomingmodern/machine/text3/rogers1926column.pdf
Online tool: Shows aerodynamic drag, rolling resistance, power required & est. MPG. (n.d.). Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com. Retrieved January 1, 2023, from https://ecomodder.com/forum/showthread.php/online-tool-shows-aerodynamic-drag-rolling-resistance-power-6341.html
Model entries for 2009 Prius with passengers:
A Watt is the metric unit of power vs. the traditional English units or in metric units. 115 Watts equals 0.15 Horsepower. One kilowatt is approximately equal to 1.34 horsepower.
The metric unit for work is a joule. A joule is equal to one Newton-meter, which is the amount of work needed to move a 1 Newton force a distance of 1 meter. When you divide work by time, you get power, measured in units of joules per second. This is also called a Watt.
The English unit for work is the “foot-pound.” This comes from multiplying the distance measured in feet by the force measured in pounds. The unit for power comes from dividing by time, in seconds. The power unit is the “foot-pound per second.” A more familiar term is the “horsepower,” which is defined as 550 foot-pounds per second.
From 3 ways to calculate power output. (2010, July 30). wikiHow. Retrieved December 30, 2022, from https://www.wikihow.com/Calculate-Power-Output
“Lower fuel consumption is not directly proportional to reduced exhaust emissions due to the complexities of ICEs, however, an in-depth study in London found 20mph did reduce vehicle emissions, again mainly because the lower speed is more conducive to less erratic, smoother driving which reduces exhaust emissions.”
Wallis, B. (2022, July 18). Seven reasons Einstein would support 20mph. Bristol Health Partners. Retrieved January 1, 2023, from https://www.bristolhealthpartners.org.uk/news/seven-reasons-einstein-would-support-20mph/
Will Rogers 0n Auto Fatalities & Progress - Syndicated Column, April 4, 1926. (n.d.). America in Class: Resources for history & literature teachers. Retrieved January 1, 2023, from https://americainclass.org/sources/becomingmodern/machine/text3/rogers1926column.pdf