Thursday, May 08, 2008


Should People Slow Down? Maybe, An Envelope Calculation

31556926 Seconds/Year (Google search)
303824646 Population of US (2007 est, cia world factbook)
$13860000000000 Annual GDP of US (2007 est, cia world factbook)

(13860000000000 $/year) / (303824646 people*31556926 seconds/year) = .0014456 $/person-second [That's $4.97 an hour]

30 miles/gallon at 55mph (source, est from graphic)
23 miles/gallon at 75mph (source, est from graphic)
25 miles/gallon at 70mph (source, est from graphic)
28 miles/gallon at 65mph (source, est from graphic)
3.62 $/gallon May 4th US average (source)
1.63 person/vehicle (source)
3600 second/hour

(3.62 $/gallon * 55 mi/hour) / (30 miles/gallon * 3600 second/hour * 1.63 people) = .001131 $/person-second

(3.62 $/gallon * 75 mi/hour) / (23 miles/gallon * 3600 second/hour * 1.63 people) = .002012 $/person-second

(3.62 $/gallon * 70 mi/hour) / (25 miles/gallon * 3600 second/hour * 1.63 people) = .001727 $/person-second

(3.62 $/gallon * 65 mi/hour) / (28 miles/gallon * 3600 second/hour * 1.63 people) = .001432 $/person-second

Of course, this assumes that your daughter and grandmother are as likely to be the 1.63 occupants as you and your wife.

(Also, I don't like that consumption curve. That 55mph tripe is based on old vehicles, and may also include big trucks. I know my 2002 Mazda protégé is at 3400RPM at 75mph, still in the flat part of the torque curve.)

It also doesn't cosider the unpaid value of peoples' time. I know I value my free time at least as much I'm paid for work (obviously, I'd work much more if I didn't, especially since I get time and a half).

At my pay, with just me in the car, and using that ridiculous consumption curve; at 75mph gas would need to cost $9.18 a gallon to wash out the cost of my time.
Using estimated after-tax $s (25%), gas would need to cost $6.88.
The author needs to take some remedial math classes!

That 15% figure he bandied about applies to the decease in speed, and not a decrease in gas mileage!

(55mph/65mph) x 100 = 84.6% or 100% - 84.6% = 15.4% decrease in MPH.

The decrease in MPG is actually less than 7%!

(28mpg/30mpg) x 100 = 93.3% or 100% - 93.3% = 6.7% decrease in MPG.

Now, let's translate the costs in gasoline of going 65 over 55 versus cost-benefits of going 65 over 55.

On a 325 mile trip, at 55 MPH, you'd be sitting in your car for almost six hours.

At 65 MPH, the trip would take five hours flat.

In other words, you'd spend an about a whole hour less.

To me, that's HUGE!

OK, what about relative gas costs?

At 55MPH @ 30MPG, you'd use 10.83 gallons of gas.

At 65MPH @ 28MPG, you'd use 11.61 gallons of gas.

That's a difference of .77 gallons, at $4.00/gal, it comes out to an additional $3.11.

SO, let's recap.

For the measy cost of three bucks, you get to spend a whole hour less behind the wheel.

Sounds like a deal to me.
Yup, what a great deal!

What is the 15% you are referring to though? Is that from the freakonomics thread?
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