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Ask the Experts!

2 July 2008

“I am trying to decide if a carpooling situation that is available to me is really going to save energy/gas $. If I drive to work on my own it takes me about 25 minutes, 16.5 miles, mostly freeway time at 60 MPH. If I carpool – I drive about 25 minutes to the person’s house, 6.5 miles, mostly side streets/stop lights at 30 MPH. We then drive another 5.5 miles together, less than 10 minutes, mostly highway at 60 MPH. The car I drive is a Saturn SL1 – gets about 30-34 miles to the gallon. With the carpooling I am driving less miles, but spending more time on the road – just wondering if the energy savings is significant.” – Heather

What did our experts say?

The Saturn SL1s with automatic transmission had EPA estimated mileage of 29 miles per gallon for city driving and 35 miles per gallon for highway driving.  From the information you gave us, if you carpool your gas use (one way) would be: (6.5 miles/29 miles per gallon ) +(5.5 miles/35 miles per gallon) = 0.38 gallons

When you drive by yourself on the highway your gas use (one way) would be: (16.5 miles/35 miles per gallon) = 0.47 gallons.

Besides that, your friend has to drive to work separately, too.  Assuming that person has a similar car their gas use one way would be: 5.5/35 = 0.16 gallons

So going separately you use 0.47+0.16 = 0.62 gallons each way and going together you use 0.38 gallons.  You reduce your gas use by almost 40% ((0.62-0.38)/0.62).  Of course this is based on the EPA estimated mileage, so it’s not exact, but it’s certainly close enough for decision-making purposes.  If you carpool every day it should save you and your friend combined about 2.4 gal/week or about 115 gallons/year depending on how many vacation days and holidays you have.

Answer provided by Martha Hewitt, Director of Research at the Center for Energy and Environment

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