Geek Challenge: Summer Road Trip Riddle

July is the heart of summer road trip season.  And with gas at about $4 per gallon, that trip to the national park can be pretty expensive.  So, this month’s geek challenge could help you optimize your trip.

Everybody knows that at highway speeds the faster you drive, the lower your fuel mileage.  Also, we know that driving faster also saves time getting to your destination.  The question is how many dollars in fuel cost does each hour of time saved cost?

Published data on the fuel mileage of cars at various speeds is exceedingly scarce.  So, to help answer the question, I collected some of my own data.  I happened to be driving from Detroit to Chicago on a perfectly calm day with moderate temperatures.  The light traffic and higher Michigan speed limits allowed me to set the cruise control at a variety of speeds for significant stretches, while using the car’s computer to track mileage and elevation changes.  I derived the following fuel consumption curve for my Mercury Milan Hybrid:
 

Given this fuel consumption curve, how much do I save per hour by driving to my road trip destination at 65 MPH instead of 75 MPH?  Is it:
A. $6 per hour
B. $9 per hour
C. $11 per hour
D. $15 per hour

Extra Credit:  Collect data on your own vehicle and calculate the incremental cost of driving faster with your car.

The correct answer with the best engineering content will be this month’s winner. Submit your responses to: geekchallenge@dmcinfo.com