To entertain myself on the long drive from my Mom’s house in the Bay Area to my home in LA, I conducted a science experiment. How much more efficient was my 2003 MINI Cooper at various speeds and how much money did that translate into?
Step 1. Measure the average miles per gallon (a statistic my car tracks) at six different speeds for a distance of two miles. Repeat twice so there are three sets of data for each speed (see chart below).
Step 2. Average the miles per gallon at each speed.
Step 3. Extrapolate the cost of the trip per mile at each of the six speeds based on the average miles per gallon for that speed. I paid $2.63 a gallon when I filled up before driving home so this is the price I used. The formula looks like this:
1 mile x 1 gallon/average miles at a particular speed x $2.63/1 gallon of gas =
Driving one mile at 85 mph costs $.095.
Driving one mile at 80 mph costs $.085.
Driving one mile at 75 mph costs $.086.
Driving one mile at 70 mph costs $.075.
Driving one mile at 65 mph costs $.068.
Driving one mile at 60 mph costs $.068.
Step 3 raised a few questions, namely why the cost of driving at 75 mph was slightly higher than driving at 80. And is my car really as efficient when I drive at 65 mph as when I drive at 60 mph? More testing is needed to answer these questions.
Step 4. Determine the time it takes to drive one mile at each speed. The formula looks like this:
1 mile x 1 /miles per hour x 60 minutes / 1 hour x 60 seconds / 1 minute =
At 85 mph one miles takes 42 seconds.
At 80 mph one miles takes 45 seconds.
At 75 mph one miles takes 48 seconds.
At 70 mph one miles takes 51 seconds.
At 65 mph one miles takes 55 seconds.
At 60 mph one miles takes 60 seconds. (This is also the speed at which trucks will pass you en masse.)
Step 5. Compare gas costs with time savings.
Driving at 85 mph costs $.027 more per mile than driving at 65 mph but saves you 13 seconds.
For a 30 mile commute, this adds up to $.81 more at a savings of 6 minutes and 30 seconds.
For a 400 mile road trip, this adds up to $10.80 more in gas but cuts one hour and 27 minutes off the trip.
For a driver who puts 10,000 miles on their car each year, driving at 85 mph costs an extra $270 but saves 36 hours.
Conclusion: The cost of driving fast – at 85 mph instead of 65 mph – is roughly $.03 more per mile, or about 40 percent more. The actual difference, $.027, looks small but adds up quickly. Slow down and drive at 65 mph, which is the speed limit on California state highways (with a few exceptions), and you will save roughly 30 percent.
During the tests the terrain was mostly flat and wind was minimal. I collected data while driving but did the calculations in the safety of my non-motorized home.
Questions. What are the indirect costs of driving fast or of driving slow? Did you cause an accident or get a ticket for driving at 85? If you slowed down to 60, did other drivers honk at you? I noticed my car was so much quieter at 60!