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For even more fun, try calculating how much additional time you will spend during a month of commuting at 55mph compared to 70mph. Then take your gas savings and divide by those extra hours to find out how much your free time is worth.

At 1,000 of freeway driving, people will spend an extra 3.5 hours per month driving if they limit themselves to 55mph rather than keeping up with traffic at 70mph. And they will save 5 gallons of gas by getting 25mpg instead of 22.5mpg at the higher speed.

So they are effectively pricing their free time at (5 gal * $3.50 per gal / 3.5 hours) = $5 per hour.

The higher-mileage vehicle means even less money saved due to driving slower, so your time becomes worth even less. Driving a Prius slowly might mean giving away your free time at a rate of $3 an hour.

You have a valid point when comparing 55mph to 70mph or greater. I’m not going to drive 55 unless traffic, road conditions or speed traps demand it. But I find it interesting that one car in a balance of speed/mileage may get the optimum mpg at 67mph or 73mph or whatever.

I used to have a rule of thumb that I could set the cruise and drive 9mph over the limit with impunity as long as I was not standing out in the crowd, i.e., passing everything in sight. Now I tend to judge more by the traffic flow than the speedometer.