Well, it may help the environment, but does it really save you any money? Charts of MPG vs Speed are tough to come by, but here’s one from 1997;
Now, I know things may have shifted, but I doubt the curve has steepened much from here. At 55 MPH, we are looking at 32 miles per gallon. At 65 MPH that drops to about 27. The math from here should be simple. At 55 MPH you would go 55 miles (in exactly an hour of course) and use 1.57 gallons of gas. At 65 MPH, you go 55 miles in just under 51 minutes, and use 2.04 gallons of gas. So you’ve traded 9 minutes for .47 gallons of gas saved. With gas about $2.75 per gallon, that’s $1.28 saved. This computes to $8.56 per hour. There’s often a trade off between time and money. Of course driving an hour round trip to save $1 on a grocery item doesn’t make sense, but these are the choices people sometimes make without much thought. If your highway miles are minimal, no big deal, but the difference between 55 and 70 when one drives 12,000 highway miles per year is 47 hours. I’m sure we’d all like to grab that time back just when vacation time has run out.