The theory is if you drive slower you will get better gas mileage. For the past month I decided to test this theory for myself.
Normal driving for me is around 65mph, but this often times requires speeding up to 70 to pass people going 60 because they speed up as you are trying to pass them. This type of driving averages about 25 mpg in my Subaru.
I made a conscious effort to not drive much over 60 mph for one full tank of gas. I found it difficult to maintain that speed. This type of driving did indeed yield about 1 more mpg for 26 mpg.
I then decided to do another test, this time setting cruise control for about 62mph. This yielded the best results, giving me an average of 26.5 mpg.
How much does it save?
As to be expected, driving slower saves no time at all.
Assume a 10 gallon tank:
At 25mpg the is a cost of $0.146 per mile.
At 26mpg the is a cost of $0.140 per mile.
At 26.5mpg the is a cost of $0.137 per mile.
For a total difference of $0.009 per mile. Over a years worth of driving (14,000 miles) that equates to a savings of $126.
Worth it? You decide.
For me, it was nice just setting the cruise control and not worrying about trying to pass everyone and then get back over in the right lane so others could pass me. I will probably stick with the 62 mph cruise control setting for a while and see what happens.