One of the problems of maintaining a maximum speed of 55 m.p.h. is that it seems to waste a lot of time. Of course, use of the mobile phone, listening to books on tape, etc. can make this time less unproductive than it might seem at first. But it's a constant challenge to find ways to be productive during commutes, whether on a professional basis (phone conversations with business associates, etc.) or a personal basis (books on tape).
But what about the time itself? How much is actually lost? I drive about 60 miles per day on average, I estimate that 36 of those miles are spent driving at 55 m.p.h. when I could drive 70 m.p.h. That's faster than the speed limit but probably about what would be considered "normal." At 55 m.p.h. I spend 39 minutes, 16 seconds driving more slowly than "normal." If I were driving at 70, I would spend 30 minutes, 51 seconds driving the 36 miles. So I seemingly lose 8 minutes, 25 seconds per day.
However, I also only have to refuel about every 8 days instead of every 5.3 days (on average). I estimate that pulling off the road, fueling and getting back on the road takes about 8 minutes, so I save an average of 31 seconds per day stopping for fuel less often. Thus, the grand total loss is 7 minutes 54 seconds per day. In the course of a year, I lose 33 hours and 11 minutes behind the wheel (assuming 252 such days per year).
Looking at this time another way, my company values the 7 minutes and 54 seconds at about $12.66. An interesting comparison is that, on a given day, I save about 1.4 gallons of fuel compared to what I would have consumed using my pre-experiment driving style, which, at current Southern California prices, is worth about $4.57.
Or, yet another way is that I try to sleep 7 hours per night. Thus, the extra time spent driving amounts to 0.77% of my waking hours.