Also, engines have a particular rpm where they run most efficiently. For small cars it's often somewhere between 3000 and 5000 rpm, so running the engine for short periods in its optimal efficiency range conserves more than running longer durations outside optimal range, even if that's a lower rpm.

I myself have experimented with P&G some but I have such a long commute that I can't tolerate much less than 65 miles per hour average. So my pulse/glide experiments have focused on using the technique without dropping below 60 mpg, and revving back up to 75 miles per hour. At this cycle I have not found it to be measurably more efficient than simply holding the car at a constant 70 miles per hour. The serious P&G folks of course will coast down to speeds much lower than 60 and probably don't often go as fast as 75, so they do much better than I do.