I've continued to learn small details about how to get the most economy from our Mazda5 5 speed, so I thought for the benefit of other 5 owners I would summarize it here (though much of it can be generalized to any manual transmission vehicle).
1st off, when the motor and weather is cold, there just isn't much to be done that will improve economy significantly until the motor warms up. I've tried P&G, lowest rpms possible, and just plain not worrying about it, but on my 2.5 mile commute to and from work, fuel economy barely fluctuates outside of 25-28 mpg. It's pretty clear why after watching the Scanguage, as the GPH display holds a low rpm idle consumption at nearly twice what it consumes once warmed up. No doubt some of that carries over into higher rpm operation.
As for general maximum fuel economy without special techniques, the lower the shift point you can use, the better, so long as the next gear doesn't drop the rpms below roughly 800 or so. Cruising in 5th gear at idle speed (about 19-20 mph) on flat ground produces over 50 mpg. Even at 25 mph, economy still hovers around 50 mpg. The same speed in 4th drops economy down into low 40s. Shift at the lowest rpms possible as soon as possible.
Now as far as pulse and glide technique is concerned, I've found that attempting to utilize it at anything under 30 mph average speed is simply not productive. Because the rpms in 5th gear are so low already, it's difficult to make up any ground there unless one takes the extra measure of engine-off coasting. Even so, under 25 mph or so, even EOC'ing doesn't return too much to economy, as the bump start at the low end triggers a rich-start condition that negates much of the benefit. For the wear and tear on the car, I don't consider it worth the effort.
Where P&G seems to produce the most benefit is at average road speeds of 40 mph and above. A steady cruise in 5th at 40 mph nets 42-44mpg. P&G bumps that up by about 2-3 mpg. EOC P&G adds another 2-3 over that. Unfortunately it's a bit more difficult to assess the benefit at highway speeds due to traffic on the local two lane roads, but using a combination of 2+ second drafting and limited P&G I have been able to attain high 30s-40 mpg on a local 15 mile trip we do regularly with 75% highway driving. Sustained cruise control at 55 mph nets about 32 mpg without any other aids. Fortunately this car is so slick aerodynamically, economy still stays around 30 mpg at 65-70 mph. Not bad for a 6 passenger wagon!
Regardless, a bad day of traffic and light sequencing can bring my best effort in town down into the mid-20's. Short of just runnin' those reds, there's only so much you can do.
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