My observations (with the automatic transmission) is that the peak efficiency speed varies with the grade or slope of the road. On downhill sections peak efficiency happens at a very slow speed that is often not practical on roads with other motorists. With a fully warmed engine and a nice downhill grade probably 35 mph is peak efficiency. On sections that are uphill higher speeds provide more efficiency (perhaps around 50 mph) but this number will be lower if the hill is not very steep and the passenger weight is minimal and the number will be higher for steeper hills and higher cargo/passenger weights. On a completely flat road with no headwind peak efficiency is probably around 40 mph but most roads have enough variation in elevation that slightly higher speeds are better. The good thing is efficiency doesn't take a big hit until cruising at speeds high enough to cause serious aerodynamic drag (around 60-65 mph). That's because the engine is actually more efficient at moderate power outputs (more efficient in terms of fuel/hp). To a point, the decline in mpg due to increasing aerodynamic drag at higher speeds is somewhat countered by a rise in engine efficiency at slightly higher engine outputs. That is to say, an engine is not at it's most efficient when it is loaded at low rpm's (and this is especially true of a cold engine).
I've also noticed this engine is so efficient that the engine warms up slower than other, less efficient, engines. Fortunately, Mazda engineers appeared to have taken special consideration to insure cabin heat is available without undue wait. I believe they designed the water jackets and the entire cooling system with a priority not only on maintaining optimum combustion temperatures but also on getting the heat to the cabin (when desired) as quickly as possible even while the engine is not producing much heat (due to it's high efficiency). On cold mornings I've seen the cold engine light go out after a couple minutes of running only to come back on after an extended downhill section of road in which it was not necessary to push the accelerator to maintain the speed limit. The engine uses the momentum of the vehicle and gravity to continue to turn (while reducing the amount of fuel injected).
The optimum way to warm this engine from full cold in terms of efficiency is to drive off immediately after starting the engine (ideally onto a road with high enough speeds or enough of an uphill grade) so the engine has a moderate load for at least the first two minutes of warm up. Only when the engine is fully warm can it be driven in a high efficiency, low rpm, low load mode.
But most of this is merely academic with a vehicle that gets such good mileage anyway (when no special precautions are followed).
