Ahh, jees, wasn't my answer already complicated and technical enough?
OK, yes, you get the best power to fuel ratio at peak BSFC, however, that chart alone won't provide optimum shift points either.
They don't take into account the effect of gear ratio. Optimizing power per unit fuel (fuel economy) is not the same thing as optimizing distance traveled per unit fuel (fuel mileage). You will go a farther distance on the same amount of fuel at peak BSFC in 5th gear than you will at peak BSFC in 1st gear. Therefor it makes sense to attempt to minimize time in a low gear and maximize time in a high gear.
Given that (as is shown in your link) best economy (or specific fuel use) is generally relatively low RPM (1500-2000 in the sample they used) with roughly 3/4 throttle position, along with the leverage advantage of higher gearing, along with the fact the most people aren't going to look up a BSFC chart for their car, along with that most people tend to shift fairly late in the car's RPM range, I'm sticking with my original (albeit overly simplified) answer of "shift early, accelerate quickly, but never to a higher speed than you really need to go"
I respectfully disagree with your gear ratio argument. If you ignore torque and distance and just think about power, it's simpler. You change gear ratios and throttle to move the operating point of the engine around to match your engine load and road speed. Gears and throttle let you move from one point to another on the BSFC chart.
Consider this BSFC chart for Volkswagen TDI, a relatively low-RPM engine.
Power curves are in blue, with legend on the right. Increasing throttle moves you up the chart, and the engine puts out more power (blue lines); Downshifting increases RPM and moves you to the right.
This shows that the lowest fuel consumption, per unit of work, is the "197" point, at about 1750RPM. The blue line shows the engine is putting out 50HP at this point. This corresponds to a car accelerating quickly in a low gear, or fighting through a lot of air drag in a high gear.
I think of BSFC at two operating points: Accelerating, and Cruising.
When I'm accelerating, I have a fixed amount of work to do, to reach my target speed (0.5mv^2). I can do this work close to the point of max efficiency (50HP). Or, I can do it even faster at 80HP, in even lower gears, but I'll consume 20% more fuel. Follow the 80HP line up, and you'll see it crosses the 250, 230, and 220 lines. Or I can do it slower, in higher gears. Following the 10HP line (slow acceleration), the lowest BSFC I can get is about 230, which is 15% worse than the BSFC minimum of 197.
Now, let's look at Cruising. You have your target speed, and you don't need much power to get there. Assume 20HP on this curve. You can see that the best RPM is around 1100-1400 RPM. If you have high enough gears to get there, use them, but only if you're consuming 20HP. If you need to accelerate, you need more power, so shift to a lower gear to bring the RPMs back up towards the BSFC minimum (197).
Back to my driving rules for Accelerating and Cruising. In my car, the torque peak is around 2800RPM, so I shift just past this point when accelerating, and I keep the vacuum gauge around 5 (80% throttle), to stay as close as possible to the minimum BSFC. Then, once I reach cruising speed, I use the highest gear and minimum throttle to maintain that speed.
Clear as mud? :)
