I'm going to say: It depends!(TM) !
Partial answer: It's more important to get into top gear as soon as possible, vs. getting to cruising speed as soon as possible.
1) It depends which transmission you have.
5-speed: If you have the manual, that's easy, just short shift (keep the revs generally below 3000 RPM) and put it into the highest possible gear for your cruising speed always. With its silly short gearing, the Micra will happily cruise in 5th gear at 50 km/h on a level road. (Shifting into top gear ASAP is a major reason why it's so easy to beat the 5-speed's official fuel economy rating.) If you need more power, downshift!
Automatic: It's harder to accelerate briskly AND efficiently (low to moderate RPM) because you have less control over gearing. The automatic is most efficient when in 3rd or 4th because the torque converter locks up in both gears, getting you out of lossy slushbox mode.
- Automatic upshifts/lockup under light to moderate acceleration happen at roughly:
19, 35, 41 & 59 km/h .... (12, 22, 26, 37 mph) ... when the engine is warm.
If you're accelerating briskly, the shift points happen at higher speeds. But you can still get the car to shift into top gear "early" by briefly easing back (not off) on the throttle once you pass 59 km/h, wait a second for the shift, then pressing down again. It will stay in top gear/locked while you continue moderate acceleration as long as you don't floor it.
2) It depends what you're going to do next.
It is definitely true that moderate or even high load (but always low/moderate RPM) accleration can be more efficient overall than glacial blue hair granny slow acceleration. (Look up "brake specific fuel consumption" if you want to know why.)
But your results will be terrible if you brake immediately after accelerating like that. The blue hair granny is going to get better fuel economy even though that acceleration is technically less efficient because she'll still be rolling along behind you while you do the 2 pedal dance. Only fools rush in ... to avoidable stops & slowdowns.
3) It doesn't really matter
In the big picture, assuming you're not a worst case jackrabbit-then-jam on the brakes type of driver, the pedal to pay the most attention to is the brake, not the accelerator.
I've actually done a fair bit of "eco-driving" coaching and I hardly ever talk about acceleration technique. The key is in looking & planning well ahead to avoid rushing into avoidable slowdowns/stops. Doing that saves the average person the most fuel. Coast as much as possible into those situations instead. The brake is the devil's pedal.
Clear as mud?
View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...