for my car persona vvt, the sweet spot is truly 2.9k-3k rpm to upshift if you are still planning to accelerate gently.
Basically, the car needs 2k at least to do decent acceleration. Upshifting from 2.5krpm will make the next gear start at around 1.8k rpm, which will bog down the car's acceleration.
It will struggle for a while, before picking up again (after 2k rpm).
This is especially apparently for 5th gear. If I'm in 5th gear below 2krpm, will gently decelerate. Need to press accelerator around 40% in order to maintain speed. Which is rather fuel consuming.
So during 4th -> 5th gear, i may even pull up to 3.2k rpm so that i can enter 5th gear in 2.2k rpm, where i just need to feather 15-20% to the gas pedal to maintain speed.

When I first gotten this car, i tried to upshift as soon as possible.. keeping the car RPM around 1.5k-2.5k rpm. but i ended up getting a worse FC and very poor experience, as the car feels sluggish at all times. Then sometimes as I was rushing, i held on the gear for longer periods, and during that week, i actually gotten better FC. that's when I realized, it's better to accelerate asap to your desired speed, than to slowly crawl to your desired speed at higher gears.
another thing that aids fuel consumption, is downshifting to lower gears during hill descend. Higher RPM, but engine braking actually keeps FC low.
Tried descending with 4th gear (2krpm) vs 3rd gear (3k rpm). Generally get better results with 3k rpm. 2nd gear (4.2k rpm) too noisy. didn't bother/like doing that during descend (unless i'm already in the 2nd gear at the moment)

p/s: i've read back and noticed ppl was discussing over saga blm mt. Well my wife is driving saga blm mt, and i can only say, the car characteristic is very different from my persona vvt mt. Her car can easily move within 1.5k-2.5k rpm. When driving her blm, i never needed to rev till 3k. Tend to upshift a lot earlier. Persona VVT needs at least 15kmh to move smoothly in gear 2 (without needing to hold onto the clutch that long), as opposed to her BLM, which can easily slot in 2nd gear at 5kmh and move smoothly with clutch fully engaged.

This post has been edited by tctham: Nov 2 2020, 11:07 AM

yep. depends from car to car. Bigger engine and certain transmission definitely can afford to cruise at higher speed. For small engines like my persona and saga, even going 120kmh is worse than going 90kmh. Much less 140kmh. The engine needs higher RPM perhaps due to lack of power / lack of higher overdrive gears to cruise comfortably at high speed.