I think the gauge is irrelevant in this case, it is just a reference for you to keep at a same given speed. I say this because I tested different speeds for each plane and settled for the best rate of climb. The real 'gauge' in this case is the chronometer. The faster the plane gets to the altitudes the best is the speed for a given plane to climb. It is not rocket science in this case. From my tests, 5-10km/h might account for 6-7s at 500m, 15-17s at 100m and so on so forth. A 5km/h difference is noticeable right at the first 'check point' (500m). 10km/h makes you miss the mark by a mile. It is hard to make a mistake on the account of methodology.
It is just like the mixture. You have an optimal position, and if you go up and down and the RPM decreases, it means that you are at the right spot. And I found this optimal speed in all the planes that I tested. This is what I was saying to Unreasonable. It happens that we were talking about different gauges and the same speed, but the gauge in this case is irrelevant. What really tells you how the plane is climbing is the chronometer. And my refferece gauge was always the chronometer.
What is possible is that after 2000m there might be a leeway of inaccuracy due to the mixture control, but not in terms of being at the wrong speed, but of the mixture being off. I tested the Camel with auto mixture and all the values were higher than the ones I collected with manual mixture. I might test the others, but I don't predict much changes.
And sure, I will keep testing the others! I need to finish the bombers chart on the max speed and test the S-16, S22. Little by little I'll cover them all.