[Crumpp]

Originally Posted by 41Sqn_Banks

I know that, I can read. You asked how the engine data align with the speed graph. They align with an consistent altitude difference of about of 2,000 ft.

The altitude difference between the FTH in the charts can be explained by a different level flight speed. E.g.
Hurricane I has a FTH of 17,750 ft at 316 mph.
Spitfire I has a FTH of 18,600 ft at 364 mph.

As we see the 50 mph speed difference results in a drop of 1000 ft for the FTH.

There is nothing contradicting in the charts ...

The data matches the same fan plot in another report on a Spitfire equipped with a Merlin XX engine.

AFAIK, the Spitfire Mk I was equipped with the Merlin III engine.