Remember that fuel consumption testing in those days was not the science that it is now. The tank was simply driven around a course and the fuel used measured. Speeds and rates of acceleration were left up to the driver. No attempt was made to allow for temperature or the condition of the course, though both would be recorded. An Archer is roughly the same weight as a Valentine but with the extra power is vastly nicer to drive and so the driver can be in the optimum gear much more easily and will need to change down less for heavy going. That in itself will make a significant improvement in consumption.
As I said the difference between the different versions of the WW2 6-71s is basicly that the more powerfull ones get more fuel and are allowed to rev harder.
David
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