You can count the number of teeth on a pinion and crown wheel or try and count the revolutions "in" and "out" to check the differential.
(I'm not 100% up to speed on the history of PV444 but thought that by the time of the 544 only the commercial versions had the 4.5 rear end)
I believe both the three speed and four speed gearboxes have a final drive ratio of 1:1 so if you're talking about top gear speed comparisons the difference in gearbox doesn't have an effect (only for lower gears)
There are many on line tyre and transmission combination calculators out there to help you figure out the potential top speed or a road speed at a particular engine speed if you don't fancy calculating the values yourself (simple maths really)...
...but as mentioned above a flat road and a navigation nanny / clever telephone will be pretty accurate for an estimation of true road speed.
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