going by your maths in relation to your diffs, i'd say you're correct in deducing that you have 3.54:1 ratio diffs fitted, which will give you an error at your speedo head as it is driven from the back end of the gearbox...
a possible correction that is used in G.B. is to find a speedo from an old Triumph car (probably the Triumph 2000/2500) with approximately 1100-1140tpm (Turns Per Minute), your standard speedo i think will have about 1400tpm
it might be worth you asking for verification on the speedo tpm on the Series 2 Club forum
hope that helps
p.s.
a good looking truck

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