I have an 110S in my 850 with a Rod Change gearbox part number 22G1832 and a number stamped 5/11485.
What is the likely final drive ratio and how can I test exactly what it is.
I was thinking of jacking up one front wheel and turn the engine over until I have one complete revolution of the front wheel.
Multiplying the engine revolutions by two should give me the final drive ratio, I think
Any ideas

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Most rodchange boxes except the Moke ones got a 3.44 diff ratio. I'd say that is 9/10 rodchange boxes.
Every rodchange box I've stripped over the years had a 3.44 diff in it.

But really, as someone has built yours up with the 1100s on, it could be anything in there.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.