Just work out the overall ratio.
Spur / pinion x internal ration = overall ratio.
So 81 spur with a 23 pinion
(81/23)*2.6 = 9.16
So for the 78 spur
(78/pinion)*2.6 = 9.16
(78*2.6)/9.16 = pinion
22.13 = pinion 22 pinion is the closest
So 81/23 gives an overall of 9.16
AND 78/22 gives an overall of 9.22
So that the closest you can get switching from the 81 spur to the 78 spur.
MiCk B. :-)
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