If the M41 “thinks” you have a 4.30 rear end (i.e. the speedo gear is correct for a 4.30), and you couple it to a 4.10 rear end, the speedo/odo will be in error by approximately 5%. You will be traveling 5% faster than the speedo registers and the odo will register 5% less distance than you have actually traveled. You get this by dividing the new ratio (4.10) into the old ratio (4.30), giving ~1.05. This assumes everything else is equal, particularly tire size.

For you to see a ratio of .833, there must be something else at play.

Cheers,

--
Barry -- 1967 122S 'Betty'