Going to a larger rear sprocket

[Ward P]

I've got a 43 on the back. reads 125km at 110 on the Gen1 already optimistic speedo.

assuming your speedo was correct with the 40 tooth. a 45 tooth would read 45/40 x the correct speed. ie: 100 would read 112.5
200kmph would read 225kmph. and 300 would read 337.5.

an indicated 300 would actually be 267kmph.

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but definitely wouldn't cause any stutter.