On my '18 I run a 38t rear down from the 41 stock and have the same trouble. I use a motrax converter the same as Steve. All I do is remember I'm going around 10mph faster than what the speedo says. This isnt the case of course as its a % increase but it keeps me out of trouble.

On the F3 I dont have a speedo at all, I work on the.. if i'm being overtaken im going to slow, if i'm doing the over taking im going to fast, and if the little clock says 13k and there are no more gears im going to prison principle.
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If I have to take the carbs off once more...