Larger Gearbox Sprocket vs Speedo Readings

[Fredl]

Evening,

I have up geared my A7 from a standard gearbox sprocket and gone 1 tooth larger - amazing, it's like a new bike!

My question is - seeing as though the speedo drive comes from the gearbox and not the rear wheel - having changed
the gearing will my speedo readings now under-read as revs ratio has altered? (ie the indicated mph is actually higher,
as seems to be the case) which would make sense - any idea what the ratio / % difference might be?

I've read that Chronometrics are usually pretty spot on accurate but when the gearing is altered do the readings also
need (mental!) adjustment?

Interested to know any thoughts on this and whether I have my science all jumbled up?!

Many thanks,
Fredl

[muskrat]

G'day Fredl.
Std ratio 19/42 = 2.21, your new ratio 20/42 = 2.1 so it's near enough to 5% difference. So at 100 MPH your speedo will read 95 MPH.
Cheers

[wilko]

Just add some weights to the balance wheel to increase chrono reading.

[Fredl]

Thanks for the calculation - very much appreciated and that makes sense now!

Not sure I am adept enough to hang weights etc so will just be wary of the speed limits
and sit slightly under them and all will be well.

Cheers!
Fredl