Gearbox ratios

[Piggy]

RPM gauge deffo correct?

This is my only thought... Duno how to check though tbh.
I assumed they worked or didnt

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You say 3-4% out but at what speed, is it 3-4% at 70 or 100mph (where most people check) but what about at 20mph or 50mph?

For testing you could go out and hold the car at every 500rpm then note the speedo speed and gps speed (on a flat road of course) and then that'll show if and where any major discrepancies are.

Nah its same across the whole range. Only 1mph or so at 30/35