Since swapping over the speedo drive parts from my mk4 gearbox in to the replacement mk5 gearbox ive noticed the speedo is under reading my speed, ive only driven when using the satnav since the speedo was working. At 30mph on the satnav the van is saying I'm only doing 25-26mph. If I do 30mph on the clocks the sat nav says 35mph.
When pushing it up to 60mph on the satnav the clocks only say 50-51mph !
Ive got some borrowed steel wheels on it atm which are fitted with 195/14's and my alloys where only fitted with 185/14's so obviously this will make a small difference to the speedo reading as they'll be very slightly taller. But I don't think this will cause that much of a difference will they ?
I went from standard 27" tall tyres to 30" tall off road tyres on a Vitara and it didn't make anywhere near that much difference to speedo reading.
Any ideas what the problem is ? Are there different drive gears for the speedo drive ?
Ive fitted my mk4 one which fitted perfectly, and have also retained the mk5 speedo sensor aswell.