speedo drives

Ok i re-built a t/box out of 2 one a disco the other a DEF , to put on my DEF 300tdi with 238/85-16 tyres and my speedo read spot-on before i did all this , So i fitted a BLUE 20 tooth drive which should be correct . No miles out . mmm
Then fitted a 21 tooth GREEN , after removing mine from the original box to check the colour which was a green . Nope still out .
So i counted the teeth on the so called green out the original box and blow me its got 22 teeth ??? , so much for the colour guide . It should be a YELLOW ???????????? which i now have fitted and counted the teeth

OK WHY is the question why so far out , and what is an acceptable speedo over-read . i realise over is better than under , but surely i should not have to fit 2 drive gears up the range to get the speedo reading properly .

I've got this problem/issue.
My speedo says i'm doing 70 when the GPS reports 60 ..... which way should I go ?

MORE TEETH , that much i now realise . i believe 5mph over-read is accepted . I do believe bigger tyres will also do it . 31 inch in my case . depends what your set-up is , but more teeth on the speedo drive will do it .

i dont get why mine needs more teeth than 20 its meant to with my 235/85-16 fitted .