speedo worm drives

I recently rebuilt my transfer box lt230 mostly with new gears in a disco case , noticed today on the m/way a rare occurrence for me , but speedo said 70 but i was actually doing 65 , it was this 5mph out from 20 up .
what have i done , as i had 2 boxes in bits , one a defender one a disco . Speedo was fine before . Dont think i have somehow changed the ratio as its still as angry at 70 as it was before , not very scientific i know . Anyone help me sort it out . thanks gerry

Did you change the speedo drive?

(What motor & tyres you running?)

bongo1 said:

I recently rebuilt my transfer box lt230 mostly with new gears in a disco case , noticed today on the m/way a rare occurrence for me , but speedo said 70 but i was actually doing 65 , it was this 5mph out from 20 up .
what have i done , as i had 2 boxes in bits , one a defender one a disco . Speedo was fine before . Dont think i have somehow changed the ratio as its still as angry at 70 as it was before , not very scientific i know . Anyone help me sort it out . thanks gerry

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That is about the usual variance I belive, it is normal for an analogue speedo to read a bit high. It is possible to change the drive gear to get it more accurate.

Hi thanks for your replies , I put a blue worm , and thought the other shaft worms in the box were the same ?. DEF 300tdi R380 box , 235/85-16 tyres . I only used one gear out the old disco transfer box , all the rest are new gears . i didn't use the older DEF T/box gears because the teeth were the early squared off type not the later sharper to aid changing hi/lo , As you know . I must of dropped the ratio ? or im now just thinking i must have you start doubting yourself , Like i said It was always unhappy at 70 and still is ! , and that's at 3000RPM . Just did a mpg check and it was 30mpg bit of motorway at 70 , town ,stop/start , lanes . What im trying to say is, I am driving it as normal and if i had dropped the ratio the mpg would of dropped , if anything it is better than normal , but that could be down to the new turbo doing its job .
My old t/box i took off and replaced with the re-build the speedo was spot on , thats why im baffled

Maybe or not, a slight difference in for drive in the box used depending of the disco transfer box, the early disco EFI V8s used a transducer from the box via a short speedo drive cable and then electrical connection up to the speedo head, later (around 1995) both petrol and diesel had the transducer bolted to the box and then an electrical connection direct up to the speedo head.