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1991 vn speedo giving false readings

arc angel vn ss

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hi ive got a 1991 vn commodore its an ss replica and yer

i have a serious problem with the speedo sum time when i start the car the speedo give me the corecct speed im doing or close to it but then if i accelerate hard or have alot of ppl in the car the speedo goes out by 10 or 20 its never been out by more then 25 but its a problem as i was on my way home from werk thinkin i was doing the correct speed 80 when pulled over and fined 250$ and told i was doing 101 and losing 3 demerit points and only being a red p plater means ive only got 1 left i need help does any1 no what could be causing this
 

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vn calais manual

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hasnt had a diff change has it? sum1 might have changed the diff gears to a different ratio throwing it out.
 

arc angel vn ss

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not that i know of i only got it last october for my birthday and yer it doesnt appear to have been changed
 

vn calais manual

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id look into it, remove the rear diff cover, drain the oil out, and look on the side of the crownwheel for a diff ratio stamped into it.

standard is 3.08:1.

youll need some oil and an $11 gasket, takes 20 minutes if that.

if the ratio is anything but 3.08:1 youll need a speedo corector kit.
 

arc angel vn ss

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yer i just called the guy i bought it off and he said that he hasnt touched the rear diff and the ratio hasnt been changed

its just weird nobodys herd of a speedo doing that before
 

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dont worry mate - think its more common than ya think - my vp is an ex chaser with 90k on the clock , and the speedos out - would hate to think how many speeding fines came out of my car , and mines absolute stock as a rock besides the body kit and some wheels, and wheels wont throw it out too much
 

arc angel vn ss

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yer but like my speedo isnt out like permently out like 10 and it stays that way sum times it werks sumtimes its 10 under and sumtimes 20 under it changes
 
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