I have swapped my 3.62 ratio diff to a 3.92.
Could anybody tell me how to work out how much faster my speedo will be reading than the actual speed?

Cheers

3.92 / 3.62 = 1.08

=> you speedo is showing +8% faster than your really going

e.g. your speedo says 60, but your really only doing 55.5

or try this http://locost7.info/gearcalc.php



[Edited on 3/8/09 by mcerd1]

Ok, I am going to make myself look like a complete t#*t but I have to ask this......... I changed my diff from a 3.92 to a 3.38. Has this affected my speedo?

yes

3.92 / 3.38 = 1.16

=> you are going +16% faster than your speedo says (assuming it was right before the swap)

so if your speedo says 60, your really doing 69.6

try the gear calc program above ^^^

[Edited on 3/8/09 by mcerd1]

Nat, great post. in a word yes, your actual speed will be more than your indicated speed.
cheers
Craig

You will need to recalibrate your speedo, so as not to pick up any speeding tickets.

[Edited on 3/8/09 by aka Keith]

Bloody hell, you learn something everyday! That will explain why when I took my dad out he was commenting and how it feels like we were goin much faster than we actually were!! From numpty of Guildford!

quote:

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Originally posted by asl
Bloody hell, you learn something everyday! That will explain why when I took my dad out he was commenting and how it feels like we were goin much faster than we actually were!! From numpty of Guildford!

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well I'll throw a spanner in the works, I can change my diff without having to recalibrate my speedo.
why you might ask, cos I take my pulses from the driveshaft which only requires rolling distance on the acewell,

tony

Depending on where your speedo pickup is of course. If it's on the axle or wheel then diff changes will make no difference. If it's on the prop or transmission then it will make a difference.

Edit: ^ beaten to it

[Edited on 3/8/09 by matt_claydon]

quote:

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Originally posted by mad-butcher
well I'll throw a spanner in the works, I can change my diff without having to recalibrate my speedo.
why you might ask, cos I take my pulses from the driveshaft which only requires rolling distance on the acewell,

tony

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quote:

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Originally posted by mcerd1
3.92 / 3.62 = 1.08

=> you speedo is showing +8% faster than your really going

e.g. your speedo says 60, but your really only doing 55.5

or try this http://locost7.info/gearcalc.php
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Thank you very much for the reply.

Forgot to say that I am running cable driven speedo on Type 9, that I know was deadly accurate from rolling road session.
Could I just swap the drive gear in gearbox? If so, could you tell me which gear I would need?

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