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rayjackson1950
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S3 - Diff Ratio

Can I check the diff ratio without removing the diff.

I assume I can when in the air by counting wheel turns against prop turns, but all I have found is how to do it with the diff removed.

I'm interested as the speedo against Google maps suggests the speedo is about 10% out.

Jonathan Kay
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Yes.

Jack up one rear wheel and leave the other on the ground with the brakes OFF and the wheel chocked. (Corrected, see below.)
Put the gearbox in neutral.
Mark both the prop shaft and the rear wheel to make it easier to count turns.
Turn the lifted wheel 10 times (other options are available) and count the turns of the prop shaft.
Divide that number by ten because you've turned the wheel ten times (or whatever) and then by two because you've only been turning one wheel.

Or something like that.

Jonathan

Jonathan Kay
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"I'm interested as the speedo against Google maps suggests the speedo is about 10% out."

That's the instantaneous speed display on a smart 'phone rather than some distance over time calculation?

Jonathan 

simon_h
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Speed reading 10% high or low? High is correct and within tolerance so you are more likely to be within the speed limits. Low needs addressing. 

John Vine
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#2:

...with the brake on...

I think that should read "off", JK?

JV

Jonathan Kay
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I knew that something was wrong! Thanks for the correction.

Brake OFF, wheels chocked.

Jonathan

Youds7
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The ratio is stamped on my diff. 

SLR No.77
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The ratio is stamped and written in felt tip on my diff, 3.62, but I'm convinced from rpm/mph readings it's a 3.92.  On the list of things to investigate this winter.

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