Re: VDO problems

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

From: "Theodore R. Harlan" <trharlan@one.net>

>I sent my speedo to VDO about two weeks ago for a recalibration and a
>replacement of the hair spring.

>1. Will it be difficult for them to make that adjustment? they ARE VDO.
>But they suck at inspiring confidence.

It shouldn't be.  All they need to do is look up the divider ratio.  This is 
the number of rotations of the speedo cable that equates to one mile (or 
kilometer if it is a metric speedo).  This ratio is usually written on the 
speedo.  On some it is written on the face (something like W=1750).  On my 
Avant, it is written on the rear (6644).

>5. How far off will it be if calibrated for 185s

You also need to specify the aspect ratio.  I think Audi 5000s use 185/70 as 
standard.  That gives an (approximate) tyre diameter of (14 * 25.4) + 
(2*185*0.70) = 615mm

195/70 gives (14 * 25.4) + (2*195*0.70) = 629mm, i.e. +2% change.

If you are using 195/60, then you will be worse off: (14 * 25.4) + 
(2*195*0.60) = 590mm, i.e. -4% change.

If you are using 195/65, then you will be very close: (14 * 25.4) + 
(2*195*0.65) = 609mm, i.e. -1% change.

I'm not sure what the tolerance for the accuracy of a speedo is (in the UK 
we have some silly rule about it being accurate at 30mph within a few 
percent (5%?), but it is certainly not going to be as good as 1% over its 
entire range.  2 or 3% is more realistic.

Tyre wear can easily cause a 2% difference.  Also apparantly, some tyre 
manufacturers aren't very precise with their tyre sizes.

HTH.

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant (with self-calibrated electronic speedo - now 2% 
accurate)