From: mark@wdcwdc.sps.mot.com (Mark Shaw)
Subject: Re: Corrado Speedo Error
Organization: Motorola Western MCU Design Center, Chandler Arizona
Date: Fri, 20 Nov 1992 00:56:19 GMT

In article <dchill.721941258@cansqnt>, dchill@sequent.com (Duncan Hill) writes:

|> I recently purchased a 90 Corrado with the BBS wheel upgrade
|> and the 195 55 R 15 Sport Contacts.  I swear the speedometer
|> is reading about 15 KM ( 9 miles ) per hour too fast.  I have
|> paced by car with a friends GTI and this checks out.  I had
|> the car in for service and had it checked, they said it is fine.
|> The only way to be sure is to know factory specs for what
|> the speed should be in a specific gear at a specific number
|> if rpm's.  Does anyone have this info?  The dealer can't
|> seem to provide it. Has anyone else experienced this same
|> problem.  The tire size should not be an issue because the
|> speed is measured at the drive shaft.  However, if the 
|> speedometer was calibrated for 205mm tires and then the 195mm
|> tires were put on it could cause some discrepency. My 
|> calculations show that the 10mm difference in width translates
|> into only a 5% difference in overall wheel circumference and
|> therefore only a 5% difference in speed.  I am experiencing
|> a 15Km/h error at 100km/h. Any ideas would be welcome.
|> Thanks,

I see an assumption in your analysis that your speedo (and your
frien's) were accurate to start with.  When you look at the SAE
recommended practice (J678) you will find that it is acceptable
to have the speedo within +/- 4 mph (6.4 km/hr) OR to have it
+6 mph (9.6 km/hr) and - nothing.  It is entirely possible that
the gear ratio in your transmission is for the more common 
185/60r14 or 195/70r13 tires/wheels and usual final drive ratios.

Most VDO speedo's I have seen usually have a number printed on
the face that gives the design speedo cable revolutions per
unit of distance (usually around 1500/mile for water-cooled
cars).  This number may not be visible in the cluster and 
may be behind the light shrouding.  

Your calculation for tire size seems overly pessimistic? I have
seldom found the circumference to be meaningful.  What you want
to look at is the loaded rolling radius.  This can be approximated
by multiplying the tire width by the aspect ratio and adding it to
half the rim diameter.  A 205/55HR15 tire would be approx. 11.9 
inches and the 195/55HR15 would be approx. 11.7 inches.  This is
a difference of only 2%.  On the other hand a 185/60HR14 has a 
11.4 radius and a 195/70HR13 has a 11.9 inch radius.  Actually going
from the 185's to your 195's should make you look slower, whereas
the 195's are almost the same as the 205's.

Rather than gage your car off your friend's (which may be reading
slower than normal) try measuring the distance your odo reads for
a known measured distance at least 10-15 km long.   This will tell
you if the gearing to the speedo is wrong.  Based on the ratio of
your actual measured distance to official distance you can then
estimate what your odo should read.  Then drive at a fixed speed
for about 1 km and measure the time it takes.  Now you can more
accurately determine where the error is.  

I have seen speedo's which have enormous amounts of zero offset,
that is it is the same amount fast at every speed.  This would be
the most common error.  Errors that are linear with speedo (always
the same percentage) would be due to dirt between the drag cup and
the magnet on the speedo shaft or incorrectly magnetized.

I'd check the odo measurement first for the wrong gear/tire/axle
ratio first.  If that matches the problem is usually in the 
actual speedo itself.

Mark
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