speedo accuracy

[SCIROCCO_SPEED@msn.com (Brandon Smith)]

Okay guys, lets keep it real simple and do some math with the Distance/Time
equation.  Measure out exactly 1 mile.  At the very beginning of that one
mile, lay down a simple pressure sensitive cable, or light emitting object
with a reflector on the other side of the road, and either one rigged to a
timer.  Down the road at the end of the one mile, have the same thing that
stops the timer.  Just get a good start and have a constant velocity of say
75mph for example so that when you pass through the first photo-gate it
would (for all intents and purposes) be instantaneous, or even go by RPM.

Okay so maybe its not that simple, but aren't there a few physics teachers
on this list?  They should have adequate equipment to accomplish this.

This gets me thinking about the Tire Slip idea.  It makes sense and I
understand it, but wouldn't it take place more so when accelerating or
decelerating, seems that when traveling at a constant velocity it wouldn't
happen that much to affect a speedo.

just my tired thoughts,

-Brando


----- Original Message -----
From: "Rabbit16v" <Rabbit16v@attbi.com>
To: "Scirocco List" <scirocco-l@scirocco.org>
Sent: Saturday, December 14, 2002 1:00 PM
Subject: Re: speedo accuracy


> Your only fault in all of this is that you haven't had it radar checked.
> I'll bet if you did have it checked with radar that it would be off just
> like everyone else's.  I guess the problem that we need to figure out is
> where our calculations are going wrong.  At this point I think we need to
> question the charts we have access to on the internet?  Anyone have a
chart
> on paper in a manual or something that they can compare to the charts on
> Scirocco.org?
>
> Dave
>
> >
> > My $.02 CDN on the speedo issue (along the same lines as Ron's)  Both
the
> > Scirocco and the winter-beater Jetta have 7A transmissions in them.  The
> > Scirocco has the stock 220KM/h speedo, and the tach is a W-field
> > alternator driven unit from an 89 Jetta TD (ranging from 0-5000rpm)  The
> > Jetta has a 16V speedo (260km/h) and tach (0-8000rpm)  The speedo drive
> > gear matches the R&P of the transaxle (3.94 in the 7A)
> >
> > These two cars read as dead on as I can reckon by eye for RPM vs.
> > indicated speed on the speedometer - ie. 100KM on the clock is 2575 rpm
on
> > both tachs (admittedly harder to pinpoint on the 8K tach due to
scale...)
> > which impresses me, especially as the Scirocco's tach runs from the
pulses
> > on the alternator, not the ignition, which means it's more prone to
error
> > (belt slip, different pulley diameters etc)
> >
> > I know that the Scirocco's actually speed will be off very slightly
> > (compared to the indicated speed) because it has 185/60R14, which
results
> > in a slight increase in travelled speed (the rpm difference at 100km/h
is
> > less than 50 though)
> >
> > Personally I'm impressed by the consistency of VW's speedos, even if
they
> > are optomistic (which I have not had occasion to determine.)  The fact
> > that these two cars with different tachs and speedos come up with the
same
> > readings as can be calculated from gear ratio and tire diameter is cool.
> >
> > Drew
> >
> >
> >
> >
> >
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>
>
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