Peter,
You're data corresponds exactly with the data I did on my '72.
You are also right about the accuracy of the GPS, except make sure the test
area is as straight as possible. The speed shown on the GPS is a "fix to
fix" time and distance calculation. So if the road twists and turns the
straight line distance will be shorter and the speed display in-accurate.
How much depends on the amount of time between "fixes" and I don't have a
clue how fast that is. It's possible it's fast enough to not matter.
Thanks for making me trust my speedo checks a bit more
Steve
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Peter Zaborski
Sent: Tuesday, July 01, 2003 9:37 PM
To: 6 Pack (E-mail)
Subject: speedo calibration -- data
Hi 6-packers,
Every once in a while a discussion occurs regarding the accuracy of the TR6
speedo. Well, today I calibrated mine and wanted to post my results as one
data point for future consideration.
First, my set up (in case any one asks):
- 76 TR6, stock wheels and Michelin Redlines (185-15)
- stock diff
- 4 speed with J-type OD
- stock unmodified Smiths speedo (MPH version)
My calibration consisted of highway driving with a friend's GPS unit
(Magellan Merdian Color). I tried various speeds and in all cases (up to 75
mph) the speedo was DEAD ON. I saw no discrepancy between the GPS speed
(which I understand is extremely accurate for velocity data) and my speedo.
Just thought I'd share that in case in the future people question the speedo
accuracy.
FWIW, my motorcycle's speedo (87 BMW K75S, again all stock) reads 10% high
at all speeds. Admittedly this is a well known design feature of that
particular bike's speedo. My 87 VW GTI is also high, but only by about 4%. I
think most newer vehicles have optimistic speedo calibrations -- not sure
why but I suspect it might have to do with a more litigious society compared
to the '70s and earlier.
Happy Canada day!
Peter Zaborski -- CF58310 UO
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