The best way to determine any error in speed vs. rpm, is to verify the
speedo to actual distance. Have you checked this against the Mile /Kilometer
marker posts on the side of the freeways? (Assuming you have such things where
you are located at. Here in the US, we have mile marker posts on the side of
the freeways accurately placed at each mile. Suggest that you travel at least
10 miles, or 16 kilometers to determine the error rate of your speedo as
determined by the mileage traveled from beginning of test t end of test compared
to actual (number of posts past). if this is off only by a small amount, then
this can be attributed to the difference in tires. if it is off by more than 3
or 4% like 11% or so, than this would indicate that someone may have changed the
rear gears to around the 3.2:1 range and not have changed the Speedo drive gear.
However, the Tach's themselves are not particularly accurate, so 1,000
indicated could easily be 20% off....very easily be off by that much. You
should attach a more accurate type of tach to do this. I always used the SUN
Super Tach // for my tests. They were guaranteed to be accurate within 1% (as I
remember). Simply the best tachs I have ever used. Mark Olson has some
modifications to make the stock Tach more accurate and stable thanks to more
modern technology! You should check out his web page for more information on
that. Unfortunately, I don't have that link handy. It's listed on most of the
major Sunbeam web pages..
Rich
-----Original Message-----
From: Colin Mills <Colin.Mills@pop70-2.bluewin.ch>
To: tigers@autox.team.net <tigers@autox.team.net>
Date: Sunday, August 09, 1998 8:19 AM
Subject: Slow Tiger
>Given that the tach may be faulty, I have the following question (and
>please forgive the metric measurements!):
>Current set up is as follows:
>I am running Goodyear Eagles NCT70 (185/70R13s), with a rolling
>circumference of 179cm. This is about 2½% less than the stock Dunlop
>5.90x13 RS.5s (183.54cm).
>I am assuming (I don't know for sure) that I have the stock 2.88:1 rear
>axle.
>The Workshop Manual tells me that, at 1,000 rpm in top, I should be
>doing 38.5 kph. However, the measured road speed of the Tiger (using a
>reliable modern car, fixed distances and several attempts) at an
>indicated 1,000 rpm is 30.7 kph. - 20% slower!
>Anybody any ideas please?
>Colin Mills
>B9473407
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