RE: IML: Speedometer on '68

[dardal@xxxxxxxxxxxxxxx]

Quoting Ed Buitenwert <mster-ed@xxxxxxxxxx>:

> The tires are radials (surprise to me) P235 75R15.  
> 
> Ed

You have the "right" size tires.  The reason why the right is in quotatation
will be evident as you read this.  The total tire width (235 mm) is very close
to the stock 9.15 inches.  Some/most in the list will tell you that the stock
tire aspect ratio was 78-84%, which is substintially taller than the modern
75%.  I however (and I am the minority) think that the stock profile was closer
to 70% (in numerous factory sources, the tires are labeled "low profile", and at
that time the first 70% profile tires had appeared.  Also, the factory claimed
gearing of 28.4 mph per 1000 rpm in high gear which is in agreement with the
70% [once you run the calculations], while the gearing with the 235/75's has
been measured by me at 30 mph/1000 rpm).  So, if I am correct, the modern
radials should reduce somewhat the stock speedometer errors.

Anyway, the speedometers always had some error (even in modern european cars
with electronic speedos, there is deliberate error).  15 mph is fairly high
though, 5-8 is normal for the 70-80 range (over-reading) for the 67-68 Imps. 
Usually, the odometers were fairly accurate, over reading only by up to 2%.  If
your odometer is also way off in the same direction, may be you have the wrong
gear.  My LeBaron for example, has a transmission from a later model, so I know
that the speedo gear is off because the odometer reads 12% low.  Incidentally,
the speedometer is accurate in the 70-80 range (built-in errors compensated by
the wrong gear, apparently), but it under-reads in the 100 range.  My Sedan has
the correct gears, and the odometer is accurate, while the speedometer is about
7 mph fast in the 80 range, but only 3-4 mph fast at 120 indicated. 

For your case, its possible that somebody replaced the differential ratio.  A
3.23 instead of the 2.94 will produce an additional 10% error over what the
stock speedometer would have.  Again, the odometer error will help guide you.
D^2


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