Pathological speedometer

[George Hovis <gnhovis@domain.elided>]

[I think someone answered this question a few months ago, if so, please 
drop me a line directly.  Thanks.]  

I've always had problems with an inaccurate speedometer on my '74 GTV, 
but was once told to "multiply the tach by 2" to get an estimate of my 
speed in 5th gear (eg 3500 RPM in 5th gear is about 70 MPH.)  I have no 
idea if this is accurate.  If it were true, I suppose it would only work 
with the correct sized tires.  I was told the car came with "metric 
radials" which someone wrote down in my owners manual in 1974 as being 
165/78R14s.  (Also don't know if that is right.  The "78" always seemed 
odd.)  If that is true, then the 185/70R14s (my current tires) should be 
fairly close to the original rolling diameter.  (math below)

So here's my question:  Cruising in 5th gear (mostly on I-5 in Northern 
California which is about as boring a drive as they come) I'm usually 
going about 4000 RPM and my speedometer reads "65".  I make a 210 mile 
one-way trip twice a week.  At the end of each trip, my trip odometer 
reads 300 miles.  If my assumption about 2xRPM=speed is correct, then I'm 
driving about 80 MPH (honest officer, the thing said "65") and my speedo 
is off 30%, but my odometer is off by 50%.  If they're both driven off 
the same single cable, how can the be off by different amounts?  Am I way 
off base here? Any help appreciated.

I was once told of a proverb that a man with one watch knows what time it 
is, but a man with two watches is never sure...

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[((cross section*aspect ratio) * 2) / 25.4] + wheel diameter
All H rating
165/78R14  =  24.13 Standard
195/70R14  =  24.75 Current
195/65R14  =  23.98 Alt.
195/60R14  =  23.21
185/70R14  =  24.19 Proposed
185/65R14  =  23.47
195/60R15  =  24.21 Alt.

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