Re: alfa-digest V7 #1225 - Alfa Speedometer

[AlfaNeely@domain.elided]

    The shop manual for the mid 1980's Alfa spider says the car should travel 
22 mph per 1000 rpm.  I have found this to be quite accurate in my 82 spider 
with 4:10 rear end.  22 mph x 4.11 / 4.56 = 19.83 mph for your 4:56 gears.  
As both cars came with similar rolling diameter tires, this conversion should 
be accurate.  That is, 2 times the tack reading should be close.
    To test your speedometer, drive on the interstate and time mileage 
markers.  Obviously, 60 mph = 1 mile per minute = 1 mile marker per 60 
seconds.  To convert other speeds, divide 3600 by the number of seconds to 
get miles per hour.  Or divide 3600 by the desired speed to get the number of 
seconds required to go a mile at that speed.  Do it several times for several 
miles.  I always wonder how accurate those mile markers are, but they seem to 
be pretty good in my experience.
    So if 4000 rpm gives a mile marker every 45 seconds that is indeed 80 mph.

    Palo Alto Speedometer (www.paspeedo.com  650-323-0243) advertises in Alfa 
Owner.  They converted my 82 speedometer from 85 maximum to 140 and rebuilt 
and calibrated it.  The total cost was about $280 as I remember.  It would 
have been cheaper to just use an earlier speedometer with the chrome trip 
painted black to match my dash.  My speedometer is off about 5 mph at 60, but 
that is fine as it slows down others (like my kids) who occasionally drive 
the spider.
    I did not check your tire conversions, but 165 78 14, 185 70 14, 205 60 
14, 195 60 15 and 225 50 15 are all about the same diameter.
Ciao,
Russ Neely
Oklahoma City



In a message dated 12/17/1999 8:21:15 PM Central Standard Time, 
owner-alfa-digest@domain.elided writes:

<< 
 Date: Fri, 17 Dec 1999 08:28:45 -0800
 From: George Hovis <gnhovis@domain.elided>
 Subject: Pathological speedometer
 
 [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 >>

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