[FWDLK] Speedo adjustment..?

[David Homstad <Dhomstad@xxxxxxxxxxx>]

Mike,

This is a good process for checking the accuracy of the Odometer
(measurement of miles), but not the Speedometer. The odometer can be 100%
accurate, and there can still be a sizeable speedo error. I once had a
disagreement with an officer of the law, which resulted in me having my
speedo accuracy verified on my 65 Coronet to prove my point. The speedo shop
first connected a counter on the cable in place of the speedo and drove a
measured mile. The cable should make exactly 1000 turns per mile. Then they
plugged a machine that would spin the speedo in the car at precision rates
in 10 mph increments. The combination of both the cable and speedo errors is
the total error indicated by the speedometer. The cable error is mostly
affected by changes in tire size, rear end ratio, and transmission swaps
with a different speedometer drive gear in the trans. Most 50s cars are
known for being optimistic (e.g. they read high). The speedo in the dash may
have different error percentages at different speeds.

It is very easy to verify speedometer accuracy. When cruising down the
highway at a steady 60 mph, you should pass the mile markers (little green
signs on the Interstates) exactly every 60 seconds. For every 1 mph error,
this will affect the time by 1 second. If your indicated speed is 60 mph and
the time between mile markers is 58 seconds, you are traveling 62 mph and
your speedo error is 2 mph lower than actual. This method is accurate up to
5 seconds error. At 30 mph the error is half what it is at 60, or 1 mph.
This method should be repeated several times and an average taken.
Repetition helps compensate for minor variations while trying to hold a
steady speed, reading the time to a second, and sloppiness in the highway
department's placement of mile markers (some states are worse than others).
For larger errors, pick an indicated speed that you think is close to a true
60, and repeat until you narrow it down.

I drive a 92 Crown Vic that is a retired highway patrol car. The 140 mph
speedo (says "Certified" right on it) is so accurate that at 60 mph
indicated, I get exactly 60 seconds between mile markers. Once on a trip,
the odometer measured 100.3 miles for 100 miles according to the mile marker
signs. Not bad for a car with 187,000 miles on it.

Dave Homstad
56 Dodge D500

-----Original Message-----
From: Forward Look Mopar Discussion List
[mailto:L-FORWARDLOOK@xxxxxxxxxxxxx]On Behalf Of Mike & Sharon Higgins
Sent: Friday, February 08, 2002 3:28 PM
To: L-FORWARDLOOK@xxxxxxxxxxxxx
Subject: [FWDLK] Speedo adjustment..?

You calibrate your speedometer to show the new, accurate speed.

This can be done by a speedometer company, or actually is pretty easy on
your own, if you live near an expressway (interstate).  Travel along until
you see a mile marker, and check your odometer at that point.  Count off
until the next mile marker, and compare. I'll usually go further, about ten
miles, to get a more accurate reading.  If you read the same as the mile
mark when you pass it, your speedometer is accurate.  If you read higher,
your speedometer is too fast, and needs a drive gear (in the transmission)
with more teeth to slow down the cable. If you are reading less than actual,
leave it, and remember that you're not going fast enough.

(Why add extra miles until you sell it?)

Mike Higgins
1955 Belvedere Sport Coupe (with 55,000 actual
miles).....................really

Do I need to add a disclaimer to this??

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-- -- -- -- -- -- -- --
Calendars are still available.  Don't miss the chance to get yours now!
Details for ordering may be found at:
http://www.forwardlook.net/calendar2002/index.html