Re: [Chrysler300] Speedometer gear correction

[John Spiers <spiers@xxxx>]

Hi all,
Here's a website regarding tire size vs speedometer error (and lots more)- the only downside is that only modern tire size designations can be input, i.e. 235-75R14 is ok, but not 850x14 or H78-14 - you would need to convert to an equivalent modern size. This site was particularly handy when I converted from 15" to 16" wheels on my van, and determined a new tire size that affected my speedo less than 1/2%.
http://secret-secret.com/turbo/DIY/tirecalc/turbotire.html
Regards,
John Spiers
Warren R Anderson <wranderson@xxxx> wrote:
-

>Just took the C out on the freeway. Seems the speedometer is 10 MPH
fast(reads 80 really only going 70). Somewhere I have seen a chart with the
different speedometer pinions that go with the different rear ends(assuming
that is the problem). Can't find it any more. Anyone out there have this
information? I'm running 235X75X14 tires. I don't think this would be the
problem.


Each tire size and each tire design will revolve a certain number of
revolutions in one mile. A speedometer, on the other end of the rear axle
gearing and the transmission speedo drive and driven gears, is looking for
1000 (lets use that number to agree with another post that may well know
what Chrysler std was in those days; my memory is that that standard number
should be 1020 revolutions every mile). Speedo gears make this change.

The speedo cable in a car should therefore revolve an easy to measure 37 7/8
revolutions in a straight ahead measured 200 feet or half that in 100 feet.
If the number comes up short in number of revolutions, the pinion would need
fewer teeth and so forth.

A speedometer shop can build ratio boxes to make corrections with the above
collected data in hand. We just bought one for a truck and it cost us about
$60. I have done this and it works not only in theory. The tires have to be
correct and inflated properly and the odometer will change calibration as
the tires wear from the calibration point.

This revolution per mile calibration will help check and calibrate the
odometer. The speed registry will have to be checked and calibrated off the
car on a test stand. My experience has been that my speedometers are way off
after extended down time. This I would surmise is because of a number of
things in the speedometer head assembly but the magnet/speed cup
relationship is where I have felt the problem was centered. I had a real
talented fellow go through my C300 speedometer and it did work just fine
right after calibration only to change some after a long down time.

HTH

Warren Anderson
Sedona,AZ


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