Re: [DML] Speedomter Calibration
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Re: [DML] Speedomter Calibration

Tom Niemczewski wrote:

> A speedometer that is within 10% of actual speed is considered VERY
>accurate! And guess what... 99% of speedometers will actually show 10% more
>than your actual speed.
>The thing that can affect the reading would be different tires than those
>used originally at the factory. Even if the size is correct. Tires from
>different manufacturers differ in size. That will affect the speed as well
>as odometer reading. The smaller the tire the higher the reading will be. In
>your case where the odo is lower than you expected and speedo is higher I
>would say that it's perfectly fine. The speedo is perfect and odo in your
>OTHER car is probably going faster than it should.
>I hope this helps
>Tom Niemczewski
>vin 6149 (in Poland!)
> <- home of the convex mirrors - available now
This really depends on the make and model of the car, and can vary even
from car to car itself. However. I think that a nearly 5mph differece
going at only 30ish speeds is a bit much. I know most of the mechanical
speedometers in the vehicles i've driven in the past have usually
differed by that much at highway speeds (70mph or so) not at 30. I know
my 98 dodge intrepid is nearly spot on, showing 69.8 on the gps with the
speedometer showing 70, but it's speedo is entirely electric, no
mechanical parts in it beyond the pickup in the transmission, and it's
probably hall-effect.

I can think of only 3 things that are changable to affect displayed speed.
1) tire size, you'd have to go larger diameter to get a more correct
reading if the numbers posted are correct.
2) angle drive. Not sure exactly what differences can turn up here. but
if one or both gear diameters aren't right then it will effect output
shaft speed, which will send the speedo the wrong wheel rpm to the speedo.
3) the speedo itself.

I guess if you had a stroboscope and a way to drive the speedo out of
the car, you could just do the math using the correct tire sizes for the
front. (which would be a circumference of 72.9 inches, or 869 rotations
per mile, by the way) So assuming the angle drive has a gear ratio of
1:1 (I don't know if it does or not. Haven't looked that up) you'd
simply drive the speedo input at 869rpm and the speedo should, if
accurate, read 60 mph. Of course doing this would just tell you where in
the system the oddness in speed is coming from. not help you correct it.
I know of no way of adjusting these speedo's mechanically. I've never
had reason to ever dismantle one.

I guess the first and easiest thing to do would be to make sure your
front driver's side wheel was inflated to the proper air pressure, then
use a tape measure and measure it's circumference. If it's less than 73
inches, then your tires are at least part of your problem. If it
measures right, or relatively close anyway. your problem probably is the
speedo, unless others can note differences they've found in different
angle drive ratios.


VIN# 03209

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