From: John De Armond
Subject: Re: GPS Indicated Speed
Date: Sat, 09 Sep 2000 22:12:26 -0400
Lon VanOstran wrote:
> Charles Piercy wrote:
> > Before life and travel became complicated with all of the toys, I used
> > to run a stopwatch between the mile markers. This gave me a 'real'
> > number to balance the speedo/tachometer against. It immediately shows
> > the errors in various vehicles' speedometers. And you know exactly what
> > speed you are at!
> > Ted Piercy
> It doesn't need to be that difficult. The speedometer will be as acurate
> as the odometer. Check the odometer against the mile markers on the
> highway for 10 miles. If the odometer shows 10.1 miles, your speedometer
> is reading 1% fast, etc.
Actually it won't. The odometer is geared directly to the wheels
(assuming mechanical speedo). No room for change. However, the
speedo needle is moved by a magnet rotating inside an aluminum cup
which generates an eddy current drag and thus force to move the
pointer. If the strength of the magnet, the strength of the
opposing spring or the distance from the magnet to the cup changes,
then the speedo indication will change but the odometer will remain
I have a friend who has a large speedo shop in Atlanta. I've
watched and talked to the old guy who does the speedo calibration
and repair. According to him, the standard is 1000 revolutions per
mile on the odometer and 1000 RPM for 60 mph. To calibrate the
speedo, he first over-magnetizes the magnet with a little
fork-shaped pulsed electromagnet. Then while spinning the input
shaft at 1000 RPM, he uses a degausser to weaken the magnet just a
little bit at a time until the thing indicates 60 mph. A little bit
too much weakening and you start over again. He said that he likes
to leave the speedo calibrated about 1 MPH fast. That gives the
customer a little cushion against the traffic cop.
If you think you have a speedo error, first check the odometer
against mileposts. This checks the speedo gearing vs tire size and
rear end gearing. If it is off, then alternative speedo pickup
pinions are available for most transmissions. If the odometer is
OK, then check the speedo. If the indicated speed is off, time for
a trip to the speedo shop.
From: John De Armond
Subject: Re: Ford speedo error
Date: Fri, 10 Nov 2000 14:15:49 -0500
> Have a class A on the Ford F53 chassis. Have noticed with both
> the GPS and following others and communicating on the CB, that
> my speedometer is off by 5 mph all across the range. ie: when
> it registers 65, I am actually going 60mph.
> Anyone know if there is a fix for this ? Not a big deal as long
> as I know what the error is, but would be nice if it was correct.
If you're sure that the offset is constant and not increasing with
increasing speed, then you have a simple offset or zero error.
Mechanical or electronic speedo? If it's mechanical, the solution
is simply to remove the speedo from the dash and move the zero bias
spring on the speedo mechanism until it reads correctly. The
industry standard is for the speedo to read 60 mph with 1000 RPM
input. That's a good place to set the bias adjustment assuming you
don't have a speedo calibration bench. An electric drill motor and
a stroboscope or optical tach will take care of the input. If you
can't round up such instruments, then you'll have to do it on the
road. Take the mechanism out of the dash. Drive on level roadway
at 60 mph and notice the difference between "60" and where the
pointer is. Stop and then move the bias spring until the speedo
needle moves the same amount. If the speedo needle is 1/4" to the
left of "60", then move the bias spring so that the indication at
rest is 1/4" to the right of where it was at rest. Don't try to
make the adjustment at speed - it is not particularly safe but more
importantly, you are very unlikely to be able to hold a constant
speed AND tinker with the speedo. This is somewhat of a trial and
error process so plan on spending some time at it.
If it is an electronic speedo, you'll have to find a speedo or other
shop with the equipment necessary to reprogram it.
I should note that the GPS speed indication isn't particularly
accurate. The old problem of computing small differences between
large numbers with any precision. My GPS can differ from my radar
gun (known to be calibrated) by up to 3 or 4 mph for quite a long
distance at a time. It will usually come back to agree with the
radar if I maintain a constant speed long enough. My GPS manual
warns not to use the speed or odometer indication for navigation
because it is not very accurate.