From: John De Armond Newsgroups: rec.outdoors.rv-travel 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 the same. 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. John From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Ford speedo error Date: Fri, 10 Nov 2000 14:15:49 -0500 Zork wrote: > 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. > Thanks > Zork 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. John |