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Triumph TR6 - Speedometer Wrong

Hi all, I noticed that my speedometer is about 10 miles per hour off. I have the correct size tires(Michlin Red Lines). Any ideas on what could be causing it? The speedometer bad maybe??

Thanks
JW BRAW

I bet you it reads to low
Most probably weakened magnets in the internal drive mechanism of the speedometer cause this.
The rotating magnets will cause so called “eddy currents” in an aluminum disc. The eddy currents will result into a magnetic field, which will try to lock to the field of the rotating magnets. In doing so it winds a spring balancing the movement of the disc. The speedometer needle is attached to the disc.

The solution is to loosen the spring or replace the magnets, perhaps also change the air gap between the disc and the rotating magnets.

I would not advice you either of these remedies, your best option is to take the speedometer to a calibration specialist.

Erik
Erik Creyghton

If Erik is right, would that affect your mpg calculations too? For instance, if you are really going 60 mph and the speedo reads 50 mph, aside from getting speeding tickets, you really going 60 miles in that hour rather than 50 miles in that hour. That is actually a 20% difference, if that example is right. (60-50=10. 50/10=20%)

Would that mean, based on what Erik suggests, that your miles per gallon would be 20% higher than what you calculate. Again, for example, if you go 100 miles and it takes 5 gallons, you would be getting 20 mpg. But if your speedo is 20% off, you might be going 120 miles, when the speedo says 100, so your mpg would 120/5(24 mpg) rather than 100/5 (20 mpg).

Or is this completely wrong?

John
JL Bryan

Yup. Completely wrong. Unless the speedo error is due to a wrong sized gear at the tranny, the speedo error will not affect the operation of the odometer, which is mechanical. By the way, I envy anyone who gets to drive a TR enough to be concerned about gas mileage.

Tony
A. J. Koschinsky

Tony
I am having dificulty with this subject.
I thought/think the trip odometer or the odometer is in direct corelation to the spedo...meaning if your spedo is in error then your actual mileage traveled is going to be in the same error (+ or-).
One thing is for sure. I really do not care what my gas mileage is. Yes it is interesting to figure out but any bad numbers sure as heck will not stop me from having fun.
Rick C
Rick Crawford

Well, here's my theory. The speedo cable will turn a certain number of revolutions per mile. The odometer will record those revolutions as miles. If the magnets in the speedo are weak, or the speedo is miscalibrated, it will not move as far around the dial as it should for the speed at which the cable is turning. But it will still turn the same number of revolutions per mile. The distance covered will be correct while the speed indicated may not be.

If the gear is wrong, the cable will not be turning the correct number of turns per mile for the odometer to be accurate, and the speed indicated will be out as well.

Tony (head starting to spin)
A. J. Koschinsky

Tony - I'm no expert, but you are correct on all counts. I took mine apart to put a dab of grease in and that's what it looked like to me. The cable spins a magnetic bar, causeing the needle to move CW against spring pressure. The faster you go, the faster the magnet spins and moves the needle up the dial.
The cable also turns a worm gear, which drives gears that mechanicaly turn the odometer drums. A weak magnet can cause an error in the MPH reading and a tire size different than stock wil throw the odo off.
JW - you can take the speedo inner works out of the case and carefully turn the needle while holding the metal plate behind the dial face still. Say it reads 50MPH when you're doing 60. Turn the plate to 50MPH and hold it while you gently turn the needle to 60. Of course, now the needle will probably read 10MPH at rest instead of returning to the "0" MPH stop post.
There is a webpage that tells how to dismantle and fix Brit speedos & tachs. Be careful - there is ample opportunity to bend and lose all sorts of tiny springs, pawls, cams and so on. You'll need nimble fingers and good eyesight...
http://mywebpages.comcast.net/rhodes/speedo.html

Jeff
74 TR6 - still not on the road
Jeff Fetner

This thread was discussed between 25/04/2004 and 28/04/2004

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