?? About Speedometer Calibration
 

I've known for some time that my speedometer reads slow. In other words, the car is traveling faster than indicated. Based on some side by side driving with family members I had it figured at about 10 mph off.

Today I decided to confirm that number by doing some timed miles using the mileage markers on the Interstate. I clocked it 8 different times and sure enough with the Speedo at 60 mph I cover a mile in 51 to 52 seconds every time. I don't know if the error is linear across the scale (probably not) but that puts me at around 70 mph.

After that long winded preamble, my question is: How is it that these mechanical Speedos get calibrated? Is it just slop in the plastic gears? From, in my case, 418,000 miles of spinning? If so, will replacing the gears with new bring me back close to spec? Or is there some other secret adjustment?

Just wondering if sending the cluster out to one of the shops that works on these things is the way to go or if I can get it fixed by replacing the gears myself?

Is your tire size for some reason not original?

Tires are 14" on the original bundts that rolled off of RBM's lot in 1980.

The cable may be slipping.

Remove the cable from the speedo/tranny and see if the ends are still square. Make sure the cable connections are tight on the back of the speedo and the transmission tailhousing. Be sure the bends of the cable are not excessive, it should be straight as possible.

The speedo portion of the gauge works by magnetic drag on a cylinder near a magnetic drum that the speedo cable turns. Only the odometer is run by gears.

there are many different size 14" tires. I run 195-70's on mine. i think stock may have been 185-70?
I got a suburban adjusted at a speedo shop- they pulled the tranny end and switched gear size. ran about $70, 5yrs ago.

He's got an error of 16%. There is no possibility of installing any tire on the vehicle that could equate to this error. Tires typically cause an error of 1-3%, or if the owner is really stupid, the error can climb to 6%. Anything above this would require modifications to the suspension or the body.

Ok, I'll check the cable ends.

So, if the cable is slipping that would affect the accuracy of the odometer as well, right?

.......right.......!

Maybe somebody in the past installed a speedometer from a car with a different rear end ratio. :confused:

Went out on another Interstate run today. This time I concentrated on tracking the odometer versus the mileage markers.
I did a 45 mile run sticking as close as possible to the 60 mph (actual 70 mph) that I did yesterday and the odo tracks dead nuts. If the cable was slipping I would expect the mileage to read 16% or so less miles. It didn't, so I'm pretty sure the speedo cal problem must be internal.

BTW, the speedo is the W116 300SD 85 MPH full scale one and I have no reasonto believe it's not the original.

It reads slow correct? Perhaps the needle is sticky and therefore does not raise as high as it should at a given speed. I would take it to a speedometer shop for service.

Call your local police and ask them if they have one of those portable radar units that they set up to warn you of your speed. It is not cop car mounted but mounted on a small trailer with a solar panel on top.

I have also found cops parked on the side of the road and asked them if they would time me as I come past. Most will.

The easiest way to correct the discrepancy is just to manually move the needle from where it is at a certain speed to where it should be. The speedo on my 115 comes out of the instrument cluster very easily. It is also a good time to clean the face and tighten the glass cover. Mine rattles from time to time.

Not trying to be a smart alec but just how would I do that, since it rests at the stop position when the car isn't moving and it's kinda tricky to do at 60 mph.

If you put the car at 60 mph and time it over a mile or so. From your time you will know how fast the speedo SHOULD read when it actually reads 60. Remove the speedo from the instrument cluster. There are 2 ways to set the speedo.

Take the cover off the back so you can reach the round aluminum magnetic pick up wheel behind the needle. Roll the needle around to 60, hold the pick up wheel and then advance the needle to the calculated speed. Or...

If you have room between the needle and the face of the speedo get a very thin pair of needle nosed pliers. Rotate the needle to 60 mph, grab the speedo shaft so that it doesn't turn, and move the needle to the calculated speed.

Ah ok, cool. I've already done the first part. I know I'm 10 mph slow when it reads 60.

I'll give it a shot. I have a spare gasser speedo that I can try this technique on first so I can get some practice.