Speedo Running ''Slow''

['68OrangeSunshine]

So I still have the original speedometer in my '68 C/10 Stepside, a truck I bought at 23, and now I'm 75. It's got 407,000 showing on the odometer. [I bought it at about 80,000. It was a 5 year old truck.]
Just lately it's been indicating ~40-45 MPH when GPS says I'm doing 50-55.
I checked the cable connections: Underdash was secure. The cable to the correction box was secure. Corrector was secure, with a little swing to it -- which I think is normal.
Next step will be to pull the little speed corrector box -- maybe it's turned to jelly inside after a number of years -- IDK.
A few years ago, I pulled the inside speedo cable out and dusted it with graphite powder as I inserted it back in. That took out the jumpiness. Not jumping now, but it won't go over 40 or 50 indicated.
Long time ago, because I went to taller tires, it consistantly read slow. I took it to a shop. They mounted me on a dyno, and they called out what speed my wheels were turning, and I noted what number was indicated. I made a Correction Scale and it's still dymo-taped to my dash above the ash tray.
If, after I pull the Correction Box, I get the old correction numbers, then we'll know that's what it is. Otherwise it's in the clockwork of the OEM Speedo -- and after only 407,000 miles. Shocking!

[pjmoreland]

It could be that the magnet in your speedometer is tired. I have seen that in most of the gauge clusters I have rebuilt. The magnet can be remagnetized, but it does require complete disassembly of the speedometer.

It will be interesting to see what happens when you remove the correction box.

[pjmoreland]

https://youtube.com/shorts/tMaFDdT_4...ETt9c4hjMTM2XI

[jeffahart]

you can try flipping the box. On edit: I meant the spedo adapter.

j

['68OrangeSunshine]

I pulled it. My indicasted speeds were waaay low. Like 35 indicated for GPS 50 MPH.
40 was 57.8.
Box was pretty greasy.
Must be Patrick's tired magnets.

[pjmoreland]

If you send your speedometer to me, I can calibrate it.

['68OrangeSunshine]

Originally Posted by pjmoreland

If you send your speedometer to me, I can calibrate it.

Deal.
Working tomorrow thru Wednesday.
I might have another Dash core on the property, I can put that Speedo in the hole in my dash if I send it.
Odometer is collaterally affected. Showing less miles than actually travelled.

[pjmoreland]

Or you could send your extra speedo if in decent shape, and I could adjust the odo to match the currently installed one while calibrating it.

['68OrangeSunshine]

Originally Posted by pjmoreland

Or you could send your extra speedo if in decent shape, and I could adjust the odo to match the currently installed one while calibrating it.

That's a thought. But it might be from a '70 with a round Red Dot for Brights On, instead of the Red Hyphen for '67/'68. Not that it matters.
I will have to root around for the extra Speedo. Not gonna cannibalize White Fang. ['67 K/10 Sub.]
Be cool if you coud add a 4 in the hundred thousands column.

Would you like to inspect the Correction Box too? It's pretty greasy. Gets motor oil off the bottom of the block, then all that graphite I poured down the cable shaft accumulated in the Box.

Can't ground #919 until next Friday 3/06, as I like to have a spare rig ready to get to work if the Tahoe won't start.

[pjmoreland]

I'd be happy to look at the correction box, too. I get it about the rectangular vs. circular high beam indicator. I care about details like that.

['68OrangeSunshine]

I call the three periods of the '67 thru '72 Chevy Truck Run, Phase I 1967/68, Phase II 1969/70 and Phase III 1971/72.
I like to keep my Phase I Orange Stepside period correct. Although a 1969 Thunderbird bucket-back bench seat [electrically adjustable] is pure fantasy hotrod. But for the rest, it's still a simple fleet truck.

On the '71 GMC Jimmy, I have long plotted to make some of the interior more First Phase like. Primarily so I don't have to think about which year truck it is while dealing with traffic. Last year, I pulled the '68's Wiper knob off in my hand trying to turn on the headlights. [Thinking it was the '71] So moving the '71's Wiper over to the left, and lowering the Headlight swiitch is planned. A retrofitted Manual Choke is already on the right. Adding Manual Throttle will be difficult. I have the Knob and Cable but getting it to actuate is beyond my technical expertise.
I have a spare 3-Spoke 1st Phase Steering Wheel and GMC horn cap, so that would drop in place.
The gullible may deduce I have a '68 MY Jimmy when that all comes about. Not sure how I'll handle that. Case-by-case is best. An expert will know it's modified.
I don't mind being thought of as a Smart A$$, but don't want to be a Liar, either.
I guess like Johnny Cash would say,
''It's Sixty-Seven, Sixty-Eight, Seventy, Seventy-One General Motors Automobile...''