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Speedometer trouble


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Hi Ratsun, first post. I recently bought a 76 620. I've solved several little issues with the truck using some great threads on here but I haven't found the answer to my speedo problem yet. It is showing about 30 mph when I'm doing more like 50. I'll use a speed app on my phone to try to get more accurate numbers if that helps, but it seems that it's way too out of whack to be the wrong pinion in the gauge. I believe the transmission is original, it is a 4 speed. The odometer is working, possibly even turning too quickly. Any insight would be much appreciated. Thanks for the help. 

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Seems like, and actual, are subject to human error. Seems like 50 could be 43, you never know. Get some real readings to work on.


Several factors affect the actual speed reading.


The pinion in the transmission. Every increase or decrease of one tooth is about 5% change in reading.


The differential may have been replaced with one from a '79 which is a 4.11 and you WILL be traveling about 6% faster than if you had the stock 4.375 in it. If from a 720 it could be even lower like a 3.70 which would be 18% faster than the stock reading


You may have larger diameter tires on your 620. Larger diameter is like having lower numerical gears. A 2" taller tire is going faster by 9% over the stock 23.65" tire



Slower when it should read faster may just be the speedometer gauge (not the cable) that is rubbing something causing a drag. The cable turns a magnet that magnetically spins the needle. There is no direct contact. The cable cannot spin faster or slower on one end that the other for long, without breaking.

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Thanks for the quick reply Mike, I went out and took it for a spin to get some actual readings.


At 25mph my speedo shows 15, at 40 it shows 25, and at 50 it shows 30. I didn't take it to 60 because I'm on back roads and my brakes are total mush (no stopping power until they're on the floor, and pulling hard to the right). 


Does the pinion in the transmission have an adjustment to change the actual speed showing on the gauge, or is the pinion removed and replaced with one that has a different number of teeth to compensate correctly?


Also, is there a quick way to identify the differential to ensure it is original? 


The tires on my 620 are larger than stock. (215/75/16) I plan on going smaller for clearance to lower the truck a few inches. Should I wait to fiddle with my speedo until I have different wheels and tires?

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217/75R16s are 28.7" diameter. So 21% at 50 MPH is 10.5 MPH or about 40 reading. Could be this, and something wrong with the gauge.


Brake to the floor is likely the drums need adjusting, this will probably cure the pull to one side.


The speedo pinion can't be adjusted only changed.


Differentials all look the same on the outside. Place in neutral and raise one rear tire. Turn wheel 10 times and count the driveshaft turns and divide by 10. Likely 43.75 turns so you have a stock 4.375 differential. 41 is 4.11, 39 is 3.90 and 37 is 3.70


This choice of rim and tire is the same as putting a 3.6 ratio differential. It must be really slow on acceleration.

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I think I'll leave the speedo alone until I get some smaller wheels, and see if that corrects my problem somewhat. If it's close I won't tear anything apart to figure it out. If it still isn't close I'll check my differential with the method you suggested, sounds like a fun experiment :thumbup: and take out the gauge to have a look around.


As for the brakes, I tried to adjust them but my manual doesn't tell me to what extent they should be adjusted to drag. I have them so that there is some drag but not sure how much drag there should be...I replaced the rears, and bled the system. The PO replaced the fronts and I haven't been inside the front drums to see what he did, maybe that's my next step if further adjustments don't solve things. 


These are some really interesting numbers, no idea how you know this stuff but I'm glad you do.

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All standard 620s had 4.375 differential ratios except the '79 which was a 4.11. 620s came with 6:00 x 14 inch tires. (these two I knew) Today the equivalent is 175/70R14 an they are 23.65" tall and 6.85" wide.




There are sights on line for quickly converting tire sizes to metric and others for comparing them by their dimensions and still others that  by plugging a few numbers in can compare speeds, speedometer errors, RPMs and differential ratios.

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I think the problem is my gauge. I heard a slight rattling noise coming from what I thought was the engine bay, but since I was looking for potential problems with my speedometer I noticed it sounded more like it was coming from the dash. I unscrewed the speedo cable from the back of the gauge and the noise stopped. I'll want to pull the gauge from the dash and poke around in it to see if there's anything obviously broken. Maybe grease it? Any tips on getting the gauge out? I'll do some reading in the meantime!

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