Speedometer

[Trod8812]

My speedometer says 20mph when I am around 33, what can cause that. All of the gauges work so I'm not sure is the cable is bad and where the cable is located.

[datzenmike]

Is the reading steady? or jump around??

 

What size tires do you have on your 620

 

Have you replaced the transmission? Or differential??

 

What year of truck??

 

 

Fill out your profile with year  of truck

[Trod8812]

The reading is stable

They are 195/70/r14 the previous owner had 18 inch Rims

It's a 1979

And no I have not replaced the transmission or differential but the pervious owner said he rebuilt the 5th gear.

[datzenmike]

The '79 was an odd year for the 620. It's the one year when a 4.11 differential was used rather than a 4.375. If it had an earlier transmission the speedometer pinion gear would be turning 6.5% slower. You are out over 30% I can't account for that much.

 

If you remove the speedometer pinion it should have a White plastic 19 tooth gear. A Red 20 tooth gear is for a 4.375 and will turn the speedometer slower, but only 6.5% slower. That's 2 MPH at 33.

 

This much out must be the speedometer itself.

[Trod8812]

Ok well when I bought the truck the guy gave me an extra guage assembly, would you recommend me putting that in? And how would I go about installing it?

[flatcat19]

Rebuilt 5 speed...what if truck never came with 5 speed? Or he rebuilt a different 5 speed then installed.

 

Never know what's happened in the last 40+ years with these things.

[hobospyder]

Gps speedo on your phone or buy speed hut gps speedo

 

Came with 14x6 bias ply tires, modern equivalent doesn't quite match up, and like flat said, who knows what's happened in 40 years

[datzenmike]

Reach up under the dash and unscrew the speedometer cable. Remove the cigar lighter, radio knobs, (I forget the rest) you'll see anyway. There is a large round 10-12 pin connector on the back of the gauge cluster that just needs unplugging.

[Trod8812]

Where is the speedometer cable on the transmission?

[datzenmike]

Passenger side at the rear.

 

It's the round thing on the bottom. This is the pinion sleeve. It mounts on the tail of the transmission and is gear driven. The speedometer cable screws onto it.

 

 

 

 

Top is the speedometer pinion. Below it is the cable end.

 

[Trod8812]

Ok so I don't have any way to jack up my truck at the moment but I can reach the cable, do you know what size wrench it is or the best tool to get the cable off?

[datzenmike]

A careful squeeze on a pair of vice grips will usually loosen it so it can be turned by hand. You'll see there are flat faces  that an adjustable wrench can get a good hold on. Generally it is not on very tight.

[Trod8812]

I really appreciate all of the help, always looking to make the truck mint. This is when I first got it.

 

And this is it today

[datzenmike]

Vast rim improvement.

[flatcat19]

All trucks eventually go black.

[sebpv]

can you show me other angles now that its painted please. I was planning on painting it black but it only looks better in my head...

[Trod8812]

 

[Trod8812]

Passenger side at the rear.

 

It's the round thing on the bottom. This is the pinion sleeve. It mounts on the tail of the transmission and is gear driven. The speedometer cable screws onto it.

 

 

 

 

Top is the speedometer pinion. Below it is the cable end.

 

What is the part called that the speedometer Cable screws into?