Speedometer Problem Issues

[1967KaiserM715]

So, this morning was cold, about 20 degrees. nothing to abnormal yet, drove truck to the gas station to get my morning coffee, everything was OK, but the minute I get to the highway(speed limit of 55MPH, but typical speeds are around 60MPH) I realized my speedometer was hovering right around 80MPH, and I hadn't even shifted into OD yet(6.5 Turbo-Diesel with NV4500 and NP208) typical speedometer reading with RPM reading of 2,000RPM is roughly 45MPH,

Also should say here, the Speedometer has always been a little off, I have 31" tires(that are measured at 28.5" actual diameter) and as such without changing the driving gear, my speedometer has been off by roughly +5MPH at 50MPH actual, or a reading of 55MPH

so anyways, this evening on the drive home I paid closer attention, it seems like the speedometer reads OK at speedometer readings under 35, but above 35MPH readings, it will read between 15MPH or 25MPH OVER indicated speed, and the needle is a little bouncy, actually it has been a little bouncy right at the 45MPH mark since I've had the truck, but steady otherwise until now.

any input is appreciated, Thanks in advance

[Factory Guy]

In my experience with other speedo's, not squarebodies, the oil in your speedo has probably gotten old and its time to send your speedo head off to a rebuilder. it could be the spring inside the speedo has gotten week as well.

[hatzie]

Calibrate it by changing speedometer gears.
The drive and driven gear tables are in the parts books.
You need the tire diameter and axle ratio.

When a speedometer is properly calibrated you'll find the actual speed is 3mph lower than the reading.

[1967KaiserM715]

Originally Posted by Factory Guy

In my experience with other speedo's, not squarebodies, the oil in your speedo has probably gotten old and its time to send your speedo head off to a rebuilder. it could be the spring inside the speedo has gotten week as well.

My dad was thinking the same thing, I have several other heads to change in(this is my Daily Driver), but over the weekend(Saturday afternoon, to this morning) the problem "solved" itself. but because today the temperature dropped significantly, and is again below 20, not to mention windchill; it has started acting up again. I'm actually leaning to the magnets getting weak in the head, vs old oil, I know cold weather can affect the magnets, although it can affect oil too.

I did notice, I think Saturday morning, I was hearing some noise from the speedo head, but not 100% sure. No insulation in the cab with a diesel makes many things hard to hear clearly.

Originally Posted by hatzie

Calibrate it by changing speedometer gears.
The drive and driven gear tables are in the parts books.
You need the tire diameter and axle ratio.

When a speedometer is properly calibrated you'll find the actual speed is 3mph lower than the reading.

I actually changed the drive and driven gears in the transfer case last summer(2018) in anticipation of changing tire size, plus it was way off anyways because I went from the stock 305/700r4/np208 to a 6.5/SM465(now NV4500)/NP208, the NP208 behind the SM465 came from a 1 ton truck. I knew I was going to change tires, and so I did the calculations in hopes of not needing to change the drive gear later, just the driven. Needless to say life got in the way and new tires got put on the back burner for now.

[sweetk30]

little light oil in the speedo cable . they get dry and can make it jump or read funny .