Speedometer abnormality?

[Pax84]

Just want to know if this is normal, my speedometer is off across the board it seems by 10 mph. When I’m going 10 on my Speedo I’m actually going 20, 5 mph is 15, 20 is 30. I didn’t go much faster than that so I’m not sure about faster speeds, it’s a rebuilt gauge cluster and I just got my Speedo cable problems worked out. My bronco has 4.11s and 33s with a 19 tooth drive gear.

 

[DirtDonk]

I’m not really good with the tooth versus tire size ratio thing, but we do have a chart floating around here or somewhere that shows what is appropriate.
But there are also 2 Different size Drive gears inside the different transfer cases on different models.
But yes, in answer your question, otherwise it is fairly normal for our speedometers to be off by quite a bit after tire changes.
These things had 28 inch tall tires when they first came from the factory, so the 33 is a big change.
Are you running a tire rated at 33 inches on the sidewall? Or are you running a metric tire that actually measures 33 inches?
There’s another difference right there.

 

[Oldtimer]

Looks like you need 18 tooth driven gear.

 

[DirtDonk]

The MPH difference should be a percentage. Not a specific miles per hour over all points.
You may have something else going on here besides mismatched gears.
And one tooth difference won’t net you a 10mph difference at 15 mph.

Did you test this multiple times? And how did you verify exact speed? Another vehicle, or GPS?

 

[Pax84]

Only really tested it once. I used a gps app on my phone which has been pretty accurate. I thought my Speedo wasn’t working because I couldn’t get it to move at slow speeds. I know have a 7 tooth drive gear as I’ve got a np205 t case. My tires are also rated 33 inches on the sidewall.

 

[DirtDonk]

Yes, something is actually wrong with the speedometer. It should start reading immediately, even if it’s reading incorrectly. Perhaps your cable is too long and jammed slightly up into the speedometer head. Or, perhaps the speedometer mechanism itself is a little gummed up.
As an experiment, try loosening the speedometer cable at the back of the speedometer.
Take a couple of spins off of the locking nut, or just let the nut/bezel hang loose.
Or is yours of the later clip style?

 

[Pax84]

I’ll loosen it a little and see what happens, I noticed when comparing my old speedo to the new one with the drill test that the new one needed a faster speed on the drill to start reading. It works fine and doesn’t bounce but like I said just doesn’t read right away.

 

[DirtDonk]

Well, the fact that it still acts wonky with the cable disconnected at the other end, and the drill motor on it, reduces the chance of the cable being too long and jammed up inside the instrument.
However, I have experienced a speedometer that would not work at all with the upper nut screwed on tight.
And this was with the original factory cable and the original speedometer.
Took the instrument cluster out once, I put it back in and the speedometer never worked right after that. If I left the cable slightly loose, it worked perfectly, and was right on the money. If I tightened it up, it acted like yours.
Unfortunately every few months it would come a little too loose and I’d have to reach up behind there and screw it back on one or two threads.
But it worked!

 

[Speedrdr]

However, I have experienced a speedometer that would not work at all with the upper nut screwed on tight.
And this was with the original factory cable and the original speedometer.
Took the instrument cluster out once, I put it back in and the speedometer never worked right after that. If I left the cable slightly loose, it worked perfectly, and was right on the money. If I tightened it up, it acted like yours.

So, Paul, are you saying you have a loose screw? Lol

Randy

 

[DirtDonk]

Only when I loosen it on purpose!

 

[Pax84]

Well, gave that a shot but still the same results or if I loosened it too much I got no reading at all.

 

[DirtDonk]

Oh well. I suppose it was worth a try. Hopefully you didn’t twist your neck or hurt your back/neck/wrist/shoulder trying to get up under there.

 

[Pax84]

No I’m all good lol, just installed my painless harness with the dash in so I was used to being under there. Guess it’s time for a new speedometer head.