Speedometer Error *NOT FROM THE SPEEDO GEAR TEETH COUNT*

[Tactical Bacon]

So as the title says, the speedometer in my Manche is off by 5 mph all the time. At 10 mph it says 15, at 30 it says 35, at 70 it says 75, and so on. Odometer tracking doesn't appear to be affected by this discrepancy. I can't find anyone with the same issue, every thread I find is people having their speedometer off because of re-gearing or changing tire sizes (which I do have but that didn't start until after I went to a new tire size and is separate from this issue (I also have the new gear to fix this)).

Any ideas?

[89 MJ]

When the engine is shut off, does the speedometer return to 0? Does your truck have a speedometer cable or is it electronic? I wonder if it isn’t an issue with the sender if you have an electric speedometer (which HOs do have, IIRC) 

[howeitsdone]

I'd start with swapping in that new gear. It's going to change your results.

[Pete M]

are the HO speedos adjustable at all? 

[gogmorgo]

Is it possible that the needle has slipped on the gauge? That's really the only thing I can think of, the gauge or needle isn't aligned properly. The speedometer is measuring the frequency of the pulses from the speed sensor, so it's unlikely for it to be off by a fixed amount at any speed.

[Pete M]

I pulled one of my HO speedometers out of the cluster and don't see any obvious way to adjust anything.  

[Tactical Bacon]

On 11/30/2022 at 7:39 PM, 89 MJ said:

When the engine is shut off, does the speedometer return to 0? Does your truck have a speedometer cable or is it electronic? I wonder if it isn’t an issue with the sender if you have an electric speedometer (which HOs do have, IIRC) 

 

It returns to 5 MPH, needle completely horizontal, which is the normal resting position as far as I can tell based on pictures of other speedos around the internet. It's the MJ in my sig so an HO with an electric speedo.

 

14 hours ago, howeitsdone said:

I'd start with swapping in that new gear. It's going to change your results.

 

Well, yes, it will, but it isn't going to fix the problem. I bought the truck with ~27.4" diameter tires which was the factory tire size for my MJ and the speedo was off by 5 mph then. I even noticed it the day I got it when I was getting passed by everyone on the interstate even though the speedo said 75. Pulled out the GPS on the phone and it said 70.

 

2 hours ago, gogmorgo said:

Is it possible that the needle has slipped on the gauge? That's really the only thing I can think of, the gauge or needle isn't aligned properly. The speedometer is measuring the frequency of the pulses from the speed sensor, so it's unlikely for it to be off by a fixed amount at any speed.

 

It's definitely a possibility, needle in the wrong spot on the spindle was my first thought, too. I had a similar issue when I replaced all the instrument panel bulbs in a 95 Ranger I owned. It required removing the needle from the speedo to get to the bulbs and when I reassembled it my speed was off. Manual said to supply a specified voltage to the speedometer then apply the needle to the gauge pointed at a specific speed. Internet said to install the speedo in the truck and slide the needle on while driving down the road with a gps or someone pacing you. What I did was guess, test drive, and guess again. This may apply here, I just can't find any information on it and didn't want to get to yanking parts before I asked to see if someone else knew.

 

2 hours ago, Pete M said:

I pulled one of my HO speedometers out of the cluster and don't see any obvious way to adjust anything.  

 

I'll probably end up yanking mine this weekend after I swap the gear and see how far I can take it down. Hopefully I find something and don't need to buy another speedometer.

[gogmorgo]

I’ve only got metric speedometers so can’t really comment on where it sits at rest, roughly 10km/h for me, but it does rest on a stop. I had one instance where the speedometer lost power past the upper middle of the gauge and dropped clockwise to straight down. Then when it got power back it pushed itself back up clockwise against the wrong side of the stop. It dropped to straight down after I shut it off and then went back up to below the stop every time I turned it back on. I pulled the cluster apart and just spun it back around to correct it, and also resoldered the broken points that made it drop out.

That’s a story for a different thread, but this is all to say the speedometer needle doesn’t stop at a specific indicated speed, it just parks itself on the stop. So if it happened to be misaligned, it would just sit on that stop regardless. And it’s fully possible if what happened to me above happened to someone else, they may have pulled the needle and put it back on above the stop, maybe your 5mph in the difference, without knowing better. 
All of the Jeeps I’ve owned have also had the speedometer out by roughly 10%. I’ve just interpreted this as the vehicle wanting bigger tires. But you see people saying their speedometer is out by X mph, likely because they only checked at one speed, and then with prompting discover it’s a percentage. It’s possible someone who knew just enough to be dangerous saw this and did something about it, 5mph under at 50, move the needle to incorrect it instead of getting a new speedo gear.

But we can speculate all we want, and it doesn’t really get anywhere. As has been suggested, my action would be to put in the new speedometer gear, see where that gets you, then see about popping the needle off and moving it if you still need to. 

[Tactical Bacon]

Late follow-up, but I found the source of the problem. For anyone else with a similar issue, check your vehicle speed sensor. The nut where the VSS screws onto the speedo gear housing like a cable would was loose. I tightened that to snug and my speedometer read accurately with the exception of the variance caused by tire size, subsequently resolved by changing the gear.

 

In hindsight, it would've been obvious had I known what the VSS looked like on these, I've never actually put eyes on one before. I had almost the same problem in the past with an 80 Firebird I used to own.

[Pete M]