Inaccurate speedometer needle

bromel

bromel

TJ Addict
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Oct 11, 2018
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Montauk, NY
First off, I have factory 3.73 gearing in my 2000 Sahara and the correct speedo gear installed in the transfer case. The speedometer needle overreads by approximately 3.5mph at all speeds. The difference is not a percentage; i.e. it's always off by the same amount whether I'm going 20mph or 70mph. (The TJ does have a very slight 1% overread in addition to this, which is normal and to be expected).

I've used an OBD-II scan tool at speed, and the computer's speedometer is nearly dead on with a GPS. The odometer is similarly accurate. It's only the speedometer needle that is off. I assume I have to live with this as I have for the past 2 years, but does anyone know of a fix short of replacing the cluster? Would flashing the ECU do anything?

Chalking it up to a Jeep Thing, but curious if anyone else has had this problem.
 
I don't know for sure what the tooth count is, but the Jeep has been stock it's entire life. I imagine it's the factory spec (34 teeth I believe). If the speedo gear was wrong, my understanding is that the needle discrepancy would increase with speed, not remain constant.
 
You can tilt your head a bit more to the right while driving :ROFLMAO: (sorry couldn't help myself)

I've never heard of a problem like that... If you disconnect the battery for 15 min it will reset the computer. Maybe it will help... I doubt it though.
 
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bromel said:
I don't know for sure what the tooth count is, but the Jeep has been stock it's entire life. I imagine it's the factory spec (34 teeth I believe). If the speedo gear was wrong, my understanding is that the needle discrepancy would increase with speed, not remain constant.
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You bought your Jeep new so you know that a previous owner could not have changed the speedometer gear for if it had different size tires at one time?
 
Jerry Bransford said:
You bought your Jeep new so you know that a previous owner could not have changed the speedometer gear for if it had different size tires at one time?
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The previous owner explained that he never put bigger tires on the Jeep and kept everything stock. It's certainly possible that he wasn't being truthful, but I think the symptoms of my problem rule out a speedo gear issue since the OBD-II speed is accurate with the GPS and the odometer is accurate with the GPS. It's only the needle itself that is inaccurate.
 
You could recalibrate the speedometer with a new speedometer gear or install one of the electronic speedometer calibrators. Personally I'd just go one tooth higher on the speedometer gear tooth count to slow the speedometer a tad. If your TJ has a 34 tooth speedometer gear I'd try a 35 tooth gear. I'd physically take it out first and count the teeth before ordering a replacement. It only takes 5 minutes and a 1/2" wrench to replace it.
 
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Yes, I guess I'll put in a 35 tooth gear so the speedometer needle will at least be accurate. Only problem is that my odometer will no longer be dead on. I suppose if I ever want to sell the Jeep, the miles will pile on slower :)
 
bromel said:
Yes, I guess I'll put in a 35 tooth gear so the speedometer needle will at least be accurate. Only problem is that my odometer will no longer be dead on. I suppose if I ever want to sell the Jeep, the miles will pile on slower :)
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The speedometer and odometer are both either accurate or inaccurate. If the speedometer is made more accurate the odometer will be more accurate too. They are not separate entities since both are driven by the same speedometer gear.
 
Jerry Bransford said:
The speedometer and odometer are both either accurate or inaccurate. If the speedometer is made more accurate the odometer will be more accurate too. They are not separate entities since both are driven by the same speedometer gear.
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I understand that, but the crux of my problem is that the odometer and the speedometer needle are behaving like separate entities. That is why this is so strange. The odometer is nearly identical to the GPS at all speeds. When I hook up the OBD-II scanner, the speed it reads out is nearly identical to the GPS. It's only the speedometer needle that is inaccurate, as if it's misaligned. (Maybe that is actually the problem, come to think of it). If I'm moving at 15 mph per the GPS, the needle indicates about 12. If I'm going about 70 per the GPS, the needle indicates roughly 67.
 
So the speedometer is actually reading too slow, you said it "overreads" above which I took to mean it was indicating a faster speed than you are actually traveling. In that case I'd try a 13 tooth speedometer gear, not a 15 tooth. Or one under the count of whatever is installed. I'd count the current speedometer tooth count, who knows... the factory might have run out of 14's and installed a 15 tooth gear.
 
I apologize, I'm in a hurry and I wrote that wrong. Reverse what I said: If I'm moving at 15 mph per the GPS, the needle indicates about 18. If I'm going about 70 per the GPS, the needle indicates roughly 73.

Jerry, I'm sure I'm not effectively communicating the issue here to you, but I'm fairly positive my problem is not related to a speedo gear. My odometer is very accurate, tested by a GPS over hundreds of miles. The OBD-II readout of my speed, which spits directly out of the ECU, is very accurate compared to the GPS. It's only the physical needle itself that is not accurate, as if it was misaligned in the factory by 3.5 mph. If the speedo gear was inaccurate, my understanding is the odometer would be off by the same percentage as the needle. Moreover, the needle would be off by a smaller margin at low speed compared to high speed.

I'll post a video of my problem tomorrow so you can see what I mean.
 
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