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  #1  
Old 08-06-2011, 10:26 PM
lsupcar lsupcar is offline
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Speedometer reading low

My speedometer is reading low. When the speedometer reads 30, GPS says 40, speedo 50, gps 60ish, speedo 60, gps 73ish, speedo 20, gps 25ish.
Yes, new tires , stock sized Michelin 225/50/16 on stock rims. I swapped back to old aftermarket 16 rims and the old original Bridgestone potenza RE95's in 225/50/16 earlier today, and the speedo remained off the same way. Both sets of tires were also roughly the same size, taking into account newer tread on the new tires. I also measured the new tire's revolutions per mile, and it measured out within specs at about 836.
Speed sensor 2 in place with no visible problems. No CEL or irratic speedo operation. At idle and at 3000 rpm the alternator reads 13.7 to 13.88 volts.
Any ideas?
Can't find this issue in a search.
BTW '92 with 155K miles.
Thanks.
Bill Z
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  #2  
Old 08-07-2011, 04:54 AM
1986nate 1986nate is offline
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Re: Speedometer reading low

Is your odometer also off by the same amount? If it is not racking up a mile after a true mile, your speed sensor is probably bad. If the mileage is correct and only the speed off, it is your cluster that is bad.
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  #3  
Old 08-07-2011, 12:48 PM
lsupcar lsupcar is offline
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Re: Speedometer reading low

Well, I tested the odometer and it reads slightly ahead of the gps mile. Speedo says 5 miles, gps says 4.9 miles, speedo 10, gps 9.8, speedo 15, gps 14.7.
To be more accurate as to the difference between the speedo speed and the gps speed: speedo 20, gps 26, speedo 30, gps 41, speedo 35, gps 48, speedo 40 gps 55, speedo 50, gps 70.
I appreciate the help.
Bill Z
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  #4  
Old 08-07-2011, 07:50 PM
1986nate 1986nate is offline
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Re: Speedometer reading low

Sounds like the gauge cluster is bad then. A good used replacement and it should be fine. I have spares of various mileage if you're interested.
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  #5  
Old 08-07-2011, 09:32 PM
lsupcar lsupcar is offline
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Re: Speedometer reading low

Is a bad gauge a common failure? What could have caused it to go bad?
Would the replacement gauge contain a new odometer? If so, could the mileage be made accurate?
Can you quote me a price on your best condition gauge? PM if you prefer.
Thanks.
Bill Z
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  #6  
Old 08-07-2011, 09:50 PM
1986nate 1986nate is offline
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Re: Speedometer reading low

Quote:
Originally Posted by lsupcar View Post
Is a bad gauge a common failure? What could have caused it to go bad?
Would the replacement gauge contain a new odometer? If so, could the mileage be made accurate?
Can you quote me a price on your best condition gauge? PM if you prefer.
Thanks.
Bill Z
I wouldn't exactly cause it a common failure but it does happen. An SVX I drove home from Florida last October had a bad cluster. Sometimes would read 10 mph, other times at highway speeds, would read as much as 25 mph off. The odometer was accurate as far as I knew. I didn't have GPS, but had printed off directions, that along with my gas mileage on the trip ruled that the odometer still functioned properly.
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  #7  
Old 08-08-2011, 05:16 AM
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huck369 huck369 is offline
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Re: Speedometer reading low

Is it an original geared transmission, or a lower geared transmission?
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  #8  
Old 08-08-2011, 08:00 AM
lsupcar lsupcar is offline
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Re: Speedometer reading low

Quote:
Originally Posted by huck369 View Post
Is it an original geared transmission, or a lower geared transmission?
It has a stock geared transmission and differentials ratios.
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  #9  
Old 08-08-2011, 07:24 PM
lsupcar lsupcar is offline
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Re: Speedometer reading low

FWIW, one other thing I noticed today is that when accelerating from a stop, the speedo needle does not move or come off the peg to register a speed until the car is moving about ten mph.
Bill Z
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  #10  
Old 09-05-2011, 09:20 PM
lsupcar lsupcar is offline
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Re: Speedometer reading low

As a follow up, I replaced my combination meter with a used unit and the speedometer is now accurate once again. Now i need to have the odometer adjusted to the correct mileage.
Bill Z
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  #11  
Old 09-06-2011, 03:39 AM
1986nate 1986nate is offline
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Re: Speedometer reading low

Quote:
Originally Posted by lsupcar View Post
As a follow up, I replaced my combination meter with a used unit and the speedometer is now accurate once again. Now i need to have the odometer adjusted to the correct mileage.
Bill Z
Glad you got it sorted out. Talk to forum member "Cam" as he was able to take a cluster and zero it out when he did his engine build.
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  #12  
Old 09-06-2011, 05:39 PM
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Re: Speedometer reading low

I took my spare apart and pm'd bill on how to roll the numbers...you have to remove the 2 screws and remove the odometer cover and clear guard under it to have access to the numbers.
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