Speed Calibration
Thread Starter
On The Tree
Joined: Jul 2009
Posts: 151
Likes: 5
From: GA
Speed Calibration Been trying to troubleshoot my spedometer reading, why the reason the faster i go the more span out from what my spedometer reads to acually how fast im going, for example, Spedometer reads 40 when im acually doing 70! been programming it with HPP3 withs a 3.42 rear and 25" tires (tires + rims). Any Suggestions? Could it be the speed sensor?
TECH Enthusiast
Joined: Nov 2004
Posts: 660
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This is certainly not advanced engineering.
You are simply using the wrong tire height or gearing in the programmer. The percent error in the calculation (caused by using the wrong numbers) means that as your speed increases the difference between actual and displayed (by incorrectly calibrated speedo) will increase as well. There's nothing wrong with the sensor.
Find an online tire size calculator to verify your actual tire height.
You are simply using the wrong tire height or gearing in the programmer. The percent error in the calculation (caused by using the wrong numbers) means that as your speed increases the difference between actual and displayed (by incorrectly calibrated speedo) will increase as well. There's nothing wrong with the sensor.
Find an online tire size calculator to verify your actual tire height.
Thread Starter
On The Tree
Joined: Jul 2009
Posts: 151
Likes: 5
From: GA
I have the tire size down and the gear ratio, the issue is the span difference, the faster i go the larger the span. Basic physics even with a different rear/tire size it would be constant. I Have worked every possible combination and it still spans out.
Thread Starter
On The Tree
Joined: Jul 2009
Posts: 151
Likes: 5
From: GA
i tried the 3.23 rear and the 2.43, i replaced the speed sensor and still have the spanning out issue, could it be limited to the tranny or ecu? or can it be the volt readings from the magnetics? im not sure what would cause this
TECH Enthusiast
Joined: Nov 2004
Posts: 660
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It could only be the 5 volt reference or incorrect gearing/tire size.
Either way it's not physics, it's mathematics. I already told you that the discrepancy between actual and indicated speeds WILL CHANGE WITH CHANGE IN SPEED. If you can't take my word for it then do the math at different speeds for yourself. You will find that indeed the span is greater at higher the speeds, but that the span will always be the same PERCENTAGE (ie; 6% off). It is not constant, even though you may think it should be.
Check your 5 volt reference. It's almost certainly an isolated issue (non PCM) otherwise you'd be having problems with lots of other sensors as well. I would still expect the problem to be erratic instead of smooth if there was an issue with the reference.
Then if you don't know your gearing from the RPO codes then maybe you need to climb under the car and spin the wheels/driveshaft and compare.
Either way it's not physics, it's mathematics. I already told you that the discrepancy between actual and indicated speeds WILL CHANGE WITH CHANGE IN SPEED. If you can't take my word for it then do the math at different speeds for yourself. You will find that indeed the span is greater at higher the speeds, but that the span will always be the same PERCENTAGE (ie; 6% off). It is not constant, even though you may think it should be.
Check your 5 volt reference. It's almost certainly an isolated issue (non PCM) otherwise you'd be having problems with lots of other sensors as well. I would still expect the problem to be erratic instead of smooth if there was an issue with the reference.
Then if you don't know your gearing from the RPO codes then maybe you need to climb under the car and spin the wheels/driveshaft and compare.
u try 2.73.....they came in my car from the factory
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