TH400 Speedometer values in Hp Tuners
09-06-2007, 10:52 AM
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TH400 Speedometer values in Hp Tuners I just put a TH400 in my car and I'm unsure of the values to enter to get the speedo to work correctly. Right now it reads like 80 when I'm only going 30. The trans has:
GM8655291 GM Speedometer Sensor
GM8655315 GM Speedometer Drive Sensor Rotor
It looks like this:
The car used to a 6 speed and these are the values I have right now.
General VSS Pulses Per
Mile: 47758
Rev-Trans: 17.000
Final Drive Ratio
Ratio: 3.90
Trans/Revolutions/Mile: 817.8515
Tire Size: 28.00
Gear Ratio 3.90
Calculated Tire 96.17
I think I have to change the Rev-Trans from 17 to 40? Because the reluctor wheel in the T56 had 17 teeth and this new one has 40. But other than that I m not sure of the calculations I need to perform. Thanks for any help you guys can give me.
GM8655291 GM Speedometer Sensor
GM8655315 GM Speedometer Drive Sensor Rotor
It looks like this:
The car used to a 6 speed and these are the values I have right now.
General VSS Pulses Per
Mile: 47758
Rev-Trans: 17.000
Final Drive Ratio
Ratio: 3.90
Trans/Revolutions/Mile: 817.8515
Tire Size: 28.00
Gear Ratio 3.90
Calculated Tire 96.17
I think I have to change the Rev-Trans from 17 to 40? Because the reluctor wheel in the T56 had 17 teeth and this new one has 40. But other than that I m not sure of the calculations I need to perform. Thanks for any help you guys can give me.
09-07-2007, 12:26 AM
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I believe you are correct.Using the 40t reluctor you will see 40 pulses per complete revolution of the driveshaft.
You're going to need to correct your VSS Pulses/Mile field due to the reluctor t count change. The easiest way to do this is 40/17=2.35*47758 (that should have been your original figure) You could now populate that field with 111514.93 if you like.
Due to your gear ratio your tire will require 3.9 driveshaft revolutions to complete one revolution.
The circumference of a 28" tall tire is approximately 87.92"
There are about 63360 inches in one mile
You will need to move 87.92" right around 720.65 times to cover one mile
Since your output shaft will have to spin 3.9 times in order to cause your tire to make one complete revolution I'd assume you'd then use 720.65*3.9 which gives you 2810.5548. SO, your transmission output shaft would rotate 2810.55 times for every mile traversed assuming everything on the road is perfect and your tire stays exactly at its specified height (this will not happen in the real world but it should be good for GM PCM land so we can be happy)
You'll notice this figure is relatively close to what you get if you divide the original reluctor wheel tooth count by the original VSS Pulse/Mile count.
I hope some of this information helps.
You're going to need to correct your VSS Pulses/Mile field due to the reluctor t count change. The easiest way to do this is 40/17=2.35*47758 (that should have been your original figure) You could now populate that field with 111514.93 if you like.
Due to your gear ratio your tire will require 3.9 driveshaft revolutions to complete one revolution.
The circumference of a 28" tall tire is approximately 87.92"
There are about 63360 inches in one mile
You will need to move 87.92" right around 720.65 times to cover one mile
Since your output shaft will have to spin 3.9 times in order to cause your tire to make one complete revolution I'd assume you'd then use 720.65*3.9 which gives you 2810.5548. SO, your transmission output shaft would rotate 2810.55 times for every mile traversed assuming everything on the road is perfect and your tire stays exactly at its specified height (this will not happen in the real world but it should be good for GM PCM land so we can be happy)
You'll notice this figure is relatively close to what you get if you divide the original reluctor wheel tooth count by the original VSS Pulse/Mile count.
I hope some of this information helps.
