# Speedometer Milage Reading



## firebird (Feb 6, 2011)

can anyone tell me from which wheels, front or rear, is the milage on the car calculated?

website to wheels and tire sizes change and how it will effect the speedometer and milage...
http://www.performanceprobe.com/ind...nceprobe.com/misc/index.php?do=calc&page=tire


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## HP11 (Apr 11, 2009)

firebird said:


> can anyone tell me from which wheels, front or rear, is the milage on the car calculated?


If I'm understanding what you're asking, the speed sensor is mouted in the rear housing of the transmission on the 6 spd and I believe it's inside of the automatics.


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## firebird (Feb 6, 2011)

reason i'm asking is because i have 235/40/18 on my 06 and according to factory description, it came with 245/45/17. i am not sure if the person before me changed these and if speedo was corrected. i will have to use the gps to find out. if speedo was not reprogrammed to new tire size, the car is getting more miles on the clock than it should as the diameter of the 18is smaller than of the 17...

also i am trying to figure out from what wheels is the sensor getting the info to how many miles it will put on the clock. is it front or rear. thinking of putting 255/45/18 in the rear but if sensor is reading from the rear wheel, it will read less miles without being reprogrammed. if it's the front wheels then i would match the front to 245 40 18 to correct the speedo and throw a bigger one in the rear without thinking twicee about it.


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## AlaGreyGoat (Jul 6, 2006)

235/40/18 are about 25.4 OD and the 245/45/17 are about 25.7 OD.
Not much of a difference. I thought stock 18 were 245/40/18 that are
the same as 245/45/17. There are speed sensors on each wheel for
AntiLock and Trac Control. Don't know which ones run the speedo.

Come to think of it, it must use the fronts, as the speedo doesn't go up like the
tach when I'm smoking the rears.

Larry


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## LS2 MN6 (Jul 14, 2010)

Mileage is calculated off the TOS signal (Transmission output sensor), but if that has an issue then it uses the wheel speed sensors. The stock system uses stock tires and rear end for calculations based on the sensors. If you change them you need to update the ECM and the ABS module or else the TCS and Speedo will be off. Typically these are changeable values in the module although it's best to try and keep everything close to stock.


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## firebird (Feb 6, 2011)

It's not off by much, for every 10K miles these add up to 110 miles on the clock, but i would like to get close to stock or for it to read slightly less miles on clock. 

thanks LS2 for this info and for the earlier info you provided. i guess i will look into 245 40 18 for the front and 255 40 18 for the rear, which will put me in oposite direction when it comes to miles. am i correct? this set up will show less miles on the clock?


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## AlaGreyGoat (Jul 6, 2006)

I was wrong, the speedo goes up with the tach when you smoke the
tires. Must use the rear sensors.
Proved it a few time going home last night.

Larry


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## HP11 (Apr 11, 2009)

The answer actually in post #2 with a more complete explaination in post #5. That would still cause the speedo to go up when you smoke 'em.


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## Poncho Dan (Jun 30, 2009)

According to Description and Operation in the service manual: "The [DIC] receives vehicle speed information from the PCM. ...The PCM receives vehicle speed information from the vehicle speed sensor located in the transmission assembly."

So its not taking/showing signal from the sensors on the axle stubs, but rather in the trans.
I learned something today. 

You can pull up a PID called "undriven wheel speed" in HPT. It must be taking that from the ABS module...


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## HP11 (Apr 11, 2009)

That's where I read it.....


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## Poncho Dan (Jun 30, 2009)

Description and operation can be a great starting point for beginning diagnosis. I wish the book would call them out better, it took me a while to find it.


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## LS2 MN6 (Jul 14, 2010)

GM typically has 3 speed signals.

Vehicle Speed from the ECM (this is calculated from the TOS)
Driven Wheel Speed from the ABS (this is calculated from the Vehicle Speed Signal, and uses the Wheel Speed sensors as a backup if Vehicle Speed isn't available).
Non-Driven Wheel Speed from the ABS (this is calculated from the Wheel Speed Sensors directly)

The Wheel Speeds themselves are also sometimes broadcast by the ABS module on GM LAN as well. Also technically the ABS is called an EBCM (Electronic Brake Control Module). GM uses ECM (Engine Control Module) not PCM (Powertrain Control Module) as the official name of the engines computer as Automatic Transmissions have a separate TCM (Transmission Control Module). Typically PCM refers to a module that acts as both the TCM and the ECM.


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## firebird (Feb 6, 2011)

great info


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## Poncho Dan (Jun 30, 2009)

LS2 MN6 said:


> GM typically has 3 speed signals.
> 
> Vehicle Speed from the ECM (this is calculated from the TOS)
> Driven Wheel Speed from the ABS (this is calculated from the Vehicle Speed Signal, and uses the Wheel Speed sensors as a backup if Vehicle Speed isn't available).
> ...


So if you've got an automatic (say 4l60e), you're receiving your vehicle speed signal networked through the TCM, or is that a direct signal to the ECM?


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## LS2 MN6 (Jul 14, 2010)

Poncho Dan said:


> So if you've got an automatic (say 4l60e), you're receiving your vehicle speed signal networked through the TCM, or is that a direct signal to the ECM?


The TOS is typically going to the ECM. The TCM handles stuff like shift control and reporting the gear the transmission is in. Things that the manual transmission has as well (like the TOS) GM tends to route directly to the ECM. This is to make the wiring harnesses on the ECM identical.

EDIT: I've just been reviewing the wiring. It seems on the Auto there is a different TOS (on the GTO called Vehicle Speed Sensor) than on the Manual. The Automatic TOS communicates directly with the TCM which creates the VSS line (that's the hard line speed signal that aftermarket Nav systems tap into on the cluster) the ECM will most definatly still create the CAN message, but it will create it based on the VSS line coming from the TCM. I'm in the process of looking it up on the manual.

EDIT2: In the manual, the ECM gets the TOS signal direct form the sensor. Also this would mean that in the GTO with the varying systems that the primary spedo signal isn't even on the vehicle network but rather the VSS line which goes form the TCM to ECM to Cluster (or just ECM to Cluster on Manual car).


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## firebird (Feb 6, 2011)

another question, sorry for the wrong topic, but it seems i might get an answer from you guys quicker in this thread. bare with me as i am still learning this car...

my car sits in the garage, and i open the door, the clock is always on(so i hear the clock on is normal), and i press the on button on the radio, just out of curiosity and it turns on.

so the question, is the radio supposed to be able to work with ignition off and open/closed door all the time when you press the ON button? or someone jack something up?


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## LS2 MN6 (Jul 14, 2010)

firebird said:


> another question, sorry for the wrong topic, but it seems i might get an answer from you guys quicker in this thread. bare with me as i am still learning this car...
> 
> my car sits in the garage, and i open the door, the clock is always on(so i hear the clock on is normal), and i press the on button on the radio, just out of curiosity and it turns on.
> 
> so the question, is the radio supposed to be able to work with ignition off and open/closed door all the time when you press the ON button? or someone jack something up?


Radio always has power. On many cars you can turn on the radio now with key out (my 2007 Saab 9-3 radio would come on if you hit the power button).


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