Originally posted by DemonSVT:
Originally posted by warmonger:
Look at it logically: The speedo only measures axle rotation via the differential. The diff and the tires spin the same speed all the time as they are locked together. The speedo is driven off the diff.
In this situation nothing has changed by changing final drive gears, therefore the speedo will still report the same number of wheel revolutions as it did before.

On my chart, 180 tire revolutions is 13 MPH. So even if the diff had a different final drive, once it reached 180 revolutions the speedo will read 13MPH.
I hope you can see what I mean after this, I can't seem to put it any clearer and the explanation isn't that great.
BTW:
On our cars that final drive ratio slot in the computer is worthless.



Woo hoo I get the argue linear logic vs computer programming...


The PCM most definitely uses the VSS signal for all it's speed calculations. We both agree on that.



Quote:


I also agree the diff itself will spin at the same speed while the output shafts (CV's) will spin slightly slower with the 3.84 gear.



I assume you mean the input shaft and internal gearing here? The axles and CV's will turn the same speed as the diff as they are plugged into it.
Maybe our error here is in defining our frame of reference. If your frame of reference is based on the same engine rpm with the 4.06 as it is with the 3.84 then this may be correct. However, I don't think that is the proper approach for this.

Originally posted by DemonSVT:

However that wasn't my point...

The PCM uses the figure entered in the axle ratio data point to calculate numerous functions of the PCM. (Yes based on the VSS signal and sometimes other sensors as well)
Since this figure is used for such things as odometer, speedometer, ABS wheel sensors, transmission shift points, etc all the calculations will be wrong since the actual output speed is slower with the taller gear ratio.
Since these calculations are now using a number that is no longer correct there will be incorrect/faulty readings because of it.




ONLY IF it uses engine RPM as part of the calculations...which it may.

Originally posted by DemonSVT:

BTW there is also a data point for tire diameter as well. It's called "tire_revs_per_mile" and it also affects all the same calculations.
More food for thought.




This would definitely be usefull and would affect the speedo output.


I can't help but feel that I am failing to get my point across. Other than the statement about the speedo being wrong with a final drive change, I don't have any disagreement. I'll try again as I'm sure you're going to do the old slap the forehead and say "I get it know" when I get you to see what I mean.

--The speedo is driven by the computer...agreed.
--The computer gets its signal from the VSS and calculates the speed as you have described....agreed.
--The VSS is driven by the speedo gear mounted on the diff housing....agreed???
--The diff speed is what the VSS actually is measuring....agreed???

***Assume stock wheels and tires***
--The diff speed is measured by the VSS in some fashion by corresponding to each individual revolution. It could be one electrical pulse per revolution or 2,3,4 pulses/rev though I suspect it is probably only 1 or 2....Agreed???

--Since the diff speed in Revolutions per minute is the same as the tire speed in Revolutions per minute, the VSS is directly measuring the tire speed in revolutions per minute. ...Agreed???

---Now, here is the point I'm trying to make:
the VSS is always measuring Tire revolution irregardless of engine RPM.
AT 53 MPH in 5th gear, the tires and the differential are turning 735.7 Revolutions per minute.
***with a 4.06:1 axle ratio, the engine RPM is 2300
***with a 3.84:1 axle ratio, the engine RPM is 2175

With the different final drive, the # of revolutions per minut of the tire is still 735.7 in order to get 53MPH, but you only have to go to 2175 engine RPM to get there.
Therefore the speedo drive system and the speedometer will always remain functional on this type of setup irregardless of the final drive you use.
******Another logical assumption*********
The PCM can't use engine RPM in its speed calculations. If the car were to stall out on the freeway, the key would still be on. Would the speedometer stop working? I hope not though I haven't tested it. This tells me that the speedometer displays only the tire rotational speed and that can only be taken from the VSS.

As far as all the other calculations I can only agree and it should not hurt to change it in your computer if you have acess to it. However, the bottom line will remain that the speedo should not be in error with just a final drive change.

( yes this is fun! )

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black