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.

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.

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.

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.


Now the difference between 4.06 and 3.84 is only ~5% so it's not going to jump out and make a huge difference.

Heck the deviation of the stock Speedo is probably that bad above 60-70mph anyway...

2000 SVT #674 13.47 @ 102 - All Motor! It was not broke; Yet I fixed it anyway.