Originally posted by DemonSVT:
News flash.

The axle ratio is programmed into the PCM.

I would be surprised if the 2 gears were different actually. We are not talking old school direct drive cables to a mechanical speedometer.


Yes that also means for all those that installed the 3.84 final drive your speedometer and odometer are now off. Well unless you let the PCM know via a chip of course.



I won't argue that the gear ratio is in the computer, or that you can change the speedo by changing the ratio. However, nothing else changes if you change final drive gears and leave the computer alone. You will not get speedo error. What you will get with a different final drive is lower engine rpm AT the SAME speed.

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.

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