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DemonSVT
Hard-core CEG'er

Reged: 09/13/00
Posts: 9602
Loc: KC, MO
Re: Speedometer Gear
#421721 - 10/04/03 04:31 AM

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.

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. * Re: Synchro part numbers OutlawdSVT 01/04/05 04:18 AM
. * Re: Synchro part numbers dukeofsho 04/07/05 03:19 AM
. * Re: Synchro part numbers Bugzuki 04/09/05 04:27 PM
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. * Re: Synchro part numbers ssmumich00 03/05/04 04:46 PM
. * Re: Synchro part numbers Swazo 03/05/04 06:33 PM
. * Re: Synchro part numbers Blackcoog 03/09/04 10:19 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ myfastse 03/02/04 11:04 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ CJ98 02/06/04 01:51 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ ssmumich00 02/06/04 04:37 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ CJ98 02/07/04 12:11 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ Blackcoog 02/17/04 09:19 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ Blackcoog 02/24/04 05:36 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ ieffem 02/26/04 01:38 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ ssmumich00 03/01/04 08:58 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ DemonSVT 02/06/04 08:43 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ bigerik 12/19/03 05:21 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo warmonger 12/19/03 01:35 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo bigerik 12/19/03 02:30 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ ssmumich00 11/24/03 05:39 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ Judge 08/18/03 08:58 PM
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. * Re: Part #'s Upgraded MTX 75 Transmission shift fo shotwell 08/22/03 06:34 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo Rev. Po-Jay 10/01/03 02:58 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo ssmumich00 10/01/03 04:03 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo Rev. Po-Jay 10/01/03 08:13 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo warmonger 10/02/03 02:07 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo Rev. Po-Jay 10/02/03 05:44 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift fo warmonger 10/02/03 08:34 PM
. * Speedometer Gear DemonSVT 10/03/03 03:48 AM
. * Re: Speedometer Gear warmonger 10/03/03 11:43 PM
. * Re: Speedometer Gear DemonSVT 10/04/03 04:31 AM
. * Re: Speedometer Gear brianl703 12/22/03 11:49 PM
. * Re: Speedometer Gear warmonger 10/04/03 02:34 PM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ Blorton 07/24/03 04:53 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ 1314 05/20/03 03:59 PM
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. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ warmonger 05/21/03 02:25 AM
. * Re: Part #'s Upgraded MTX 75 Transmission shift forks+ fordrule 06/25/05 09:16 AM
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