Readjusting speedo for SVT tires/rims

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09-15-2010, 08:15 PM

fordguy427

Thread Starter

Join Date: Nov 2008

Location: Pa.

Posts: 21

Readjusting speedo for SVT tires/rims

I have a 07 GT and put a set of new car take-offs from a Shelby 500 and am in the process of readjusting my speedo. I have the Pro-Cal from Ford racing to do the adjusting. The front tire/rim revs/mile are 746 and the rear revs/mile is 748. My speedo is off 2 M.P.H. on the fast side. My question is....do I use the front or rear number to redo the speedo. I realize it is only 2 numbers off, but I might as well get it right rather than take the average between them.
Thanks in advance,
Lee

09-15-2010, 08:26 PM

SpartaPerformance

4th Gear Member

Join Date: Apr 2010

Location: Long Island, New York

Posts: 1,291

Rear tires

09-16-2010, 07:27 AM

Nuke

6th Gear Member

Join Date: Jun 2007

Location: PA to KY ('07) to IL ('09) to MS ('10) to FL ('11)

Posts: 16,182

+1 on the rear. And most handhelds only allow adjustments in 10 tire RPM increments. Add to that that the tire's RPM's per the specs are not always accurate so you may need to experiment until the speedometer matches your GPS.

09-17-2010, 08:57 PM

goose5

1st Gear Member

Join Date: Sep 2008

Location: British Columbia

Posts: 83

My speedo is 6km/h off (on the fast side) with the factory wheels and tires.... lol

09-17-2010, 09:22 PM

howarmat

s197 Junkie

Join Date: Sep 2006

Location: IN

Posts: 16,087

its the same size as factory basically (.2 of an inch off) so its not worth messing with it. you could actually end up further off than you were to begin with

09-18-2010, 10:13 AM

157dB

Cut & Paste Expert

Join Date: May 2007

Location: NW Arkansas

Posts: 13,322

Quote:

Originally Posted by howarmat

its the same size as factory basically (.2 of an inch off) so its not worth messing with it. you could actually end up further off than you were to begin with

+1
Unless you are extremely **** and just need to do something with the tuner.
Grab the tape measurer and see the ACTUAL RevsPerMile of the tires...
There are 63360 inches in a mile.
Divide by the tires ACTUAL circumfrence in inches and thats RevsPerMile.
Use the rear tires RPMs as they are the driven tires and are directly
connected to the VehicleSpeedSensor on the tranny via the driveshaft.
The fronts are not hooked to any VSS, just ABS sensors.

You can mark out a mile and start at one end with a mark on your
rear tire, drive to the end of the mile and have someone count the
revs. A magnet, reed switch, battery and counter could count the revs
at speed as the difference between rolling slowly and driving high speed
effects the rolling diameter of your tires. Once again, how **** are you?

Or you could just use your GPS to show speed and put tape over
the incorrect 2 MPH off OEM factory speedo if you are mister over the top ****...

Error on the top side (shows faster than you are going) and it might save you
from a ticket some time.