You need to make sure the GR in the Gryphon is correct. Once it is, I would use 2709 cm circumference as a starting point and confirm distance or speed with your GPS.
If it's off, then use the formula:
(GPS miles/Truck miles) X 2709 = New Tire Size (cm, circumference)
Note that your speedometer will ALWAYS read 2-3 MPH fast at 65-70 MPH, even if your tire size and gear ratio are exactly correct. My odometer is dead nuts, but at 65 MPH (on my CTS), my speedometer reads 67-68 MPH.
I know this process seems confusing, but once you get the hang of this, it's really not hard (in case you change tire size again in the future).
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"Good judgement comes from experience. Experience comes from bad judgement." - Colin Powell to new batch of 2LTs (while serving as V Corps Commander)
Factory manuals here: http://www.motorcraftservice.com
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Tue, October 11th, 2011, 09:18 AM
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