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4th Gen Ram Tech2009 - 2018 Rams and the 2019 Ram Classic: This section is for TECHNICAL discussions only, that involve the 2009 - 2018 Rams and the 2019 Ram Classic. For any non-tech discussions, please direct your attention to the "General discussion/NON-tech" sub sections.
Unless/until he gets his speedo recalibrated he will not be able to get a accurate number. Although I would guess there may be a calculator on the net somewhere to make adjustments and calc it by hand. His odometer will be low (as his speed would be reading low with the larger tires).