Hi Craig
While I've not made a tire replcement I have exactly the same factory
setup as you and was thinking of replacing with that 31x10.5 too. My dealer
told me that any tire available as factory standard or option can be reprogrammed
for your speedo by the dealer. He said it's such a simple and quick task
that they don't even charge labor for it. I don't know why he limited his answer
as I would expect all tires diameters would be possible??
This diameter tire will turn your engine slightly slower at all speeds.
Whether it's enough to show a small mileage increase or not might rest with how
you drive. My truck delivers a flat 20mpg whether I drive 55mph or 65. Faster
than that it starts to fall off.
The reduction in gear ratio quoted to you (I don't know if that figure is
correct or not) simply takes into account that the larger diameter tire turns
fewer full revolutions per mile than your existing
As for advice, there's certainly nothing "harmful" to the truck. If your
truck had a manual transmission then it's factor standard axle ratio would have
been 3.21 and of course those same original tires. Your factory rims are at
least 7" wide which should be satisfactory for the tire you're considering. You
do not need to raise the suspension to accommodate this tire under your
fenders I don't think. This is a Dodge standard tire size on some Dakota models.
If it's only with 4wh dr then those trucks do stand higher than yours if yours
is 2 wh dr. Check your dealer for advice
Paul Sahlin
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