md
Dodge Dakota
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9/29/2003
11:30:00
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 Subject: Tire Heighth VS Ride Heigth/RPM
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Message:
I'm going to a a taller tire after installing 4.10'2 in my 2000, 4.7, auto, Dak. I sometimes pull a boat that weighs approx. 2700 lbs going down the road, so RPM in 3rd and OD is important.
I currently have the OEM 255/65/15's - listed tire diameter is 28", but ride heigth is about 27"
I notice that using Randy's Ring and Pinion calculator, and plugging in 28" for the diameter and .75 for the overdrive, at 65 mph it shows an rpm of 2399. In real life I am running at 2500 rpm.
When I plug into the calculator the ride heigth (diameter taken with weight of the truck on the tire) of 27", I compute out to just a tad under 2500 rpm.
Question: When determining the tire size I can reasonably go to, all else being equal (like sufficient clearance), should one use the actual ride heigth of the tire and wheel, as opposed to the tire manufacturers listed diameter??
In this example it makes a difference of 100 rpm - that becomes significant when pulling a boat up a grade.
Any thoughts?
Thanks,
MD
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