md
Dodge Dakota
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10/04/2003
09:55:11
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 Subject: Question: Tire Diameter/Ride Height/RPM
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Message:
I have a question regarding the use of rpm calculators, such as the one at Randy's Ring and Pinion.
I have a 2000 Dak, CC, 4.7, auto, 4:10 gears. I am getting ready to put larger diameter tires on it. The affect tire diameter has on the true final drive ratio is important to me because I ocassionaly pull a boat weighing about 2500 lbs loaded - not a great load, but it gets "heavier" when pulling in the mountains on my way to northern Idaho. Consequently, 150 rpm loss or gain can be a bit more important to folks like me than someone driving the "flats".
When using the various calculators there is no mention on any of the sites I have found that discusses tire diameters published by the tire makers and actual ride height diameter (tire diameter as it sits under the weight of the truck).
My truck came with 28" diameter tires according to the manufacturers specs. When I plug that number into the calculator I get a different RPM than if I plugged in the ride height diameter, which is 27" the difference is just over 100 rpm at 65 mph. The ride height calculation is exactly what my tach reads. For my purposes, 100+ rpm is significant.
Have any of you had similar experiences using these RPM formulas? Is actual ride height of the tire the correct diameter figure to use in these formulas? I wonder why we don't see a discussion of these 2 ways of defining and measuring tire diameter on any of these sites - an inch difference, or slightly more on heavier vehicles, makes for a significant difference in RPM calculations.
Thanks,
MD
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