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Discussion Starter · #1 ·
Tried a search and couldn't find what I was looking for, So...
I know this is a dumb question, but what is the diameter of a 33x12.50 tire. I am using an RPM calculator and I need this diameter to get an answer. Thanks for any help.

Nick
 

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33"...unless you're looking for the true diameter, not just the number on the side of the tire. Some manufacturers will post these sizes on their websites. For instance, 33" MTR's are only 32.7".
 

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Discussion Starter · #3 ·
I think I need the true diameter...or do I? Maybe I am confused. Any one know if this calculator needs the "true diameter" or will entering "33" in the tire diameter box give me the desired answer? Here is a link to the calculator:

http://www.quiknet.com/~ke6vut/rpmcalc.html

Thanks
 

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unless you compensate for loaded radius at your running inflation psi and any parasitic losses due to clutch slippage or torque converter slippage (unless its locked up in the gear in question), then you will only get close anyway, so just use 33"
 

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All tires have different total heights... My old 34" swampers were about 33", my 35" BFG's are about 34".... From what I've seen most 33" tires are between 31-32" in real life.

Easiest method to figure it out is to just go measure the tire.
 

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Jack it up and measure it, if you measure it on the ground, it is inaccurate cause the tires sitting on the ground with a load on it.
 

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Why is this inaccurate? I think it would be more accurate to take into account the sidewall flex instead of the "ideal" diameter of the tire with no loads on it.
Think about the tire spinning at highway speeds and warming up. They will expand and take out the majority of the buldge. However I would also have to go along with everyone that 33 is close evnough, unless it is a bias ply claw they run almost 2 inches short.
 

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FJ40Trevor said:


Why is this inaccurate? I think it would be more accurate to take into account the sidewall flex instead of the "ideal" diameter of the tire with no loads on it.

Either way, I agree with everyone else, 33" is close enough.
well if you want to be super accurate, you would find the actual CIRCUMFRANCE and go from there

if you squish a circle into an elipse then mesure diagonloy thru the center you will allways get a diferent mesurement .

also keep in mind, new mud tires are generaly 1-2" taller than a bald tire....
 
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