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Old 06-19-2005
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Tire sizing and RPM

My truck has the 4.10's and when I cruise on the highway at 75, my RPM's are at 3,000 with the stock 235/75/15 tires! I want to stepup to some 31s or 32s. I basicaly want to know how much lower i can get the rpm's or how low they are gonna go. Is there some sort of conversion, or chart that anyone has, or what RPM does your truck run at? thanks
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Old 06-19-2005
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it should be around 2000ish. are you concerned about it being low or are you wanting to lower it? because running 31's or 32's wont hurt your truck at all. hell i run 32's on a 4cyl
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Old 06-19-2005
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I want the rpm's to be lower, if they are 2,000 then hopefully i'll save a lil gas (until i hit a hill lol)
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Old 06-19-2005
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you should be fine go get some 32x11.5x15 you will love it and never want to go back i know i that way.
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Old 06-19-2005
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bigger tires would definately help keep the rpm's down. Right now with 3.55's and 30" tires it reads exactly 2k RPMs at 60, which is 65 when you factor in speedo error. I've got some 25" burnout tires and they kill my top end, I hit the governor and people are still passin me like nobody's business. With the bigger tires the governor doesn't kick in till 100 actual mph.

here's a decent calculator for figuring out speedo error and stuff. And here is one for figuring out optimum rpm/tire size/gears. Hope this helps..
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Old 06-19-2005
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thanks guys, im just a lil concerned about braking now if I go upto 32's....
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Quote:
Originally Posted by a311fanam
thanks guys, im just a lil concerned about braking now if I go upto 32's....
YOU WILL BE FINE.....
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Old 06-19-2005
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To figure the new RPM @ 75MPH after the tire switch, use this formula:

(old diameter/new diameter) x current RPM @ 75MPH.

You can get the actual tire diameter of a P-metric tire like your 235/75R15 from one of the calculators listed in the posts above.

Remember that most "inch size" tires are usually somewhat smaller than the marked size implies. A "31" is often 30.5~30.7" in diameter, for example. You can get an accurate diameter number from the manufacturer's website specs of the replacement tire you have chosen.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The required brake torque increase due to the tire size change will be equal to

[(new diameter/old diameter) - 1] x 100%

Your truck will still stop but you will need to apply about that much extra pressure to the pedal to make it happen.
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Old 06-19-2005
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Thanks bob! rpm's should be around the 2,700 range
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Old 06-19-2005
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ok know that you know why isnt the tires on yet
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Old 06-19-2005
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Quote:
Originally Posted by Redneckstone
ok know that you know why isnt the tires on yet
I need about $650 for some BFG a/t 32", and about 400-500$ for some alloys, does R-F have a loan dept? lol j/k
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thanks
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Old 06-19-2005
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Quote:
Originally Posted by Redneckstone
YOU WILL BE FINE.....
My 32s do increase my brake wear, I'm at 46K miles and am soon going to be turning the rotors for a 2nd time.
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Quote:
Originally Posted by Wowak
My 32s do increase my brake wear, I'm at 46K miles and am soon going to be turning the rotors for a 2nd time.
yes they do increase brake wear but he still will be able to stop fine...
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