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Old 12-08-2017, 05:39 PM
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twistedoak's Avatar
Join Date: Aug 2014
Location: phila
Posts: 443
tire size,tire size under weight & RPM

needing to up size my tires
I'll begin with where I'm at

2014 freedom edition auto, just upgraded to 4.10s from 3.21s on stock 32"SRAs(31.25 actual)
thusly my highway cruising RPM at 70 is now up to 2700 from 2100
with the same stock 32" GY SRAs(31.25 actual).

ideally I'd like to get my 70mph RPM down in the 2500 range
so I figured on bigger tires to get me into this goldilocks zone .
I don't really want to lift and I'm also trying to keep the tires I put on inside the fenders
I'm wanting to mount some toyo open country ATs
on a set of 17X8.5 rims with 5.2" of backspace
here's where I'm getting a bit confused.

I'm currently looking at toyo open country AT's in either
285/70R17 listed at 32.8" basically a 33" tire
285/75R17s that at 33.8" listed is basically a 34"tire

how much RPM drop will I get per inch of tire I go bigger ?
is 100 RPM per inch a plausible guideline?"

and secondly being my jeep uses the actual tire size to calculate speed
is it safe to believe any tire will lose about 1" between listed tire size and the actual under weight.

am I even barking up the right tree with this?

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Old 12-10-2017, 08:05 AM   #2
YelloJ74's Avatar
Join Date: Jul 2013
Posts: 408
Look at Grim Jeepers website. It has a cool spec sheet where you can input all your numbers and it gives you the calculations you are needing.
87 YJ, 98 Cherokee, 04 WJ Overland, 74 CJ-5
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