Gearing Gurus - Change from 33.5” to 34.5” with 3.42? - Dodge Cummins Diesel Forum
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post #1 of 24 (permalink) Old 11-18-2019, 09:11 PM Thread Starter
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Gearing Gurus - Change from 33.5” to 34.5” with 3.42?

Ok guys so I’m one of those bunnies who doesn’t want to lug or lose much performance if I go up in tire size.

I don’t know how to do the math but I have 3.42’s and I have the 285/60/20 (33.5”) tires

I am almost set on 275/65/20 (34”) tires just because I know they won’t rub and will give me a touch more height (I’ll only be running a level and want an OEM+ look)

Just so I can cover my bases and if I find a tire in 285/65/20 (34.5”) that won’t rub at full lock (I find that they all do past the 34” height for the most part, at full lock), can anyone run my gears and tire height into my approximate rpm at 70 mph for the 34” and 34.5” size?


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post #2 of 24 (permalink) Old 11-18-2019, 09:16 PM Thread Starter
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And yes I am a pussoliah - I am getting too old to just deal with a “little rub” at full lock

Since I enter and exit my driveway at full lock like that annoys me to no end

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post #3 of 24 (permalink) Old 11-18-2019, 11:36 PM
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Check out GrimmJeeper.com

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post #4 of 24 (permalink) Old 11-19-2019, 12:20 AM
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On stock wheels I find that as long as you stay under 12.5 wide tire they will not rub. You don't want mine or @Jimmy N. opinions on 3.42's gears....
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post #5 of 24 (permalink) Old 11-19-2019, 09:18 AM
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Yeah - personally I like the 3.42 gears for interstate driving and just dropping into 5th when the load demands. With the lower 1st gear of the aisin, I think it works fine, but then I'm not approaching the upper weight limits. I do think, unless yours is a grocery getter, that the tire size increase will leave you feeling overgeared. Anyhow, just take the two tire diameters, multiply them by Pi 3.1415 is close enough, then divide which ever one you're considering going to by the one you have (works for both going up or down in tire size. The answer will be the percentage change in your RPM, so just take your RPM now at the highway speed you like and multiply it by the percentage you just calculated. That will give you the RPM you'll have with the new tires. I suspect with the 68RFE, if you're empty and cruising 80 MPH interstate you'll be happy, but if you work that truck at all, you'll not like the higher gearing.

Finally - I got to put that ol high school math I learnt long ago to use.

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post #6 of 24 (permalink) Old 11-19-2019, 09:22 AM
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Quote:
Originally Posted by SRich12 View Post
On stock wheels I find that as long as you stay under 12.5 wide tire they will not rub. You don't want mine or @Jimmy N. opinions on 3.42's gears....


Like you had to tag him. Hes been on the hunt for a gear thread for a couple days. He knew about it before it was posted lol
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post #7 of 24 (permalink) Old 11-19-2019, 12:33 PM Thread Starter
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Originally Posted by SRich12 View Post
On stock wheels I find that as long as you stay under 12.5 wide tire they will not rub. You don't want mine or @Jimmy N. opinions on 3.42's gears...…….
This is the only thing I disagree with in bold, I have combed through dozens of threads and posts over the past month and yes there is no rub in normal driving with up to a 35x12.5 on stock wheels, but at or close to full lock it rubs -

This is what I want to avoid or deal with is any rubbing in any circumstance, on stock wheels, and I'm trying to find the magic size that allows that - so far that seems to be the milder 275/65/20 (34"x11") - my factory tires measure 33.5" x 11.2" so the rubbing *at full lock* has as much to do with added height as it does with increased width - from all my research this is why some guys can go to 295/60 (34x11.6") without rubbing *at full lock*, depending on the tire type

I have also spoke to guys running 285/65/20 (34.5"x11.2") and they rub *at full lock* so its entirely dependent on tire type, sidewall lugs sticking out, and the height just as much as the width

If I was not concerned with rubbing *at full lock* I would just go to a 295/65/20 as it looks amazing. But I am also concerned about lugging on 3.42's, economy, and added unsprung weight. Hell, I may stick with the stock 285/60/20 size just in a more aggressive looking tire, if I can't get what I want out of a different size.

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post #8 of 24 (permalink) Old 11-19-2019, 12:41 PM Thread Starter
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Using the GrimmJeeper tool it cant be right? Says projected rpm change 1440 from 1492 using the inch taller tire?

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post #9 of 24 (permalink) Old 11-19-2019, 02:17 PM
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I am running nitto grappler something or others. 295/65-20. I get a little, and I mean a little rub on the driverside lower track bar or whatever it's called. It only hits going in reverse with full lock to the left.

I just ordered another pair to replace the most worn ones and get a proper tire on my spare. They seem to wear a little quick, no fault of my own I am sure.

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post #10 of 24 (permalink) Old 11-19-2019, 03:21 PM
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Im running 295/70-18 on stock wheels and they have yet to rub anywhere!


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post #11 of 24 (permalink) Old 11-19-2019, 03:32 PM
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Quote:
Originally Posted by SRich12 View Post
Im running 295/70-18 on stock wheels and they have yet to rub anywhere!


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That's technically only a 34.5" tire which matches what he is looking for (295/60-20)

295/65-20 is a 35.5" tire.

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post #12 of 24 (permalink) Old 11-19-2019, 04:48 PM Thread Starter
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Quote:
Originally Posted by RoyBell View Post
I am running nitto grappler something or others. 295/65-20. I get a little, and I mean a little rub on the driverside lower track bar or whatever it's called. It only hits going in reverse with full lock to the left.

I just ordered another pair to replace the most worn ones and get a proper tire on my spare. They seem to wear a little quick, no fault of my own I am sure.

Thanks Roy for the great pic on your truck! The Ridge Grapplers look awesome on it! I will have to look up their specs - how is your rpm's at 70mph since upsizing to a true 35" tire height?

Quote:
Originally Posted by SRich12 View Post
Im running 295/70-18 on stock wheels and they have yet to rub anywhere!

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Not sure if the factory 18's have the same offset as mine - I know the BAP wheels that I and Roy above me have are +55 - Even if yours were 10mm less offset on those factory 18's would make a difference with rubbing at full lock. But Roy gives me a good reference.

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