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Discussion Starter #1
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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Discussion Starter #2
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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Cummins Freak
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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...…….:stirpot:
 

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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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Discussion Starter #7
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...…….:stirpot:
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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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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I’m running 295/70-18 on stock wheels and they have yet to rub anywhere!


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Discussion Starter #12
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?

I’m 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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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?



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.
I will have to check it out on the way home.

Here are 2 pictures while towing/under load. Both locked out in 5th gear.





I have a post going about gearing as well. I will likely be going to 4.10s mainly because of my 16K lb camper and planning to pull to the west coast. I just ran it to the east coast and wish I had more gear.

With that being said, I have been running this setup for about 2 years and live in IL/flat lands. I pulled my 8K lb camper pretty easily and never really wish I had more gear until I started to tow twice the weight.

IMO 3.42s are under geared from the factory and really should of had 3.73s with 4.10s as an option. It looks as though the 2019s have 3.73s from the factory which should tell you something about the 3.42s. Luckily with the torque from the cummins it wasn't as terrible as one might expect.
 

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Discussion Starter #14
Thats helpful - in 5th at 70mph looks like you are just under 2k rpm's - thanks so much!

I agree with you on the choice, if you're towing, get the 4.10's, for pavement princesses like me, although I will never get the investment back, with 35" tires, I believe 3.73's are very beneficial to keep the rpm's in the sweet spot of powerband and economy for highway cruising.
 

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Discussion Starter #17
You're only talking a 3% change. That's about 45 rpm at 1500. I think you're overanalyzing it.
I am and thank you! I went over the calculator numbers with our helpful forum member friend Gordon the other night - even to go up to 295/65/20 is barely 75rpm

I theorized and think that when people go up in size, and then they complain about how the "truck lugs" etc - This is all to do with adding unsprung WEIGHT - going from factory 57lb tires, some Ridge Grapplers in the standard (non metric) sizes or other mud tires can be 75 POUNDS! Once you add more than 10% additional unsprung weight per corner, all bets are off and that is the lugging feeling that some are getting.

I just have to double check my clearance and if I find a 285/65/20 that is light enough (Cooper AT3 XLT) I may pull the trigger on the slightly larger size. Also looking into 35x11.5x20 on the Nittos, just have to check their weights.

I would stick with the stock size but most of the more aggressive tires in the stock 285/60/20 end up adding 4-5lbs per corner anyway, so I may as well get some more wheel well fill up!

Thanks guys and yes I am the overanalysis king around here.
 

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My wheels are heavy as well. I dread having to rotate them :(. I opted for the size/style I did for maximum benefit of towing and looks.
 

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Hmm. Found a thread involving gearing...yum.

Anyway, two things I'd like to bring up: Go by a tire's static loaded radius, or revs per mile, not by what it says on the sidewall. Nominal sizes, as on the sidewall, can vary quite a bit both in what the resulting gearing is and if they physically fit.

Second, I like the simple approach to determining tire and gearing changes. If decreasing the revolutions per mile by going to a taller tire by 5%, then the tach will read 5% lower at a given speed.
Yes, it really is that simple.

Between 3.42s and 3.73s, there's about a 10% difference.

Lastly, while on the return trip from CA yesterday, and despite having tow/haul activated, the trans decided to keep the rpms all the way down to 1,500 a couple of times. Even with an empty trailer that really hampered performance, and to me it also sounds and feels bad, so I downshifted manually to get the engine into the happy spectrum of its powerband again.
With 3.73s I spun the DRWs a couple of times when taking off from stoplights in the wet, but that was my own doing.
Much better than only being able to use 4th and 5th gear during regular highway driving, I think.
 

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Discussion Starter #20
^^Thanks Jimmy and yes I noticed that - So for the 285/65/20 I'm about 3.3% larger and if I was up to a 295/65/20 (doubtful, for concerns of rubbing at full lock), would be 5% - at 1,500 rpm's at 70 as my truck runs now, the 3% or so brings me down maybe 50rpm so not the largest deal in the world.

I'll keep the hive apprised - Apparently finding a setup that doesn't rub at full lock on stock wheels (for every 25 posts of larger tires on stock wheels, especially the 20's, there are 23 of them that say 'oh yeah a little rub at full lock') is harder than anything, when going with a taller tire, which I'm starting to theorize is more the issue with the "full lock rub" than necessarily the width - and yes I know thats due to tire variances (some tires run wider in the same size or taller in the same spec), but I'm trying to find my "happy fit" - so far it seems the 285/65/20 in either something not as aggressive like a Terra Grappler G2, or a Cooper AT3 XLT, should work well - the Ridge Grapplers run a bit wider in the same size (34.8x11.5 as opposed to 34.6x11.2) and are a couple pounds heavier, but look awesome.
 
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