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Discussion Starter · #1 ·
I am upping my tire size from 285/70/17 (32.8"), to 315/70/17 (34.5").


I've been getting a lot of "you need to re-gear" like statements. But i don't see why..?


Comparing my current tire/wheel combo to the upcoming combo:

Stock steelie is 32 or 34 lbs.. i'll go with 32 for now

Stock: Stock 17" steelies (32 lbs) and 265/70/17 Bridgestone Dueler H/T (37 lbs)
TOTAL: 69 lbs

Current: Stock 17" steelies (32 lbs) and LT285/70/17 BFG A/T's (58 lbs)

TOTAL: 90 lbs

New combo: Stock 17" steelies (32 lbs) and 315/70/17 BFG A/T's (64 lbs)
TOTAL: 96 lbs


Tire weights from tirerack.com


Honestly, The only time i ever noticed a loss in power when i went from the stock meats, to the BFG's, is when i used to disconnect the VSC and gun it off of every corner (when gas was $3/gal :lol:). That was the only time..


So how would a re-gear benefit me? Going from the 285 to the 315 to me (using the numbers provided) does not look like a big difference. I don't see why i would want to re-gear. If weight is not the only reason for the re-gear, and more of tire sizing alone, let me know.. because i don't know.


Thanks for the help :cheers:
 

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For gearing... It's really all about the rpm/mph/power band. The larger diameter tires/wheels will lower your rpm at any given speed/mph, it's as simple as that. If you have all the power/torque you want, when you're doing what you want, then you're good to go. If you get outside the power band and lug the engine, you'll lose MPG. The additional unsprung weight will effect handling, but gearing will have no effect on that. Ron

I am upping my tire size from 285/70/17 (32.8"), to 315/70/17 (34.5").


I've been getting a lot of "you need to re-gear" like statements. But i don't see why..?


Comparing my current tire/wheel combo to the upcoming combo:

Stock steelie is 32 or 34 lbs.. i'll go with 32 for now

Stock: Stock 17" steelies (32 lbs) and 265/70/17 Bridgestone Dueler H/T (37 lbs)
TOTAL: 69 lbs

Current: Stock 17" steelies (32 lbs) and LT285/70/17 BFG A/T's (58 lbs)

TOTAL: 90 lbs

New combo: Stock 17" steelies (32 lbs) and 315/70/17 BFG A/T's (64 lbs)
TOTAL: 96 lbs


Tire weights from tirerack.com


Honestly, The only time i ever noticed a loss in power when i went from the stock meats, to the BFG's, is when i used to disconnect the VSC and gun it off of every corner (when gas was $3/gal :lol:). That was the only time..


So how would a re-gear benefit me? Going from the 285 to the 315 to me (using the numbers provided) does not look like a big difference. I don't see why i would want to re-gear. If weight is not the only reason for the re-gear, and more of tire sizing alone, let me know.. because i don't know.


Thanks for the help :cheers:
 

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It is not a matter of weight, it is a matter of actual rotational mass and the fact that the tire is taller and it changes your rear end gear ratio. For every 1" you go up in tire you lose .10 off your diff ratio.

For example, the stock auto diff is a 3.727, or 3.73 for sake of argument.
The truck comes standard with a 31.5" tall tire.
When you go up to a 33" tire your ratio goes down from 3.73 to 3.58. that is a loss of .15 of your ratio.
When you go up to a 35" tire you lose .20 of your ratio. A 35" tire yeilds you a 3.38 rear end ratio.
Going up to a 4.56 ratio you will gain all your ratio and power loss from the tires back and get more push technically because the car will require less power to rotate the tires.

hope that helps :)
 
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Discussion Starter · #4 ·
It is not a matter of weight, it is a matter of actual rotational mass and the fact that the tire is taller and it changes your rear end gear ratio. For every 1" you go up in tire you lose .10 off your diff ratio.

For example, the stock auto diff is a 3.727, or 3.73 for sake of argument.
The truck comes standard with a 31.5" tall tire.
When you go up to a 33" tire your ratio goes down from 3.73 to 3.58. that is a loss of .15 of your ratio.
When you go up to a 35" tire you lose .20 of your ratio. A 35" tire yeilds you a 3.38 rear end ratio.
Going up to a 4.56 ratio you will gain all your ratio and power loss from the tires back and get more push technically because the car will require less power to rotate the tires.

hope that helps :)


Definitely did help.



But, will it destroy my mileage? I'll be running higher RPM's at all times, correct?
 

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I've been running 285 BFG AT's for on-road driving and 305 MTR's for off-road excursions. I've got an automatic that came with the 3:73 gears. I haven't had a problem with the stock gearing and was perfectly content with it (both on and off-road), except for the fact that I was in the group that might be sporting defective rear ring and pinion gears. For that fact alone I decided to re-gear. I went with 4:10's since that got me back close to the stock gearing with the 305's on. If not for waiting around for the rear end to explode I would have kept the stock gears.

Jim
 

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would not 35's be better on gas mileage when cruising the freeway? And crappier when drive the streets? I would think Dom does more street driving than freeway. (But I really don't know.) If me, I would regear. Less strain on the drive train over the long haul.
 

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Definitely did help.



But, will it destroy my mileage? I'll be running higher RPM's at all times, correct?
you will more than likely get better gas mileage after the re-gear because it will be easier to rotate the tires instead of more load on the engine and constant downshifting on the freeway.

Jim D, 4.10 would actually be perfect to take most of us back to just over the stock ratio. it would give better mileage and a slightly lower gear for more crawl ratio and offroad power
 

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would not 35's be better on gas mileage when cruising the freeway? And crappier when drive the streets? I would think Dom does more street driving than freeway. (But I really don't know.) If me, I would regear. Less strain on the drive train over the long haul.
actually, it still would hurt mileage no matter what because it is hard to get the tires rolling off the mile and hard to keep them rolling after that
 

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ok, my last question:


When you re-gear the rear, do you do the front too? :confused:
YES! absolutely. If not then when you are in 4wd the front will be turning slower than the rear. You can only imagine how much stuff that can break...
 

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we're looking to regear soon with our 35" swampers... I've notice roughly around a 10% difference in mpg, odometer reading, and speedometer reading as well... I never know how much gas I have either... annoying...can't wait to re-gear actually
 

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Dominc, with just the LTBs (not 35s), I can already feel the power loss. On hills that I use to cruise up in top gear, now the AT downshifts.
 

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3.90:1 is the correct gear.

33/31.5=1.047619047619047619047619047619
1.047619047619047619047619047619*3.73=3.907619047619047619047619047619

4.10s are more fun.
 

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Where does one go to do a re-gear to 4.1s? What does it usually cost? Let's say SoCal for my "window" of where I'd go...
 

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Dom actually I would not go to 35's. I want to talk you out of it because there are more negatives than positives right now.

I am lucky to get 13 mpg on the freeway and it is all due to 34.88" dia with 13" tread width. Today I am gonna pump up to 60 lbs. and see if that helps my $1000/mo. Shell bill... and I don't commute!

My marshmallows are nice on the trail, but not so much nicer that it's worth the monstrous mechanical cost and fuel cost. In fact, definitely not. Yes, they track a little straighter, let me go a little faster, roll over the bumps a little better. Then I get out to the highway and watch the needle till the next station.

I won't mention my almost $5,000 rear axle. That plus the front gears, installation work, cost of the new tires, and it is just not worth it. If you spent that much bucks on a real hottie instead of Ursula - now that would be a good investment return
 

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Dom actually I would not go to 35's. I want to talk you out of it because there are more negatives than positives right now.

I am lucky to get 13 mpg on the freeway and it is all due to 34.88" dia with 13" tread width. Today I am gonna pump up to 60 lbs. and see if that helps my $1000/mo. Shell bill... and I don't commute!

My marshmallows are nice on the trail, but not so much nicer that it's worth the monstrous mechanical cost and fuel cost. In fact, definitely not. Yes, they track a little straighter, let me go a little faster, roll over the bumps a little better. Then I get out to the highway and watch the needle till the next station.

I won't mention my almost $5,000 rear axle. That plus the front gears, installation work, cost of the new tires, and it is just not worth it. If you spent that much bucks on a real hottie instead of Ursula - now that would be a good investment return

let me know when you're ready to swap FJs.
 
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