Regear because of weight or size?

[808inthe949]

Hey all,
Second post here. Messed up and put my first post in the KK forum. I have an 05 -_-
Anyway, I know that regearing is necessary when upping tires above 31" but is that because the bigger tire weighs more or is it because of the effort it takes for a full revolution? I ask because I dont want to regear just yet. I have the 17X7.5 factory aluminum rims that weight 27lbs I think. My jeep has the proper lift setup 2.5-3" so I can fit a 245/75/17 (wheel spacers are also installed, I know shoot me for using spacers right?). the tires im looking at weigh 37lbs. I have no body armor right now and my plans are pretty light weight. Would that total wheel weight/size require me to regear?

I understand that regearing is a matter of preference and not completely necessary. Id like to not have my ****** **** out early on me due to the erratic shifting. Gas mileage is a close second concern. I know I will need to stay at stock size to stay at stock mpg but will my proposed setup be a huge difference or just 1-3mpg?

Tires are GY wrangler AT Adventurer w/ Kevlar 112T

 

[JeepJeepster]

Its both weight (moment of inertia) and the bigger tire.

 

[duderz7]

I had 265 75 16 mudders for a short time, when I put my 245 75 back on there was a noticeable difference, but its not a race car so my next set will be 265 70 or 265 75 on 16s. I would love to regear, but the cost is what holds me back.

 

[808inthe949]

I wish I had 16's. I know the price difference between a decent/cheap set of rims w/ AT tires is considerably close to a full set of AT on 17's, but try explaining that to the ole ball and chain. I have the height needed for the 245/75 and im going with light rims and tires so im hoping the difference from stock isnt too terrible. Im also wondering if I could regain that loss by upgrading exhaust/intake, possibly a ported throttlebody, ect...

 

[808inthe949]

jeepjeepster- Iunderstand it takes more force from the motor to move a tire with larger contact area, wind resistance, yadda yadda. But it seems like the general tip is that you regear when you get bigger/HEAVIER tires. If my tire is the same or very close to the stock weight, just as an example, but is 2" taller, would it make the same effect as tires that are the same size but heavier?

 

[duderz7]

I don't remember the equation or ratio per say, but a larger diameter tire effectively makes EVERY gear higher and more than one would think. So while getting more power (which there isn't a lot that can be done with these engines) could mask it you are still asking your trans and diffs to push a higher gear than they were intended for. I think a lot comes down to how you drive your jeep and how you expect it to perform.

 

[lfhoward]

You can visualize the force needed to rotate a tire by imagining your arm is the tire radius and your shoulder is the axle. The force of the axle rotates the tire like your shoulder muscles lift your arm up. It will take more force either with a weight in your hand (heavier tire) or lifting the same weight with a longer/fully extended arm (larger tire radius). It’s levers & physics. So, both the tire/wheel weight and the tire diameter matter.

 

[JeepJeepster]

jeepjeepster- Iunderstand it takes more force from the motor to move a tire with larger contact area, wind resistance, yadda yadda. But it seems like the general tip is that you regear when you get bigger/HEAVIER tires. If my tire is the same or very close to the stock weight, just as an example, but is 2" taller, would it make the same effect as tires that are the same size but heavier?

I would think the diameter has a bigger role than the weight... But thats just my educated guess...

 

[Redonthehead]

I second that. Moment is the primary factor. The lever analogy is correct. Think of the force you have to apply to a short wrench vs a long wrench. Or think of your old 10-speed bike, where using the larger front sprocket makes it a lot harder to pedal even though the sprocket is only an inch bigger. The 2.5" difference in tire diameter from stock requires more force at the driveshaft unless your differentials are geared down in proportion to the increased tire diameter.

I am giving up on my 245/75r17 Rubicon wheels and tires because the mods to make them work are just too much. Tom Mudd has posted that he thinks you need a 3-4" lift to make them fit as the spacer adapters push the tires out into the flares, etc..

 

[808inthe949]

I believe you guys are right. The science is sound! Which is a little unfortunate for me as I was hoping I wouldn't lose MPG's or low end torque, but it seems I will. I do plan to regear at some point so I guess those plans got expedited. I have been trying to find a 4banger that's junked or cheap but no luck. Actually have had a hard time finding a 4banger liberty period. It would be awesome if someone had all or some of the needed parts for a regear to 4.10 and was willing to sell them (ya know if their libby died or was sold).

 

[mercdudecbr600]

You can find sets of 16" stock alloys pretty much everywhere but 16" AT tires are starting to become more rare. Just so you know, stock 16" alloys weigh about 21-22lbs (according to my scale) and the 245/75/16s weight somewhere in the 34-37lbs range. So all in all, you'd save a few pounds by going to the smaller wheels but gain more sidewall for offroading.

 

[808inthe949]

That's a solid idea! I think I'll plan for that when I need a new set of tires. Just looked up some 16's and tons of factory ones around.

 

[huntert98]

Just a heads up ive been running 265/75R16 for almost 2 years now and tons of added weight. It can get by without a regear

 

[nbwajdr]

Hey all,
Second post here. Messed up and put my first post in the KK forum. I have an 05 -_-
Anyway, I know that regearing is necessary when upping tires above 31" but is that because the bigger tire weighs more or is it because of the effort it takes for a full revolution? I ask because I dont want to regear just yet. I have the 17X7.5 factory aluminum rims that weight 27lbs I think. My jeep has the proper lift setup 2.5-3" so I can fit a 245/75/17 (wheel spacers are also installed, I know shoot me for using spacers right?). the tires im looking at weigh 37lbs. I have no body armor right now and my plans are pretty light weight. Would that total wheel weight/size require me to regear?

I understand that regearing is a matter of preference and not completely necessary. Id like to not have my ****** **** out early on me due to the erratic shifting. Gas mileage is a close second concern. I know I will need to stay at stock size to stay at stock mpg but will my proposed setup be a huge difference or just 1-3mpg?

Tires are GY wrangler AT Adventurer w/ Kevlar 112T

Bigger tires effectively regears the entire vehicle - you'll notice the speedometer is no longer accurate and the engine runs at a lower rpm in every speed. I left the tires/wheels on my 05 CRD alone. But I had a 72 Chevy Blazer years ago that had 38" **** Cepek tires on it Going up to that size on the Blazer made my engine run a nearly idle while doing 60mph and the speedo was way, way off. I wish I'd have put 4.12 gears, or even 4.56 into it - but that was an expensive proposition.

 

[nbwajdr]

Bigger tires effectively regears the entire vehicle - you'll notice the speedometer is no longer accurate and the engine runs at a lower rpm in every speed. I left the tires/wheels on my 05 CRD alone. But I had a 72 Chevy Blazer years ago that had 38" **** Cepek tires on it Going up to that size on the Blazer made my engine run a nearly idle while doing 60mph and the speedo was way, way off. I wish I'd have put 4.12 gears, or even 4.56 into it - but that was an expensive proposition.

Funny how the auto-nanny bleeped out the first name of those tires. It should have been "****" Cepek tires. Maybe this will work.

 

[808inthe949]

Funny how the auto-nanny bleeped out the first name of those tires. It should have been "****" Cepek tires. Maybe this will work.

Must have a ***** mouth! ****
I'm happy to hear that some have ran without regearing for years and no serious issues came up. I know my power band will move from where it is currently and I'm okay with that. I do have some long trips planned later this year and the savings in gas money would certainly help but I don't I'll be saving enough to warrant spending the almost $2k to regear.

 

[808inthe949]

P. O. T. T. Y.

 

[Chuck Donley]

Regear to keep the engine operating at the RPM you need to keep you in the power band. As you increase tire size the engine RPM slows for a given speed, and your rig becomes a dog-won't accelerate, too much downshifting, not enough torque for off road applications. You can use a calculator like this one https://www.differentials.com/technical-help-2/differential-gear-ratio-calculator/ to figure what gear ratio you need for a given tire diameter. Our 3.7 engines produce 210 horsepower at 5,200 rpm, with 235 lb-ft of torque at 4,000 rpm.

 

[huntert98]

The tire size alone going from 29" (stock) to 32 only decreases torque 10%. This will make a difference, but more importantly added weight and reduced aerodynamics make a larger impact. My KKs weight without passengers or inside gear is 5200lbs. The added weight (tires, front and rear bumpers, tire carrier, gas can, hilift jack, full skid plates) made more of a difference for me than just the tire diameter alone

I do recommend you regear eventually, but when you do it think about how much weight you will be carrying. 4.10 gears would put me at almost exactly stock. I plan on going up to 4.56 because of my KKs fat ***

 

[808inthe949]

So will I have any transmission over heating issues? My WK had an alert on the dash for trans over temp (got bigger tires and went cross country, mainly got so hot in the desserts between new mexico and Cali) and now I'm thinking it was partially due to getting larger tires without regearing.
My KJ doesn't have armor or trail gear yet (stock weight) but I plan to add them aver the next few months. Im getting the tires installed Thursday. I'll post up after about how big of a "seat of pants" difference I notice. It'll be a tank or two before I know an estimated mpg.