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I know you have made up your mind, but you really sound like a better candidate for 4.88 gears.
Also don't really expect much better mpg around town if it is stop light to stop light, though it might be better with the auto as compared to a manual. On the highway I do real good if I can keep my speed down to 65 or less. In fact the best mpg I have ever had was in Oregon where the speed limit was 55 for a tank, 22.3 I think it was.
 

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I don't do over 70 much. It's a lifted brick with big tires and OK brakes. No real open country roads here.
 

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I don't do over 70 much. It's a lifted brick with big tires and OK brakes. No real open country roads here.
Which is why I think you make a good candidate for 4.88 gears over 4.56 gears for 35" tires. You will gain more "pep" and your in town "city" mpg will be better. If you don't do much highway driving, if you don't typically drive over 70 mph, there is less reason to go with 4.56 gears for 35" tires vs 4.88 gears.
But you will get more pep, more power, from the 4.88 gears. And off road you will be better.
 

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My Rubicon has 37s with 4.88s. The RPM is about 100 more at 70mph than stock. A good all around gearing. 15 mpg overall. 14.5 mpg at 70. 16 plus mpg on 55 mph roads.
 

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The only downside I see to the 4.88's is if you ever want to run a set of 32-33's you would likely be well over-geared, where as the 4.56 would be a better option. But lets be real... once you get used to the look of the 35's why go back...
 

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I did the math. The 4.56's at 70 in 5th gear is the same as 3.73"s in 4th. The 4.56's will give me back my 5th for the freeway and allow 4th to be a good passing gear. The will also help in 1st gear take off. My BFG 315's are not a true 35 tall.
 

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12 JKR with 4:10 auto BFG 315 75 17 KO2s. At 70 I am turning 2400. 99% of my driving is in town and my milage is in the 15s. 4:56 would be nice but not worth the cost to switch. Performance is good enough for me.
 

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Just to let you know a 35" tire will vary by brand and model. You may see an inch difference between two 35" tires. Check your actual running size by measuring from the ground to the center of the hub and multiply by 2.
You mileage will also vary by the type of tread (MT vs AT) and weight of the tire (E rated or C rated), but the biggest factor is how you drive (Grand theft auto or old man). Do you run mostly highway or mostly in town? If highway then 4.10 gear is going to be better for you. If mainly in town driving then 4.56 gears. If you are going to run any trails then I would do 4.56.

Either one will be much better than the 3.21 gears you have.
Note you will need new carriers as well.
 

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I did the math. The 4.56's at 70 in 5th gear is the same as 3.73"s in 4th. The 4.56's will give me back my 5th for the freeway and allow 4th to be a good passing gear. The will also help in 1st gear take off. My BFG 315's are not a true 35 tall.
I'm running 4.56 with 34's and for me it's just about perfect. I also don't drive over 70 with my BRICK so r's aren't a big deal. I did go from 3.21's to 4.56 and difference was huge. No more over reving stopped on hill to get it moving. Most times I use 2nd to start and jump into 4th for around town. Actually can use 5th a lot where before it was strickly highway. My mpg dropped on the highway but actually improved in town. Over all I'd say it dropped 1 to 2 mpg but I also switched to 34's at the same time as regearing. I suspect the tires reduced mileage as much or probably more then my regearing.
 

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Just to let you know a 35" tire will vary by brand and model. You may see an inch difference between two 35" tires. Check your actual running size by measuring from the ground to the center of the hub and multiply by 2.
You mileage will also vary by the type of tread (MT vs AT) and weight of the tire (E rated or C rated), but the biggest factor is how you drive (Grand theft auto or old man). Do you run mostly highway or mostly in town? If highway then 4.10 gear is going to be better for you. If mainly in town driving then 4.56 gears. If you are going to run any trails then I would do 4.56.

Either one will be much better than the 3.21 gears you have.
Note you will need new carriers as well.

Totally disagree with the 4.10 statement when it comes to highway driving. The only time it works well is if you live close to sea level, are pretty much stock as weight goes, it's flat, the speed limit is above 60 mph and you don't have a lot of wind. In the inter-mountain west it is pretty useless. I did not swap gears for around town or off road capability though there is an improvement.
 

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Thanks for bringing that up, I generally will recommend if you are in an area that has lots of hill or step grades that you should go up to the next gear set. I don't know what being at sea level has anything to do with it though with fuel injected motors the air fuel mix is automatically adjusted. My wife's JKU Sahara with 32's and factory 4.10 gears would average around 20-21 mpg with the 3.8 motor and 6 speed. We live at 6000' and have a pretty steep grade to get up to out place from town. We lived in Illinois before that and got the same mileage. If it had 35's on it I would likely have gone 4.88 due to the grades.
 

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I am scheduled to have the gears done in the highest point of the virus here in So Cal. I have to admit I am a little nervous about that. The news makes it sound like I'll be driving around bodies and getting the virus.
 

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Thanks for bringing that up, I generally will recommend if you are in an area that has lots of hill or step grades that you should go up to the next gear set. I don't know what being at sea level has anything to do with it though with fuel injected motors the air fuel mix is automatically adjusted. My wife's JKU Sahara with 32's and factory 4.10 gears would average around 20-21 mpg with the 3.8 motor and 6 speed. We live at 6000' and have a pretty steep grade to get up to out place from town. We lived in Illinois before that and got the same mileage. If it had 35's on it I would likely have gone 4.88 due to the grades.
I assume it is about how flat the terrain is. Most places near sea level have fairly flat terrain. So people who live near sea level don't need the power to go up hills the way others do. Sure, some places near sea level has plenty of hilly terrain. But most of them don't.
That's my two cents.
 

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.....I don't know what being at sea level has anything to do with it though with fuel injected motors the air fuel mix is automatically adjusted......
Less air means less fuel which equals less power. For every 1000 feet in elevation you lose approximately 3% HP. The whole reason for re-gearing is due to the lack of power our engines produce, especially down low. In fact if I every get the chance to put a LS motor dumping my 4.88's in favor of 3.73 or 4.10 thanks to them making a minimum of 300 ft lbs of torque at 1300 rpms or so.
 

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Less air means less drag as well if we are talking theories . Your jeep is also lighter at higher elevations then at seal level. When you live in the hills the power loss is only half the time. Since the 3.8 does not have any hp anyway I am not out much.
 

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Less air means less drag as well if we are talking theories . Your jeep is also lighter at higher elevations then at seal level. When you live in the hills the power loss is only half the time. Since the 3.8 does not have any hp anyway I am not out much.
It is not theory, it is fact. It is why year in and year out the mile high nationals produce the slowest speeds and time on the circuit. These guys are down 15% in HP and with a 7000 hp motor that is a lot of lost HP.
 

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It is true that being at higher elevations reduces engine power. But you can have issues without being high enough to make a difference in power output. We live at around 500 to 600 feet. But where we live there are mountains that range from 1000 to 3000 feet. Have to go up and down those "mountains" requires downshifting, especially when the gearing is too tall. Having the right gearing reduces the amount of downshifting required and reduces the strain on the motor, regardless of altitude. At 3000 feet there isn't enough of an air density difference to explain the lack of power to go up the hill. Even at sea level the Jeep lacks that sort of power with stock gearing. And having a manual trans makes it more noticeable in my experience.
 

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It is not theory, it is fact. It is why year in and year out the mile high nationals produce the slowest speeds and time on the circuit. These guys are down 15% in HP and with a 7000 hp motor that is a lot of lost HP.
Actually at the Mile High Nationals they let the teams run more overdrive on the supercharger (spin it faster) than the other races to make up the power lost to altitude. The ones who really suffer are the normally aspirated guys, pro stock etc... Power is guessed to be between 10,000-12,0000 depending on who you talk to on a nitro car.

Thinner air also means less down force on wings, less traction etc... Kind of an equalizer for the lesser funded teams who can't make big power at the regular races on the tour.
 
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