powrsurg

I have no experience with MPG gains by adding a Super Charger or Turbo. I can say that CAI, Exhaust, and Headers, do help but not as much as you would probably want. ANother is that when you do these three changes, you will certainly drive with a heavier foot beause you'll love the sound.

WHen it comes to gearing there is something going I dont completely understand. I always thought the lowest RPM's used to maintain a given speed is what you would realy want to achieve to gain MPG's. By going from 3:73 - 4:10 or higher, you are actually increasing the amount of RPM's at any given speed. SO maybe regearing is good if you drive mostly city / local but under a long haul where your not stopping and going 3:23 should be better. RIght?

Basically, I look at regearing as a way of improving the off road experience, not necessarily a way to improve MPG's.

Sommer13

It all depends on how big you go an what you're changing them for. Most people do use it to gain back torque loss from bigger tires on the trail, and tend to go for 4.88, 5.13, and 5.38's. Some do it to gain back highway performance and mpg's from bigger tires, and don't go that big, maybe just 4.10-4.56, enough to be able to use their high gears again without slowing down

larythebaker394

All engines are engineered to operate within a certain power band. Maximum MPGs will result from maintaining that engine within the specified band. Regearing places the engine back into this power band.

powrsurg

I agree with everything you are saying. This is what I have a hard time with.

"Some do it to gain back highway performance and mpg's from bigger tires"

The performance gained is through the perceived (but not actual) increase in torque resulting in the ability to get to your target speed faster. The result however will result in a higher RPM at the new gear when compared to the lower gear. Maybe im looking at this the wrong way but this higher RPM will cause you to burn more gas then when you were going the same speed in the lower gear at lower RPM's.

Sommer13

True, but remember. People who slap on huge tires notice the loss in torque and compensate by down shifting and slamming down the gas, and proportionally burn more gas that way than they would if they regeared to be able to use the high gears with slightly higher rpms. There is a certain range of rpms that allow for steady speed and good fuel economy with each gear. When you try to force your jeep to go faster with big tires, you forego that good rpm range and burn more gas trying to stay at at certain speed because the gears can't get into that good space and stay there in a high gear.

Hope that helps, I suck at metaphors...

c17loadmaster

Let me give you some first hand advice. I have a 4 inch lift, 35 inch nittos, and had 3:73 gears. I could not for the life of me maintain 75 mph on the highway. I have a auto. It was constantly downshifting ( thereby shooting the rpm's up) not only was it annoying, I was getting 12mpg on the highway and 15mpg in town. That torque loss puts mega stress on your pinion gear. The pinion gear is larger on numerically smaller sets 3:21,3:73 etc. the higher the number the smaller the pinion gear. This res.ts in higher rpms because the pinion has to rotate more in order to turn your tires at the rake rate(speed) however it is easier on the engine and tranny to stay in that power band. I recently had 4:88's installed and now get 16 mpg in town and 18 mpg on the highway. Yeah my rpms are at 2900 at 70mph but it stays there and does not downshift. I can also use my cruise now. Less stress on the engine, better torque.

Sommer13

Sounds like you hit the sweet spot on the new gear choice, good job man

powrsurg

OK, I think im getting what you are saying. SO let me ask you this then. Regardless of tire size, what is the most fuel efficient gear the JK comes with? I have 2012 manuial rubicon which obviously came with 4:10's. WHen I added the 35's it effectively brought me down to 3:73. I know this because of the following site.
MFactory Competition Products - Forged Transmission Components

Anyway, how can Jeep Corp. advertise 17-24 city / highway when they offer 3 different gears. One gear must obviously have better MPG's than another right? IF so, which gear? A person looking to regain MPG and not necessarily gain off road ability, should regear to that level when they add larger tires. If you know the gear that is the most fuel efficient that is offered in the Jeep, the above chart will tell you which new gear you need to select given the size of tires you are changing to.

Im thinking you can target city or highway efficiency but not both.

Sommer13

Again, it depends mostly on the driver and his habits. Every car company gives wide ranges for mileage because they know that the amount of mpg's you can get is connected to how you drive your vehicle. It's why they offer so many add ons and upgrades like down sizing gears for automatics, and giving 4.10's to rubicon because its more likely to be used in tough terrain

If you have a heavy foot and like to take off after the light goes green like I do, you city mpg will suck, my highways fine cuz I just sit back and relax after I'm up to 70.

I'd say a 4.56 or 4.88 with 35's is the most reasonable to me if you want good mileage on both.

c17loadmaster

I don't think there is a magic gear ratio. There are too many factors, 2 dr, 4dt, auto, manual, lift, no lift, heavy accessories, tire weight, engine mods etc. 4:88's worked great for me, but I will never go down from 35's. My experience works great, and I will go up to 37's when my 35's go out. Just don't go too high on gears like 4:10,4:56 that won't allow,you to go bigger tires later. I almost went 4:56 and am sure they are perfect for 35's but with 37's in the future I would have been back to my old ratio if I did that.