who is running 4.10s on a 3.8 with 33's
Thread Starter
JK Junkie
Joined: May 2011
Posts: 3,213
Likes: 21
From: Grand falls, Newfoundland
265/70R17s that measure 31" actual height mounted, with 5.13 gears would run 2302 engine rpm at 60 mph and 2494 at 65 mph.
285/70R17s that measure 32" actual height mounted, with 5.13s would result in 2320 rpm at 60 mph and 2416 at 65 mph.
If you are never going to run taller tires than that, or if you are going to be driving 75-80 mph with frequency, then yes, 5.13s would be unnecessarily low.
Right now, with 3.73s, you likely run 1800 rpm with your 265s at 65 mph and just under 1800 rpm with your 285s.
With 4.10s, you'd run just under 2000 rpm with your 265s at 65 mph and just over 1900 rpm with your 285s.
With your 285s and 4.88s, you'd run just under 2300 rpm at 65 mph.
If you are never going to run taller than 285s or you are going to drive faster than 75-80 mph, then 4.88s could be a reasonable choice for you.
However, if you install 4.10s for the extra "umph" with your 285s for driving 65 mph during the summer, know that the extra 100 rpm will increase your rear wheel horsepower from 55 to maybe 60.
With power adders and premium fuel, you might get to just under 70 horsepower with 4.10s at 65 mph.
With 5.13s, your rear wheel horsepower with the 285s is still only going to be about 85 stock.
Again, the key is to run high enough rpms to produce enough power to reduce downshifting.
Actually, instead of installing 4.10s, you are probably better off just keeping the 3.73s, turning off the OD when you are in your jeep with the 285s all summer, and running 2350 rpm at 60 mph and 2550 at 65 mph. If you run the 4.10s, your rpms will be too low in OD, and around 2800 rpm with the OD off at 65 mph compared to 2550 with your 3.73s.
So, either keep the 3.73s and drive with the OD off, or go at least as deep as 4.88s.
285/70R17s that measure 32" actual height mounted, with 5.13s would result in 2320 rpm at 60 mph and 2416 at 65 mph.
If you are never going to run taller tires than that, or if you are going to be driving 75-80 mph with frequency, then yes, 5.13s would be unnecessarily low.
Right now, with 3.73s, you likely run 1800 rpm with your 265s at 65 mph and just under 1800 rpm with your 285s.
With 4.10s, you'd run just under 2000 rpm with your 265s at 65 mph and just over 1900 rpm with your 285s.
With your 285s and 4.88s, you'd run just under 2300 rpm at 65 mph.
If you are never going to run taller than 285s or you are going to drive faster than 75-80 mph, then 4.88s could be a reasonable choice for you.
However, if you install 4.10s for the extra "umph" with your 285s for driving 65 mph during the summer, know that the extra 100 rpm will increase your rear wheel horsepower from 55 to maybe 60.
With power adders and premium fuel, you might get to just under 70 horsepower with 4.10s at 65 mph.
With 5.13s, your rear wheel horsepower with the 285s is still only going to be about 85 stock.
Again, the key is to run high enough rpms to produce enough power to reduce downshifting.
Actually, instead of installing 4.10s, you are probably better off just keeping the 3.73s, turning off the OD when you are in your jeep with the 285s all summer, and running 2350 rpm at 60 mph and 2550 at 65 mph. If you run the 4.10s, your rpms will be too low in OD, and around 2800 rpm with the OD off at 65 mph compared to 2550 with your 3.73s.
So, either keep the 3.73s and drive with the OD off, or go at least as deep as 4.88s.
So in actual fact, I will get better milage from my rig after a re gear. The only reason I want better milage is for distance, not money. I will never be going any bigger than 285s. I have a 2" teraflex leveling lift on my rig, and that's as big as she's getting. Adding some armor to the bottom but I have not hit the bottom wheeling yet. We do a lot of long distance trips. 600kms in one day in the back county is not uncommon. Also, I want enough range to make 500kms on one tank of gas. I can do that now if I keep it to 100kmh with my 285 coopers. Now with the new Goodyear territory tires which are much lighter than my st maxx tires, I should be good. I'm not adding bumpers to keep my weight lighter, and just adding a winch behind the stocker and a surco roof rack to put on when we need the extra storage. I'm going to bet that when towing my camper the lower gears will be very noticeable to.
JK Jedi Master
Joined: Mar 2007
Posts: 14,800
Likes: 374
From: Midwest City, OK
I've had my Jeep for seven years now. I've diligently kept track of gas mileage and there are a few things I've noticed. By diligently, I mean I manually calculate it, the odometer is correctly calibrated, I usually buy the cheapest gas I can find (which these days means gasahol, but when I first got the Jeep meant real gas). And here is what I've concluded and observed:
1. The Saharas and X's as they come from the factory are optimized to get best gas mileage on the EPA test. Which may not be a very good representation of anyone's driving. You may think that isn't true, but consider what you're saying: Would Chrysler purposefully gear the Saharas and X's too high or low if they could both improve their gas mileage and advertise that fact when selling Jeeps? For the Rubis, they are appealing to a different market so go for the 4.10 compromise, which has lately been reduced to 3.73s for the autos, I believe.
2. There was a small hit when gasahol became the prevalent gas around the country. 100% gas does better, but the 10% number you see folks throw around would seem to be a bit optimistic.
3. As folks regear, change tires, etc, they are almost always doing other mods. And those other mods add weight and/or aerodynamic drag. I have NEVER done a mod that improved gas mileage. Never. That included regearing. But, when I regeared I also switched to 35s at the same time.
4. Folks posting gas mileage figures on any forum are all over the map. You really can't be sure how they calculated it, what their discipline was (did they recalibrate the speedometer? what gas were they buying? etc), and what their driving conditions were.
5. The only thing I've found on long distance driving that consistently improves gas mileage is slowing down. Driving 55-60 makes a huge difference over driving 70. About 2-3 MPG.
6. The wind is a huge factor. My gas mileage at 60 is destroyed by high head winds (25+). My gas mileage on the same trip can be from 11-18 MPG, carrying the same load. So, the only changed things are the terrain and weather. If someone asks me what gas mileage I get, I have difficulty saying, "Oh, 13 MPG", because it's all over the map. So, anytime I read where folks post their gas mileage, I wonder how it is they can come up with some single number. Perhaps like I do, by just forcing out some bogus "average" without explanation.
7. Oh, BTW, one funny thing I've noticed and I've commented to other people about, and they agree: Driving at high altitude improves gas mileage by a couple, three MPG. I consistently get better mileage in the mountains of Colorado than I do driving across the plains of Oklahoma.
So, at the end I guess the only thing one can really do to see if there is something that will improve their gas mileage for their vehicle weight, aerodynamics, driving style and driving conditions is to try it. But, I wouldn't want to bet that one would get better gas mileage because of this, that or whatever. There are just way too many variables for such generalizations.
1. The Saharas and X's as they come from the factory are optimized to get best gas mileage on the EPA test. Which may not be a very good representation of anyone's driving. You may think that isn't true, but consider what you're saying: Would Chrysler purposefully gear the Saharas and X's too high or low if they could both improve their gas mileage and advertise that fact when selling Jeeps? For the Rubis, they are appealing to a different market so go for the 4.10 compromise, which has lately been reduced to 3.73s for the autos, I believe.
2. There was a small hit when gasahol became the prevalent gas around the country. 100% gas does better, but the 10% number you see folks throw around would seem to be a bit optimistic.
3. As folks regear, change tires, etc, they are almost always doing other mods. And those other mods add weight and/or aerodynamic drag. I have NEVER done a mod that improved gas mileage. Never. That included regearing. But, when I regeared I also switched to 35s at the same time.
4. Folks posting gas mileage figures on any forum are all over the map. You really can't be sure how they calculated it, what their discipline was (did they recalibrate the speedometer? what gas were they buying? etc), and what their driving conditions were.
5. The only thing I've found on long distance driving that consistently improves gas mileage is slowing down. Driving 55-60 makes a huge difference over driving 70. About 2-3 MPG.
6. The wind is a huge factor. My gas mileage at 60 is destroyed by high head winds (25+). My gas mileage on the same trip can be from 11-18 MPG, carrying the same load. So, the only changed things are the terrain and weather. If someone asks me what gas mileage I get, I have difficulty saying, "Oh, 13 MPG", because it's all over the map. So, anytime I read where folks post their gas mileage, I wonder how it is they can come up with some single number. Perhaps like I do, by just forcing out some bogus "average" without explanation.
7. Oh, BTW, one funny thing I've noticed and I've commented to other people about, and they agree: Driving at high altitude improves gas mileage by a couple, three MPG. I consistently get better mileage in the mountains of Colorado than I do driving across the plains of Oklahoma.
So, at the end I guess the only thing one can really do to see if there is something that will improve their gas mileage for their vehicle weight, aerodynamics, driving style and driving conditions is to try it. But, I wouldn't want to bet that one would get better gas mileage because of this, that or whatever. There are just way too many variables for such generalizations.
Last edited by Mark Doiron; Feb 20, 2014 at 02:07 AM.
Thread Starter
JK Junkie
Joined: May 2011
Posts: 3,213
Likes: 21
From: Grand falls, Newfoundland
Ok,
Maybe a better question would be, after regearging did anyone go holy shit I can't make it to point x and before I could go to point x and beyond? similar conditions obviously. More power is always better, so if I only drop a mpg or two, im cool.
Maybe a better question would be, after regearging did anyone go holy shit I can't make it to point x and before I could go to point x and beyond? similar conditions obviously. More power is always better, so if I only drop a mpg or two, im cool.
JK Super Freak
Joined: Apr 2009
Posts: 1,331
Likes: 0
From: Dallas, TX
Auto, 4.10 w/ 305/65R17s. 3.5" lift. I hate my gears. Needs 4.88 to feel close to stock. Since adding roof rack, it needs 5.13. Gears are for more than just larger tires. They compensate for extra weight from bumpers, winches, armor, racks, etc.
Sent from some string and a few tin cans.
Sent from some string and a few tin cans.
Thread Starter
JK Junkie
Joined: May 2011
Posts: 3,213
Likes: 21
From: Grand falls, Newfoundland
Auto, 4.10 w/ 305/65R17s. 3.5" lift. I hate my gears. Needs 4.88 to feel close to stock. Since adding roof rack, it needs 5.13. Gears are for more than just larger tires. They compensate for extra weight from bumpers, winches, armor, racks, etc.
Sent from some string and a few tin cans.
Sent from some string and a few tin cans.
JK Enthusiast
Joined: Aug 2013
Posts: 338
Likes: 0
From: Elkton, Virginia
My wimpy 3.73's do just fine on my 33's ... I can think of other shizz i'd need to spend the $2k on for now... Maybe a gearing upgrade in the future, but the 3.73's on 33's really doesn't have any noticeable difference to the stock Sahara 18's on 255 duelers that i was running before.
