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Topic Review (Newest First)
12-15-2010 08:59 PM
Mr. Sinister
Quote:
Originally Posted by MTH View Post
Another regear question while we're on the topic . . . .

When one "regears", you're swapping out the gears in your differential, correct? And, if that's true, it would be an opportune time to add something like a limited slip differential or lockers?
yes, the perfect opportunity to upgrade. since the diffs have to come out anyway, labor costs above the gear swap should be nil. if your mechanic tries charging you extra for the diffs, you can call him on it.
12-15-2010 08:57 PM
Mr. Sinister as i thought more about this, i should mention that any added weight over stock from the rims and bigger tires, plus increased ride height will also slow you down a bit. there's complex mathematical formulas for figuring all that out, but the simple answer is you will notice what feels like a loss in power (you don't actually lose power, it just takes more of what's available to get you moving), but whether or not you can live with it is completely up to you. i don't think it will be a huge loss, but you will notice it.

to be honest, if you're running an automatic i'd recommend a superchips flashpaq or the like to change your shift points and tighten up your throttle response. that will probably help to offset the feel of lost power. plus it will recalibrate your speedo, which you will need to do as well.
12-15-2010 08:54 PM
MTH Another regear question while we're on the topic . . . .

When one "regears", you're swapping out the gears in your differential, correct? And, if that's true, it would be an opportune time to add something like a limited slip differential or lockers?
12-15-2010 03:13 PM
Mr. Sinister well, your engine is going to labor a little bit more to get you moving and keep you moving. your highway cruising rpm is going to drop a bit which might make you a bit more sluggish when trying to pass. the difference in rpm may be enough to make you "hunt" for the proper gear on the highway, but i can't say for sure. your mileage may suffer on the highway.

theoretically, lowering highway rpm will increase gas mileage, but the jeep lacks the low end power to compensate for the lowered rpm. the 3.8 doesn't have a huge powerband, so it's sensitive to gearing and tire diameter changes. most lower powered smaller engines are, at least more so than larger v8 engines and such that have broad torque curves down low.

around town will be a little more sluggish and will be tougher on your mileage. what's going to nail you is the rpm drop between shifts if you're on a automatic. with a manual, you can just wind it our a little further to the detriment of fuel economy, but the automatic will still shift under the same parameters as stock unless you use a superchip or something similar to change the shift points. i personally feel the jeep lugs a bit too much in top gear, so imagine lowering your cruising rpm by a couple hundred rpm, possibly falling out of your useable rpm range. you drop engine speed, but you increase load to maintain momentum.

plug your specific numbers in here to get an idea of your cruising rpm with 35" tires at a desired speed:
RICHMOND Street Performance Calculator

6 speed manual, 6th gear is .84 - 1900 rpm at 65mph.
4 speed auto, OD is .69 - 1606 rpm at 65

at 50 mph in OD on the auto, you're at 1235 rpm on a 35" tire.

so, tremendous stress? no. if you're not at all concerned with the performance now, you probably won't care too about what you'll lose when upsizing the tires. whether or not you'll have sufficient power to maintain 80mph is debatable. i'd say not without great effort.
12-15-2010 02:15 PM
Gobigorgohome
Quote:
Originally Posted by Mr. Sinister View Post
sounds like most jeeps. you do have to kick it out of OD on the highway from time to time, especially on inclines. mine is bone stock and i have to.
the 1" difference in tire height is not causing your issues. the sahara model with it's 18" wheels and tires are basically 33" tall tires from the factory.

any lift will affect mileage and power as well as you're just adding more wind resistance.

you can do a regear, but i don't think you'll be happy with the results and your wallet will be a $1000+ lighter. if you're dead set on swapping gears, a set of 4.10 gears will only increase your rpm a bit, anything beyond that is overkill for a 33" tire and your highway performance and mileage will suffer even more.
I have a sahara with 3.73 gears and the 18" wheels 33" factory tires you speak of. I really want to upgrade to 17" wheels and 35" tires (once I get a 2.5" lift). I know this will cause the jeep to seem sluggish off the line, but since I didn't buy this vehical to go fast is there any other problems associated with running 35" tires on a 3.73 gear ratio? I assume that I will still be able to drive at 80mph, but it will just take me an extra second or two to get there. I figure that it will take longer to get to my engines peak powerband, but I will save on mph. Would this combination cause a tremendous amount of stress on the vehical?
12-15-2010 02:03 PM
MTH
Quote:
Originally Posted by drcasin View Post
Hi guys, I got a JK 2009automatic transmission, and put 33` tires, want to regear .the question is according with your experience what is the best gear ratio?
Quote:
Originally Posted by drcasin View Post
hi guys,

thanks for the advice, however at this stage my automatic transmission on this tires 33`x 12 x 17 s and suspension lift 2,5 inch is driving very bad, fuel comsupsion is pathetic, can not keep 65 miles/hours on highway without shifted to 3 gear, and on off road is the worse
Have you tried a Superchips Flashpaq, by the way? That'll be a much cheaper and quicker way to give your auto a little extra pep.
12-15-2010 01:50 PM
Mr. Sinister
Quote:
Originally Posted by drcasin View Post
can not keep 65 miles/hours on highway without shifted to 3 gear, and on off road is the worse
sounds like most jeeps. you do have to kick it out of OD on the highway from time to time, especially on inclines. mine is bone stock and i have to.
the 1" difference in tire height is not causing your issues. the sahara model with it's 18" wheels and tires are basically 33" tall tires from the factory.

any lift will affect mileage and power as well as you're just adding more wind resistance.

you can do a regear, but i don't think you'll be happy with the results and your wallet will be a $1000+ lighter. if you're dead set on swapping gears, a set of 4.10 gears will only increase your rpm a bit, anything beyond that is overkill for a 33" tire and your highway performance and mileage will suffer even more.
12-15-2010 11:02 AM
drcasin
Quote:
Originally Posted by Ray - San Jose View Post
Quote:
Originally Posted by drcasin View Post
Hi guys, I got a JK automatic transmission, and put 33` tires, want to re gear .the question is according with your experience what is the best gear ratio?

hi guys,

thanks for the advice, however at this stage my automatic transmission on this tires 33`x 12 x 17 s and suspension lift 2,5 inch is driving very bad, fuel comsupsion is pathetic, can not keep 65 miles/hours on highway without shifted to 3 gear, and on off road is the worse
12-14-2010 04:35 PM
Jet6122
Quote:
Originally Posted by Mr. Sinister View Post
All the things you listed, plus increased wear on all drivetrain parts (everything spins faster), possible complete lack of top end power as you can "run out of gear" too quickly and on the jk, the 3.8 isn't exactly a top end monster, so you effectively lose top gear in the transmission. You should always gear to suit your engine's powerband, this is true in drag racing and in offroading. Where it is different is in drag racing, you want to get out of the hole quicker, hit your powerband quicker and cross the traps somewhere near your maximum horsepower rpm. When you upsize the tires for offroading, you re-gear to maintain a similar power curve you had stock. You don't always increase in revs much with a re-gear because the larger tires lower the revs because they take longer to complete one full revolution. A little extra gear may indeed help a Jeep with it's on and off road performance, but for 1 added inch of tire height, you're really not going to notice it. If he were going to a 35" tire, I would definitely do a re-gear. If this were strictly an off road rig where you'll never use top gear, a re-gear would be helpful. The decline in gas mileage and highway driveability don't justify a re-gear in this case.
I just changed from the stock Goodyear sra 255/75/17 (32.2 x 10) to the Duratrac 285/70/17 (32.8 x 11.50). I think those measurements are close. Not much taller, but a good bit heavier tire. I noticed a slight power loss, but I feel like the engine was weak before! I don't offroad, so I "think" after some searching around here, it is best sticking with the 3:73's since the power loss seems okay. At 70mph I turn 1900rpm, at 80 2100rpm. I haven't checked my gas mileage yet.

Well put Mr. Sinister.
12-14-2010 12:25 PM
Mr. Sinister it's a bit simplified, but that's the basics. it gets more complicated when you try to compensate for added unsprung weight (big wheels and tires weigh more, thus take more effort to turn) and so on.
12-14-2010 12:11 PM
MTH
Quote:
Originally Posted by Mr. Sinister View Post
All the things you listed, plus increased wear on all drivetrain parts (everything spins faster), possible complete lack of top end power as you can "run out of gear" too quickly and on the jk, the 3.8 isn't exactly a top end monster, so you effectively lose top gear in the transmission. You should always gear to suit your engine's powerband, this is true in drag racing and in offroading. Where it is different is in drag racing, you want to get out of the hole quicker, hit your powerband quicker and cross the traps somewhere near your maximum horsepower rpm. When you upsize the tires for offroading, you re-gear to maintain a similar power curve you had stock. You don't always increase in revs much with a re-gear because the larger tires lower the revs because they take longer to complete one full revolution. A little extra gear may indeed help a Jeep with it's on and off road performance, but for 1 added inch of tire height, you're really not going to notice it. If he were going to a 35" tire, I would definitely do a re-gear. If this were strictly an off road rig where you'll never use top gear, a re-gear would be helpful. The decline in gas mileage and highway driveability don't justify a re-gear in this case.
Outstanding answer. Thank you sir.
12-14-2010 12:08 PM
Mr. Sinister
Quote:
Originally Posted by rics1997 View Post
If you look at the chart provided by Ray, it will gain power from getting larger gears but will lose fuel economy. The chart actually says for a 33 the 4.10 is the best gear to use for overall performance. The 3.73 you have on it now will be best for fuel economy but you may lack just a little power then if you re-gear. So is the cost worth a little more power and a little worse fuel economy? If it is then re-gear with either 4.10 or 4.56
you don't actually "gain" any power, you only change the torque multiplication. and when you add more down low, you lose it up top. all a re-gear will do is get you into your powerband more quickly. and indeed, through the powerband more quickly. a proper re-gear for big tires should only raise your operating rpm a little.
12-14-2010 12:05 PM
Mr. Sinister
Quote:
Originally Posted by MTH View Post
Out of curiousty, what's the downside of "overkill" in a regear? Bad fuel economy? Engine hardship? Is it just the fact that you're wasting money? I really haven't heard any downsides to a well done regear, but surely they exist?
All the things you listed, plus increased wear on all drivetrain parts (everything spins faster), possible complete lack of top end power as you can "run out of gear" too quickly and on the jk, the 3.8 isn't exactly a top end monster, so you effectively lose top gear in the transmission. You should always gear to suit your engine's powerband, this is true in drag racing and in offroading. Where it is different is in drag racing, you want to get out of the hole quicker, hit your powerband quicker and cross the traps somewhere near your maximum horsepower rpm. When you upsize the tires for offroading, you re-gear to maintain a similar power curve you had stock. You don't always increase in revs much with a re-gear because the larger tires lower the revs because they take longer to complete one full revolution. A little extra gear may indeed help a Jeep with it's on and off road performance, but for 1 added inch of tire height, you're really not going to notice it. If he were going to a 35" tire, I would definitely do a re-gear. If this were strictly an off road rig where you'll never use top gear, a re-gear would be helpful. The decline in gas mileage and highway driveability don't justify a re-gear in this case.
12-14-2010 11:49 AM
rics1997 If you look at the chart provided by Ray, it will gain power from getting larger gears but will lose fuel economy. The chart actually says for a 33 the 4.10 is the best gear to use for overall performance. The 3.73 you have on it now will be best for fuel economy but you may lack just a little power then if you re-gear. So is the cost worth a little more power and a little worse fuel economy? If it is then re-gear with either 4.10 or 4.56
12-14-2010 11:41 AM
MTH
Quote:
Originally Posted by Mr. Sinister View Post
you don't NEED to re-gear for 33 inch tires, you should already have 3.73's. jk's come stick with 32 inch tires.
you won't notice much of a difference with the slightly taller tire going to something like a 4.11 and 4.56 would be borderline overkill.
Out of curiousty, what's the downside of "overkill" in a regear? Bad fuel economy? Engine hardship? Is it just the fact that you're wasting money? I really haven't heard any downsides to a well done regear, but surely they exist?
12-14-2010 11:10 AM
Mr. Sinister you don't NEED to re-gear for 33 inch tires, you should already have 3.73's. jk's come stick with 32 inch tires.
you won't notice much of a difference with the slightly taller tire going to something like a 4.11 and 4.56 would be borderline overkill.
12-14-2010 11:10 AM
Ray - San Jose Clickie
12-14-2010 10:31 AM
drcasin
Regear

Hi guys, I got a JK 2009automatic transmission, and put 33` tires, want to regear .the question is according with your experience what is the best gear ratio?

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