My brother just bought an 93 Wrangler with ford 9 inch axles front an rear with 5.13 gears right now has 37x12.50R15 on 10 inch rims, would it hurt when he wears out the 37's to drop down to say 35x14.50R15 or does he need to saty with the 37's. any help would be nice.

Need to know the drive train combination. Is it a daily driver or strict off road only?

it has an 4.0 motor been told it is an 4 speed tranny , dual purpose driver an weekend toy

auotmatic not sure what type of tranny been told two different things, one was an 4 speed chevy, other was an newer 4 speed jeep, has an B&M shifter in it. sorry that is all I know about it

Won't make a difference at all, In fact you will actually get better gas milage & less wear & tear on the drivetrain and a better street ride.

thank you grassroots, we were not sure but right now will just enjoy it on 37's , my brother just found the jeep bought with all good stuff like isaid also has an atlas 2 transfer case lockers front an rear, but he has gillens berra which is a nerve disorder an was wanting an play toy for to get some time to enjoy. I hope some time the near future can get an pic up it in woods or something.

YOU will not get better mileage, unless you changes out your ratios. 5:13 is very low for a driver, good for offroad however. But there is no other reason you cannot down size.

Won't make a difference at all, In fact you will actually get better gas milage & less wear & tear on the drivetrain and a better street ride.

Not completely correct. If the rig is set up for 37s, with a 5.13 gear ratio depending on what trans and the final drive ratio it may cause the engine to spin higher RPMs with a smaller diameter tire. The specific trans model would be very helpful. This is from Randys Ring and Pinion.
With 35" tires
http://www.ringpinion.com/Calc_RPM.aspx

With 37" tires.
http://www.ringpinion.com/Calc_RPM.aspx

For example I have a 02 TJ with 4.0 5spd 35's and 4.88s. Can I run a smaller diameter tire like a 33? Yes but my engine RPMs will be higher. Now if I run 37s with my same setup the RPMs will be lower.
With my current setup according to the calculator and mind you this is not always 100% spot on but a ballpark range I will be pulling 2623 RPMs at 70 MPH. With 37s and the same setup I will pull 2482 at 70 MPH and with 33s it will be 2782 RPMs there for my engine will work harder and not give me better gas mileage.

Overall the once in final drive and at highway speed the lower the RPMs the better for fuel economy. So you brother needs to consider things.

5.13's with 35's is gonna suck on the road no matter what trans you have in there. id run it but thats cuz i dont mind doing 45mph on the highway in the right lane pissing everybody off!
stick with the 37's get a set thats not too aggressive and it will ride just fine

Its all speculation.

I use Grimm Jeeper's Ratio Calcs to play with gear ratios

http://www.grimmjeeper.com/gears.html

Plugging in your numbers the RPM difference between 5.13s with 35s and 37s is less then 200 rpms diffence at 65 mph on the highway. The RPMs being 2241 with a 35" tire, versus 2120 with a 37". Thats not even really into the cruising powerband of the 4.0 yet. With 37s it would be even lower. Theres no reason why you can't go with 35s. You may even notice a little gas mileage improvement because theres less demand being put on the motor to get the vehicle moving.

Plus 37s on the highway is scary, the sidewalls are so fucking big and mushy any evasive manueving is a possible wipeout. Even 35s feel mushy on 15" rims.

sorry mislead on driver, just like from mknoxville,tn to gatlinburg for rod runs an to the trails near us such as windrock, royal blue an such, not an everyday drive but thank you all very much for the help

higher rpm doesn't automatically equal less gas milage

yeah it is an opinion. just like your opinion that 37's are scary. not only does that depend on the kind of tire but also the drivers preference and abilities. i personally have no problem making quick maneuvers with tall tires. but hey im comfortable going around a turn on 3 wheels too. if you are comfortable on the 37's now, then keep 37's on that ratio.

does that calculator tell ya how much more gas you will use at an extra 200 rpm's? just curious

It's absolutely true that higher rpms do not = less gas mileage automatically. EVERY engine has an area which it operates most efficiently. My Dodge truck has a max of 3000 rpms, and when I drive it if I shift and keep my rpms in 1600 range I LOSE 2 MPG over shifting in the 2000-2200 range.

It all depends, mostly speculation and opinions which don't have much fact to them. Just like 37's are soft on the road, opinion. I agree that they are softer than a 35 on the road, but it ALL depends on the tire (like a Bogger, tough ass little fawker, or a MT/R which flexes all over).

Have fun, go beat the Jeep.

Thats why I posted that its not completely true that dropping tire size will be automatic increase in MPGs rather provided some general examples of different RPMs and mine but closed in saying that the OPs brother would need to take different things into consideration.

But most importantly its a Jeep the mileage will always suck compared to a Honda, just wheel it, beat on it and have fun.:flipoff2:

yeah it is an opinion. just like your opinion that 37's are scary. not only does that depend on the kind of tire but also the drivers preference and abilities. i personally have no problem making quick maneuvers with tall tires. but hey im comfortable going around a turn on 3 wheels too. if you are comfortable on the 37's now, then keep 37's on that ratio.

does that calculator tell ya how much more gas you will use at an extra 200 rpm's? just curious

For the sake of argument, going around a corner with 3 wheels on the ground is NOT SAFE. Just because you are comfortable doing it does not make it safe either. I've driven highways on 37s and 35s, I've had to swerve out of the way and brake hard. Each time I was borderline out of control and taking my ass in my hand. They feel safe, right up until the time you are all over the road trying to keep the vehicle under control because some asshole cut you off, or a deer jumped out in front of you etc etc. People don't take built offroad rigs on the highway for a reason. Regardless its a mute point, the original poster isn't using it on the highway anyways.

Also no the calculator doesn't offer a MPG, that would be so subjective it wouldn't even be remotely accurate. But, its not a far fetched concept that the less gas you have to give a vehicle to pull a load or turn larger tires the greater your gas mileage will be.

For example: It's a widely proven fact that TJs with 4.0s that have towing package with 4.10 gears get grossly better gas mileage then with the standard 3.07s. The 3.07s work at a lower engine RPM, but the 4.10s use less gas because there is less demand being placed on the motor.

well by that logic going around a turn at 150mph isnt safe either. but how many race car drivers die compared to those simply crossing the street. for most people getting to the point of 3 wheels is past the point of control. thats why i said "I" am comfortable with it.
i understand the mpg being better with a higher rpm in certain cases. im not arguing that. but i owned a yj with 35's. knew i was gonna go larger so i geared to 5.13's. guess what... my mileage sucked at 70mph fast forward to 3 months later when i threw on a set of 37's. with that mere 200 rpm change my motor didnt sound like it was screaming and i got a few extra miles out of a tank. not much but it did improve.

on a wrangler with 35's 4:56 will give you stockish feel and mpg (but factor in the weight of the tires into a slight loss)
with bumpers, winch, gear, bla bla bla, 4:88's is an improvement and does exactly what you say it will improve the mileage because the motors not struggling to move the weight. 5.13's just too high and you start to go backwards IN MY EXPERIENCE. but that being said my friends yj gets 21 mpg on 33's and 4.10's with a damn carb. so these things differ.
ive owned a number of wranglers with different size motors, lift, wheels, tires gears, and so on so like i said i am just giving you MY EXPERIENCE. take it as you want

I've got a 92 on 37's, 4.0, 4:88's, auto (but mine is a three speed). IMO 5:13's would be too much gear for 35's but you need to find out your final drive ratio. For comparison purposes, mine hums along at about 2700 rpm at 50mph. I always feel like I'm gonna kill the motor, so I avoid the highway but drive it around town all the time. The other thing he could do is borrow a set of 35's, see how it rides and if he likes it.

Why is it that everyone seems to think that gear ratios and tire size is the main concern for getting better or worse MPG. Blacksheep is correct in what he states here.

All the calculators tell you is the RPM's that the engine will need to turn at for any given tire size gear combination. Thats all.

You can not forget, and this is a very big part, that you still have to turn those big ass pieces of meat with the same motor.

Basically larger tires have the same effect as driving uphill. Based on the tire size and gear ratio. You can lower the gears to make it easier to climb that hill or turn those tires. But it is going to cost you.

Another example. (Numbers for example only) My truck on flat ground does 1400 rpm at 65. It still does 1400 rpm at 65 going down hill. But the amount of fuel being used between the two is very different.

Of course Offroader want lower gears for crawling.

A stock 4.0 will surely turn 37's with 5.13's, it will turn them with 4.88's. Just not efficiently.

BUT WE DON'T BUILD THEM LIKE THIS TO BE EFFICIENT. I HAVEN'T SEEN 1 PRUIS ON THE TRAIL YET

I HAVEN'T SEEN 1 PRUIS ON THE TRAIL YET

haha give it time!