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Unread 07-15-2009, 04:38 PM   #1
TDmaster
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Gear Change without a Tire Change?

I'm running stock (OEM) 32 inch tires on my '09 JK X, with an automatic and 3.73 gears. In OD the RPMS are well below the power range, around 1700 which saves gas but really sucks for performance. I want to regear and eventually buy new, larger tires 35 or 37 inch, which also require a lift. I'm thinking about regearing now, and get the lift/tires/wheels later. I plan on installing 5.13 gears front and rear (D30 and D44).

According to my calculations, using the stock tires, this will bring the RPM upto 2400 in OD and nearly 3500 in 3rd gear. Thats is in the real power range of the 3.8L V6.

Apart from poor / reduced MPG's, is there anything I should be concerned about running these taller gears on an otherwise stock drivetrain...Oh, yeah, I forgot one thing, I'm running an RIPPMODS supercharger that sees positive boost at 3000 rpms.

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Unread 07-15-2009, 06:00 PM   #2
wrestlerobx
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dont regear without doin lift and tires first and also search there is a recent thread on this and many others
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Unread 07-15-2009, 07:03 PM   #3
ronjenx
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Quote:
Originally Posted by TDmaster View Post
I'm running stock (OEM) 32 inch tires on my '09 JK X, with an automatic and 3.73 gears. In OD the RPMS are well below the power range, around 1700 which saves gas but really sucks for performance. I want to regear and eventually buy new, larger tires 35 or 37 inch, which also require a lift. I'm thinking about regearing now, and get the lift/tires/wheels later. I plan on installing 5.13 gears front and rear (D30 and D44).

According to my calculations, using the stock tires, this will bring the RPM upto 2400 in OD and nearly 3500 in 3rd gear. Thats is in the real power range of the 3.8L V6.

Apart from poor / reduced MPG's, is there anything I should be concerned about running these taller gears on an otherwise stock drivetrain...Oh, yeah, I forgot one thing, I'm running an RIPPMODS supercharger that sees positive boost at 3000 rpms.
Being old school, I always thought:
4.10 gears are lower than 3.21 (because the axle turns slower);
3.21 gears are taller (higher) than 4.10, (because the axle turns faster);
1st gear is lower than second, (because the drive shaft turns slower in 1st);
etc, etc...
Is this the way the younger crowd sees it, too?
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Unread 07-15-2009, 09:47 PM   #4
TDmaster
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Originally Posted by ronjenx View Post
Being old school, I always thought:
4.10 gears are lower than 3.21 (because the axle turns slower);
3.21 gears are taller (higher) than 4.10, (because the axle turns faster);
1st gear is lower than second, (because the drive shaft turns slower in 1st);
etc, etc...
Is this the way the younger crowd sees it, too?
Well, I don't know about 'old school' vs the younger crowd, I'm 53 if it makes any difference?? When I was growing up around the Dragstrip, we used to call LOWER gears (numerically higher number) as TALLER gears because of the added torque at the launch. On the street we always envied the kid with 4.10's because they were nearly alwasys lower than the 3.21's or 3.73's we had. They made the rod stand 'tall' (which propably had more to do with the raked front-end than the gears. In any case, low gears at the strip literally made the rear racing slicks 'stand up tall' at takeoff due to the high torque which, BTW, is why you need lower (taller) gears to turn those larger (taller) diameter than stock 35 inch tires.
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Unread 07-15-2009, 10:13 PM   #5
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Why???

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Originally Posted by wrestlerobx View Post
dont regear without doin lift and tires first and also search there is a recent thread on this and many others
I have searched / read many gear related threads and I can't find any mentioned as to why one should not change gears first, then lift and tires later...

You just spit out the "traditional" way of thinking (which I already knew) without explaining WHY I should do it that way which is what the question was to begin with. WHY should I do the lift and tires first, expecially if I don't have the money to do all three things at once? I, for one, do not want to run 3.73 gears with 37 inch tires for any amount of time...the RPMs will be way to low.
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Unread 07-16-2009, 12:01 AM   #6
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Quote:
Originally Posted by TDmaster View Post
I have searched / read many gear related threads and I can't find any mentioned as to why one should not change gears first, then lift and tires later...

You just spit out the "traditional" way of thinking (which I already knew) without explaining WHY I should do it that way which is what the question was to begin with. WHY should I do the lift and tires first, expecially if I don't have the money to do all three things at once? I, for one, do not want to run 3.73 gears with 37 inch tires for any amount of time...the RPMs will be way to low.
You don't *need* to do one or the other first, but your engine as a consequence of the higher gearing at the diff will need to rev higher to produce the same speed. This balances out with a taller tire like you plan to have. The higher gearing balances becuase the added torque turns a bigger tire which is basically a bigger lever that propels you more distance per rev. A smaller tire with the high gearing will get your engine revving real high to turn a smaller lever which will move you less distance per rev.

Some math for contrast (this assumes the auto tranny):
@3500 rpm with 5.13 gears in 3rd gear (1:1) 682.26 rpm at the wheel
that rpm produces:
1.05 miles per minute on a 32" (63 mph)
1.19 miles per minute ona 37" (71.8 mph)

@3500 rpm with 3.73 gears in 3rd gear (1:1) 938.33 rpm at the wheel
that rpm produces:
1.44 miles per minute on 32" (86.4 mph)
1.64 miles per minute on 37" (98.7 mph)

To summarize: your 32" tires will run 63 mph @3500rpm with 5.13s versus 86.4 mph @3500rpm with your stock 3.73's Big difference, but its up to you on which gets done first. It just depends on what you want to put up with. Personally I'd get both done at the same time.
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Unread 07-16-2009, 01:14 AM   #7
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Quote:
Originally Posted by Broken_Soldier View Post
Some math for contrast (this assumes the auto tranny):
@3500 rpm with 5.13 gears in 3rd gear (1:1) 682.26 rpm at the wheel
that rpm produces:
1.05 miles per minute on a 32" (63 mph)
1.19 miles per minute ona 37" (71.8 mph)

@3500 rpm with 3.73 gears in 3rd gear (1:1) 938.33 rpm at the wheel
that rpm produces:
1.44 miles per minute on 32" (86.4 mph)
1.64 miles per minute on 37" (98.7 mph)

To summarize: your 32" tires will run 63 mph @3500rpm with 5.13s versus 86.4 mph @3500rpm with your stock 3.73's Big difference, but its up to you on which gets done first. It just depends on what you want to put up with. Personally I'd get both done at the same time.
I came up with the same (nearly) RPM you did using the following formula:
Gear Ratio RPM

RPM = MPH * ( (Diff Ratio * Trans Ratio * TransferCase Ratio * 336) / Tire Diameter) )

MPH = 65
Diff Ratio = 3.73 vs. 5.13
Trans Ratio (3rd gear) =1.00; OverDrive = 0.69
TransfCase Ratio = 1
RPM = 3rd 65 * ((3.73*1.00*1*336) / 32.06 ) = 2540 OEM Tires
RPM = OD 65 * ((3.73*0.69*1*336) / 32.06 ) = 1753 OEM Tires
RPM = 3rd 65 * ((5.13*1.00*1*336) / 32.06 ) = 3495 OEM Tires
RPM = 3rd 65 * ((5.13*1.00*1*336) / 34.80 ) = 3220 35 Tires
RPM = OD 65 * ((5.13*0.69*1*336) / 32.06 ) = 2411 OEM Tires
RPM = OD 65 * ((5.13*0.69*1*336) / 34.80 ) = 2222 35 Tires

What is interesting is that the RPM differences between stock 32 inch tires and 35 inch tires on 5.13 gears @ 65 MPH is less than 300 (3220 vs 3495), and that differences is even less in Overdrive (0.69 ratio): (2222 vs 2411).

So the fact that lower gears and taller tires go hand in hand, the goal is not to keep the same OE RPMs, the 3.8L in OD spins 1700 RPM or so which is well below the powerband of 2200-2400 RPM. That is why 4.88's are not recommended with the 3.8L V6 because it does not raise the RPMs enough to get the power and highway speeds.

If you have the 3.73 and the auto and like I have, you must have noticed the real acceleration lag trying to get around a car going 60 when your doing 65, you instictively change lanes and (usually) try to regain your crusing speed, but you can't without the downshift into 3rd gear (you have to put your foot into because your cruising at only 1700 RPM. Yes your MPG will be better at the speed, but if you like to have quick throttle response, you need to be running in the power range and fork up the dollars at the pump to get that satisfaction. However, since my Supercharger actually SAVES gas, while adding more HP, I can live with the higher RPMs.
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Unread 07-16-2009, 05:36 AM   #8
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I would never try 32's and 5.13 gears. At 70mph my engine is screaming (3200rpm) in 6th with the 35's. Rolling with 32's could really cook those gears in the pumpkin reving it that high. They would be fine for around town and perfect for offroading but freeway uses I just wouldnt test it... Do yourself a favor and get it done all at once or after you get the larger tires. Either that or drive around 55mph untell you can get the tires on.
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Unread 07-16-2009, 09:15 AM   #9
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He has an automatic. The manual has a lower numerical ratio. I have an automatic I have 5.13's turning 35's and at 70 mph I turn a hair shy of 2,500 rpm, which is perfect. Yes, my speedometer is correct and verified with GPS. As you save up money for the upgrades, put the money into a saving account or something where it can earn a little bit of interest and can be withdrawn without penalty. If you have the money for gears right now, put that money into the savings account now and as you have more money to spend on the upgrades, put that money in. It's not like the money will evaporate if it is not spent within a certain period of time.

Here is a true but mostly unrelated story. A couple of years ago I sold a rear AEV bumper. The guy didn't have the money to buy it. He wanted to send me payments every few weeks via money order and after I received the last payment, then ship the bumper to him. Would it not have been easier for him to just hold onto the money until he had the required amount? I had agreed to hold the bumper for him until he had enough money. He still didn't want to do that and got kind of pissy with me about it, so I agreed to his terms and sent him the bumper after I received the last of the payments.

Do you see what I am getting at?

If you want all those upgrades and don't have the money for them, save up for them. The only one I can see that you could hold off on is the wheels, as 35's will mount on the stock wheels easily.

5.13's are too low for 32" tires. I ran 35's with stock 4.10 gears for 4,000 miles. Don't do the lift without the tires because that looks goofy and in my opinion is more prone to rolling over due to a higher center of gravity. The 35's will be wider and the backspacing required for them will also make the Jeep have a wider track which also increases stability.

Personally, I think you should have gone with tires/wheels/lift/gears first and gotten the supercharger later. It probably would have provided enough of a performance increase to a lower overall gear ratio to satisfy you, at least temporarily.

You can always turn the overdrive off with a push of the button btw. That will probably be good enough for your stock gearing and 35's. If you want to run 37's, you probably are going to swap the D30 out for D44.
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Unread 07-16-2009, 10:08 AM   #10
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Best and cheapest advice at this point is from Prot:
"You can always turn the overdrive off with a push of the button btw. That will probably be good enough for your stock gearing and 35's. If you want to run 37's, you probably are going to swap the D30 out for D44."
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Unread 07-16-2009, 03:28 PM   #11
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Well I kinda worded my point weird, Im not saying that its going to over rev the engine. But the gear themselves are going to get extreamly hot when they turn that fast. This is regardless of if you have an auto or manual tranny. The gears will still be turning as fast as the road is moving under you... The only thing that will make them turn slower will be the wheel size.
Turning off OD will help, but its not the cure. There is a reason why bicycles have 2 sets of gears. If only one set of gears did the same thing as 2 they would of never installed them. Again I would wait tell you get the tires before switching out the gears.
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Unread 07-16-2009, 03:51 PM   #12
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if you dont have the money for both right now and only want to do one then take the money you have for that mod and put it away its own little spot wether it be in a shoe box or separate account, then once you have saved for the other mod then do both.

I see you are 53 and i would think this would be an obvious choice with someone of your age and maturity level, im assuming anyway. (Not trying to insult you so please do not take it that way)
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