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Unread 12-23-2012, 04:33 PM   #1
ubengineer
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Gears vs tires

I keep hearing that if I increase my tires to 35's I'm going to need to re-gear. I currently have stock 3:07. Can someone please explain to me why I would need to regear?

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Unread 12-23-2012, 04:56 PM   #2
BlueRubiconrich
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Jeep sets up your Jeep with gears to match the tire size that it came with for best MPG if you increase the tire size you hurt the gearing of your Jeep
the bigger the tire size the harder it is to keep speed when going up hills.
So 3.07 gears are not going to cut it with 35" tires and also your Dana 35 rear axle is not set up for 35" tires So a axle upgrade may be in order.
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Unread 12-23-2012, 05:03 PM   #3
ubengineer
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Well I have a ford 8.8 w 4:56 but after doing research I'm Thinking I made a mistake w those gears because I still want to take it on the highway and go 60 mph. I haven't installed the 8.8 yet but I do have it sitting there w the 4:56.
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Unread 12-23-2012, 05:08 PM   #4
ubengineer
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Can someone please let me know your setup regarding tire size vs gears and what rpms are you getting on the highway.
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Unread 12-23-2012, 05:30 PM   #5
Ironhead
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for starters, you need to fix your profile.

do you have a 4 speed auto? those didnt come in a 97
v-6 never came in a tj
what TC do you have?
what rear axle do you have?

if memory serves correct, a lot of guys go with either 4.10 or 4.56 gears with the 3 speed auto that would have come stock in 97 and 35" tires
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Unread 12-23-2012, 05:36 PM   #6
ubengineer
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I can't fix it on the app but I have a straight six auto. Everything is stock. I do have a ford 8.8 w 4:56 on the side I just have installed it yet.
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Unread 12-23-2012, 05:40 PM   #7
Oldcraneguy
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This link will let you know what to expect....ocg

http://www.grimmjeeper.com/gears.html
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Unread 12-23-2012, 05:45 PM   #8
laybackman
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Quote:
Originally Posted by ubengineer View Post
Can someone please let me know your setup regarding tire size vs gears and what rpms are you getting on the highway.
Let's say you went from 3.23's to 4.56's. Your engine has to turn the driveshaft more to maintain a given speed going down the road. In this example your drive shaft has to turn 1.23 more times before the ring gear makes one revolution. 4.56 - 3.23 = 1.23
Off road drivers want to rpm low and not stall. They go tall tires for clearance and very 'tall' gears. They may also alter the xfer case ratio to help them do that even more.

Drag racers can 'run out of engine' before they get through the 1/4 mile if they gear too high. In a perfect world they want to run out of engine just as they pass through the lights.

How does this weekend warrior get his rig to the drags if it is geared too high and he has no budget for a ramp truck? A set of real tall street tires.

I run stock 3.07's in a '95YJ 4.0 automatic. I went with 31" tires which are ~10% larger over stock tires. So my speedo says I am doing ~ 60 when in fact I am doing more like 65. The distance travelled and speed I am driving in my YJ is measured at the T-case......before the taller than stock tires.
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Unread 12-23-2012, 05:51 PM   #9
ubengineer
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Quote:
Originally Posted by Oldcraneguy
This link will let you know what to expect....ocg

http://www.grimmjeeper.com/gears.html
I was looking at that yesterday and there just too much info I don't know to fill that in
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Unread 12-24-2012, 08:18 AM   #10
stokerdog
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with a 3spd auto and 4.56's you will be at 2630rpm at 60 mph. 3K at 70 mph. Sounds perfect to me
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Unread 12-24-2012, 12:02 PM   #11
ubengineer
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3k sounds like a lot tho
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Unread 12-24-2012, 09:45 PM   #12
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Not having O.D. is going to have compromises. If you had a 4spd auto, rpms would be lower at hwy speeds. I would rather compromise my highway driving then my around town/offroad driving. A Jeep 4.0 is very happy at 3K. On the Interstate in the rockies there are many long grades with speed limits 65/75. 20 mile climbs in 4th gear at 3K are normal. my 4.0 runs cool and smooth.
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Unread 12-25-2012, 08:17 AM   #13
ubengineer
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I'm trying to google if my jeep has od no luck so far and unfortunately I think it's a 3 speed
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Unread 12-25-2012, 11:37 AM   #14
stokerdog
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A 97 is a 3spd if it has the OE tranny. The OD tranny's came later.
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Unread 12-27-2012, 03:55 AM   #15
ubengineer
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I called jeep they said I don't have od (kinda wish i did)
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