JK09TX

Im am also looking at 315/70/17 as well. They show on paper that they are around the same height as a 285/75/17 but wider. So do the JK with 3.73 work with 315/70/17 without re gearing or will re gearing be needed? I do not plan to re gear until I get enough to lock them as well. I do like to hit trails every so often so I do want to make sure my jeep can run them.

nthinuf

It's the same answer. Open the faq's, find the chart, plug in some numbers. That doesn't show all the variables, of course, like the drag from a lift, weight from any armor, vehicle weight difference between 2/4 doors, extra tire mass, specific location you drive (flats at sea level vs mountains), etc, but it is a good place to start just to get a feel for where your rpm's will be.

Use actual measured tire height when you can, not the mfg's spec. And to get OD Off rpm, just divide the number you get by .69

An example:
(this one is just for comparison purposes - a stock 3.8 6-spd rubi driving off the lot)
3.8 6-spd rubicon, 4.10, 31" (estimated actual height of stock 32's)
4.10, 31" = 2614 rpm @ 70

3.8 auto, 3.73, 31"
3.73, 31" = 1932 rpm @ 70

Move up to a 35" tire, with a 34" actual height.
3.73, 34" = 1781 rpm @ 70

Regear to 5.38
5.38, 34" = 2569 rpm @ 70

** But what about if you didn't regear, and just pressed the OD Off button???? **
3.73, 34" = 1781 (od on)
1781 / .69 = 2581 rpm @ 70 (od off)

Did you notice anything interesting there?

OD Off won't really help you around town, but will certainly let you get by on the freeway until you get around to re-gearing. So again, open the charts!! Play with the numbers. Gather all of the information about 'your' rig, and where and how you will be driving it. That will give you a much better handle on what you are likely to end up with. Put it all together, and you can start making some assumptions on how 'you' will feel with the power you will have with various gear/tire combinations.

blue project

What year and transmission do you have? I have a 2010 auto with the same size tire. I went with 4.56 has I don't have have a lot of steep hills and do mostly highway driving. You could do 4.88's with a 35" tire. A mounted 285/75/17 is about 34"s and under the vehicles weight is 33.5"s.

JK09TX

What formula is this? I am new to so I am still trying to understand this. So What RPM do I need to be looking at? If I do 305/70/17 is that a good tire size?

JK09TX

2009 Automatic transmission 3.73 gears

blue project

I am happy with my 4.56 but I live at sea level so not a lot of hills. If you have different terrain I would go 4.88. Some people will go say to go higher but it's all dependant on where you live and if it's your daily driver like mine is. If you go 4.56 they can be more expensive due to the uncommon size. I got mine from 4ws.com. No one could touch their price and they are Yukon gears. I should note that I have a 2010 with an auto. The newer engine and transmission tend to have have a different need. Also your current 3.73 will work but your transmission spends a lot more time hunting for the right gear so your power lags. Run with your current ones if you need but changing them sooner than later will make make a big difference.

nthinuf

It is the RPM chart in the FAQ's that has been mentioned many times in this thread! I was simply plugging numbers in to show how certain combinations will compare. It's that simple. Choose a gear ratio, choose a tire size, follow the lines.

The chart is built for OD/4th gear on 3.8 autos. (or 6th gear for manuals). Plug in a tire height and gear ratio, and you have the rpm at 70 mph. To get OD Off, using that chart, you divide the rpm number by .69. (Not talking about the torque converter or anything, just straight numbers to get an idea of where you'll be.)

Gear Ratios 1st 2.84
Gear Ratios 2nd 1.57
Gear Ratios 3rd 1.0
Gear Ratios 4th 0.69

Many people with a 3.8 and oversized tires think that around 2500 rpm is a good place to be for freeway cruising. More rpm = more hp = more torque. This added power let's you keep your taller/heavier rig with taller/heavier tires in OD, without constantly downshifting in and out.

Is that 2500 rpm range good for you? No clue. I don't live where you do, I don't have the same mods you do, I may not go the same freeway speeds you do, etc, etc, etc. That is what YOU have to decide. What rpm range will keep YOUR jeep, with its mods, and with bigger/heavier tires, in OD on the freeway without constant downshifts. Doesn't matter that you asked 10 different people and they all gave you the same answer, if that same answer doesn't fit 'your' specific situation/needs. Play with the numbers so you can get an idea of why people are giving you the advice that they are.

It's your jeep, you tell us.
But I'd bet that if you plugged in the numbers, you'd get a better idea of where you will be than you have right now...