I didn't know the Rubicon came with two different gear sets. I had assumed that, by definition, Rubicon had 4:10. In fact...I thought that was the point. But now I hear 3.73 is an option?
I bought mine used. I'm not sure it matters to me, but how do I know what I've got for gearing?
And for a really noob question: Are we talking transmission gearing, or differential gearing?
And why does gearing (apparently) need to change so much depending on tire size?
That's a lot of questions! If you have ever ridden a bicycle that had a 3 speed shifter or a 10 speed then by extension that forms the basis of why gears? I know that's not extremely helpful but others may chime in.
Since 2012 with the 3.6 and "new" auto transmission 4.10's became an "option" for Rubicons. 3.73's were standard. If you have a manual transmission, 4.10's are standard for 2012 plus Rubicons. All 07-11 Rubicons had 4.10's from the factory whether manual or auto.
I didn't know the Rubicon came with two different gear sets. I had assumed that, by definition, Rubicon had 4:10. In fact...I thought that was the point. But now I hear 3.73 is an option?
I bought mine used. I'm not sure it matters to me, but how do I know what I've got for gearing? ONE
And for a really noob question: Are we talking transmission gearing, or differential gearing? TWO
And why does gearing (apparently) need to change so much depending on tire size? THREE
ONE
Google string test. Tape a string to the driveshaft. Remember where the stearting point is. Jack one tire off the ground. Turn it EXACTLY 2 revolutions. Count the number of times the string wrapped around the drive shaft.
3 wraps is 3.07
3.25 is 3.21
3.75 is 3.73
4 wraps is 4.10
In round numbers.
TWO
Differential Gearing
THREE
Changing tire size effectively changes your overall gearing. Putting bigger tires is like adding overdrive. The Bigger tires turn fewer revolutions per mile than the stock/original tires.
There are charts and opinions all over the place on "what is the optimum ratio". And no one "CORRECT" answer. It mostly depends on your use. Two main factors. The ratio must be low enough that you can easily start from a stop in first gear. And you have to decide what is the optimum speed/rpm for your driving. 33s thru 3.73 might be ideal on the interstate so you can cruise at a lower rpm at 75mph. 33s thru 4.10s might not be deep enough if you are an aggressive off roading rock crawler. There really is no "CORRECT" answer for that. BUT the gearing MUST be low enough to start from a stop. 33s thru 3.07s might be a bit to high altho I did it for 15 years with no issues. And 33s thru 3.07 at 70mph gave me 25mpg. My TJR only hits 20mpg if I stay under 60mph. I'm sure my gas mileage would improve with 3.73s
With stock tires and the aforementioned gearing, I'm turning 2,500 RPM at 70 mph in 6th. The best I've managed so far is about 17.2 mpg on the highway long distance running between 70 and 80.
If you want to put more than one size larger tire on your jeep you should regear. The larger tire effectively give you a numerically lower gear ratio. This will kind of feel like starting in second gear instead of first and your fifth gear will be your overdrive. You could use sixth gear downhill with a tail wind.
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