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Q-
Can you advise on a recommended gear ratio for a 2017 Jeep Wrangler four-door with a 3.6-liter motor running 35 inch tires? … Would it be 4.56 or 4.88?
Gus-
Is it an automatic or manual transmission? How will it be used?
Customer-
Automatic. It’s a daily driver and weekend warrior off-roader. I do some off-road driving for my job during the week. I’m running 3.73s currently.
Gus-
The 3.6-liter finds its happy place cruising in the 2,000 to 2,500 rpm range, they aren’t really known for bottom-end torque. Check the graph below.
Customer-
So, 4.88?
Gus-
The 4.88s make a great, well-rounded ratio. Do you have any thoughts of going to a 37-inch tire? If so, I’d consider a 5.13.
We’re talking the same eight-tooth count on the pinion as 4.56 and 4.88, just to dispel that internet myth… but 5.13s would future proof it, reduce current drivetrain load i.e. less wear on transmission, transfer case, driveshafts etc. and its only 120 rpm more at cruise speeds.
Customer-
So, all together, this will allow it to handle everything better? Just making sure to get a good ratio if I’m going to commit to it… I see all these crazy Jeep guys running 5.38s and this and that.
Customer-
How bad is it to run 35s with 3.73 gears? Just curious.
Gus-
Here’s another way to look at that question. Knowing that the "sweet spot" is 2,000 to 2,500, you can see below that 3.73s actually put the engine below optimum cruising speed even in 4th gear.
Gus-
The end result? A lot of shuttle shifting, which generates heat and causes transmission wear as well as poor economy due to the converter unlocking under heavy loads.
Many are scared to go deep on gears due to old stories about overrevving etc… The reality is the modern transmission with overdrive has been around 40 years now and those issues disappeared back then.
You can see, completely not the case anymore, even a 5.38 on a stock tire is under 3,000 rpm at 60 mph.
Customer-
I’m trying to educate myself and get the right setup. I will be installing a 2.5-inch lift on it next week and the Jeep already has aftermarket front and rear steel bumpers adding weight, and there’s a 65-pound winch on the front.
Gus-
See, now there’s two more things a lot of people don’t take into account. They want to try to duplicate stock cruise rpms after lifting and re gearing. You drove it stock, these Wranglers aren’t exactly sports cars. Take the opportunity to improve the performance.
Many don’t consider the extra wind resistance, rotating mass, rolling resistance, and overall vehicle weight as you mentioned, this all adds up. It requires more torque to go down the road at the same speed.
There’s two ways to make that torque, step on the gas and feed the V6 more fuel or get the rpms up to where the motor makes more torque naturally, AND take advantage of the torque multiplication in the differential to REDUCE the load that the motor sees. It’s a win-win.
Customer-
Okay, that makes sense! I have to head out of work now. How do I get the part number on your website?
Gus-
https://www.randysworldwide.com/shop/?q=ygk014&mode=autocomplete 5.13 kits
https://www.randysworldwide.com/shop/?q=ygk013&mode=autocomplete 4.88 kits
