Lift Size for 40″ Tires on Jeep TJ

Adding 40” tires to your Jeep TJ will present many challenges. With a short factory wheel base of only 93.4”, a track width of 68.3”, and small factory Dana 30/35 axles, it is important to consider your goals before heading over to 4wp.com to buy new tires. 

To be concise, you do not need any lift whatsoever to fit 40” tires on a Jeep TJ. Though keep in mind the lower you stay, the more you will have to cut to fit a 40” tire. The higher in lift you go, the less you will have to cut to fit a 40” tire on a TJ. 

When considering the move to a 40” tire on a TJ, outline your exact goals and expectations of your rig. 

If driving around the city is your goal, and you plan to stay on fire services roads, national forest roads, or backcountry access roads. It could be as simple as adding a 6” Rough Country liftOpens in a new tab., and 40” tires. 

If you plan to adventure in a more extreme terrain with your Jeep, it would probably be time to look at keeping ride height low, extending the wheelbase, adding more track width, and either beefing up factory axles or installing heavier duty axles. 

Why would I need 40” tires on a Jeep TJ?

Tire size is the only factor that will increase ground clearance between the bottom of the differential housing and the ground. Larger tires will smooth out the ride quality on and off road, more importantly taller tires will offer more traction and have an easier time rolling over large obstacles. 

10 years ago, I remember 35” tires being the holy grail of tires. It was always the question of “How much lift do I need to fit a 35 tire”.  Times have definitely changed, and the 40” tire has become the holy grail of offroad. 

In just about 99% of situations, a 40” tire will never be necessary. Not only will you never need to hop a curb in a mall parking lot that requires a 40” tire, but they will also decrease your fuel mileage substantially, and increase mechanical wear and tear. 

We will put the price tag aside, Yikes. 

Why does the wheelbase matter on a Jeep TJ when moving to 40” Tires?

Put simply, adding larger tires without increasing wheel base will make your offroad vehicle more tipy and unstable; The roll center of a vehicle completely changes. 

Larger tires will also give you less space inside the wheel-well. In most cases, people try to fix this issue with more suspension lift, or trimming large portions to help clearance for turning. Stretching the front axle out will counteract these issues. 

Roll-Center: Both the front and rear axle have an imaginary point at which the weight of the vehicle interacts with gravity, or pivots from to transfer weight. A line (or axis) is then drawn from both roll points, where these two lines intersect and become the roll center. 

Although inaccurate, a measurement from the ground to the top of the bell housing can be used in calculations and is considered “good enough”.

Also it is important to keep the belly height (Measurement from the ground to the rocker panel) at about half of the diameter of your tire. If you are using a 40” tire, shoot for keeping your belly height between 20”-22”. 

https://www.racecar-engineering.com/tech-explained/tech-explained-roll-centre/Opens in a new tab.

Why does the track width matter on a Jeep TJ when moving to 40” Tires?

The typography and terrain where you’re wheeling needs to be considered when looking at track width. In the south-eastern portion of the United States, you will see offroad vehicles built with a wider track width. In the north-western portion of the United States, you will see offroad vehicles built with a narrower track width. 

Besides the area in which you’re wheeling, track width will help with stability greatly. 

Increasing track width can be achieved with less backspacing on your wheel choice, more offset, wider tires, and wider axles. 

https://www.buildyourownracecar.com/race-car-handling-basics-and-design/Opens in a new tab.

Why do axles matter on a Jeep TJ when moving to 40” Tires?

A factory Jeep Wrangler TJ from the factory was either equipped with:

  • Dana 30 in the front, and Dana 35 in the rear. 
  • Dana 30 in the front with a factory option for a Dana 44 in the rear
  • Or if your TJ has the Rubicon trim it was equipped with a Dana 44 in the front, and a Dana 44 in the rear. 

A factory Dana 30 used a 2.5”, .250 wall axle tube, 7 ⅛” ring size, and 27 spine axle shafts. This may not be an issue depending on the use of your jeep, but in many cases the axles will become the weakest link. 

Adding accessories like an axle truss can strengthen the axle and prevent bending, but then you will run into axle shafts breaking.

Adding upgraded chromoly axle shafts will prevent them from breaking, but then you can run into shattering the ring and pinion. 

Upgrading the ring and pinion will prevent it from shattering, but then you will run into hub assemblies breaking constantly. 

If in a factory equipped Rubicon TJ, while the Dana 44’s could work great with 40” tires. They are better suited for 35” tires, and sometimes 37” Tires. 

Typically most when moving to 40” tires will simply add one ton axles, so that their axles are not the weakest link while on the trail. 

A Dana 60 from a 2003-Current f250 offers a axle tube diameter of 3.¼”, .50 wall thickness, 10” ring size, and 35 spline axle shafts. 

These axles in their stock form can handle a 40” tire reliably. 

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