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If you are like a large number of Jeep owners, your Jeep is lifted with larger and likely wider tires. At Texas Tire Sales, we repair and modify a large number of Jeeps, trucks and SUVs. Quite often, customers come to us with recently lifted Jeeps running oversized tires and complaining that the vehicle just doesn't ride or handle properly. Sometimes, we discover they have a front end problem, poor alignment or steering geometry issues. Although a large percentage of the time, the cause of the problem is simply improper tire pressure. If you drive a Jeep without modified suspension and are running stock size tires, you probably won't need to read this post. The manufacturer's recommended pressures listed in the door jamb will work just fine.

Larger tires handle a vehicle's weight differently than stock tires do on almost any vehicle. Once you install those 35s, 37s or even larger, the manufacturer's tire pressure recommendations pretty much go out the window. So why is proper tire pressure important with larger tires? An improperly inflated tire will not only wear unevenly and much faster than a properly inflated tire, but replacing tires that cost $200 to $500 each can get very expensive. While proper tire pressure is important on every vehicle, it is especially important on Jeep Wranglers since they are solid axle vehicles, and over-inflated tires will often result in a ride harsh enough to rattle the fillings out of your teeth and under-inflated tires can also cause your vehicle to handle poorly.

Under-inflated tires: Tires that are under-inflated will cause increased rolling resistance and will decrease your fuel economy. The outside edges of the tread will wear faster than the center and the tire can even be damaged due to the excessive heat created from under-inflation. Another warning sign of under-inflation can be a steering pull or wandering.

Over-inflated tires: On the flip side, an over-inflated tire can increase fuel economy, but that savings will vanish when you begin replacing your tires sooner due to the tread wearing off the center of the tire prematurely. Over-inflation can also result in a very harsh ride and a vehicle that will handle and brake poorly due to the smaller contact patch of tire to the road. The over-inflated tires tend to skid and wander all over instead of griping the road as they were intended.

Properly inflated tires:  This means you are now balancing fuel efficiency, ride quality, handling, braking performance and tread wear.

In an effort to explain optimal tire pressure in the simplest terms, think of it this way: the load of any vehicle is supported by the amount of air in the tires. The thing that does not change is how many cubic feet of air can safely carry a certain amount of weight. The thing that does change when installing bigger tires is obviously the size of the container holding that air. If you have two different sized balloons holding the same amount of air, the smaller balloon would have higher pressure than the larger balloon. Therefore, a larger tire needs less pressure than a smaller tire to carry the same vehicle weight. The real question here is how much less? There are various methods you can utilize to determine ideal tire pressure for your particular Jeep.  So, what air pressure should you run on my Jeep, truck or SUV?  That is a loaded question with an answer that totally depends on the weight of your vehicle, tire size, and the terrain that you drive on most. Experiment for yourself. Below are a few methods to help you decide.

The Obsessive-Compulsive Method: Tire manufacturers define ideal tire pressures for their specific tires and for loads of various sizes. You can typically get the chart for your tire from your tire manufacturer's website.

In order to utilize this method for determining proper tire pressure, you will need to visit a recycling center, truck stop, landfill or some other establishment that will allow you to use their scales to determine the vehicle's actual weight.  Obviously, a fully modified Jeep will not weigh the same as it did the day you drove it off the dealer's lot. Considering that the aftermarket parts options for Jeeps are almost limitless, the gross vehicle weight from one Jeep to another will almost never be exactly the same. Also remember that you’ll get a more precise result by filling the gas tank, and even loading a few passengers up before heading to the scales.

The next step is to obtain the front and rear weight of your truck separately by first driving onto the scales with just the front tires and then record the weight. After you have determined the front tire weight, drive over the scale leaving just the rear tires on the scale. Now, record the rear vehicle weight. Lastly, record the weight of the entire vehicle with all four tires on the scales. If done correctly, the total of the front and rear tire weight should be approximately equal to the total vehicle weight. You will likely discover that the vehicle is heavier in the front than in the rear.

Next, divide the weight on your front tires by two to get the weight on each tire. Do the same with the rear tire weight.

Finally, refer to your manufacturer's load/inflation chart from the manufacture's website. Round the calculated weight up to the nearest tire pressure and add another 10%.

Verify your results.To check your results, you can measure the tires and add pressure to even them out. Follow this procedure to measure the tires; 1. make sure your truck is parked on flat ground. 2. Along the centerline, measure the distance between the ground and the bottom edge of the wheel on all four tires. 3. If there are differences, add pressure to even things out. The measurements should be almost the same on all four tires.

Though somewhat tedious, if done properly, this method should provide you with the optimal tire pressure.

The Chalk Method:If you don't have access to scales, or if math just isn't your forte, you can use this method which will typically provide the same results. This method involves using chalk to mark a section of your tires to see how much tread is actually making contact with the ground. The chalk line across the tire helps you establish whether the whole tread of the tire is being used. You can try different air pressures until you find the one that’s right for your vehicle/tire combination. Start by finding a very flat pavement surface. a concrete driveway may be the best choice. Make a mark with soft chalk that goes completely across the tread of the tire. Then, gradually drive your Jeep forward about 50 feet and then backward the same distance.

Look at the chalk line on the tire. If the chalk is only worn off on the center of the tire, reduce the tire pressure slightly and go through the process again. With the adjustment, you should see the chalk wear off more broadly. Keep making small adjustments in the tire pressure until the chalk wears off evenly all the way across the tread.

Repeat this process with all four tires. Once you’ve found the right street pressure, add 10% to all four tires. As explained above, you need to "verify your results" by getting a height measurement from the bottom of the wheel to the ground. Start by evening up the measurements of the front tires with each other. Then, even out the front tires again with the rear tires. Always adjust the tires with the smaller measurement by adding air.

If you go through the process correctly, you end up with something pretty close to ideal tire pressure for your particular vehicle.

 

Testing your tire pressure for under-inflation

It is always a good idea to test the results of your tire pressure settings with a heat test. If your tires are under-inflated, they will generate unnecessary friction and rolling resistance and as a result, excessive heat. You can measure how much heat is created due to under-inflation by monitoring the changes in tire pressure when driving.

This test should be done with the tires at resting temperature (cool) on a day where the temperature is between 70 and 80 degrees. Find a reasonably straight section of highway that’s about 10 miles long where you can drive at your normal highway speed for about 10 or 15 minutes.

First, measure the tire pressure of your tires when they are cool. Then, take your highway drive at your normal highway speed for at least 10 to 15 minutes. Watch your Tire Pressure monitoring information on the dash or pull over and take another reading with your tire pressure gauge. If the tire pressure has risen less than 10 percent of what the reading was cool, then you are good. No need to make any changes.

If the tire pressure has risen more than 10 percent, your tires are under-inflated. This increase in tire pressure indicates a corresponding rise in temperature. For every 10 degrees Fahrenheit, the pressure will rise approximately 1 psi (pound per square inch). In other words, a 5-psi increase equates to a 50-degree temperature increase.

Increase the tire pressure in all tires a pound or two at a time and repeat this test until your tire pressure increase at highway speed is less than 10% of the (cool) resting tire pressure..

(Note that mud terrain tires have more aggressive tread patterns and typically run a little hotter at the ideal pressure in comparison to street or all terrain tires.)

Once you complete this test, drive the vehicle and pay attention to the ride, braking and handling... particularly around corners. Always use your personal judgment to determine if the tire pressure is appropriate for your particular vehicle and style of driving.