If you're looking to improve the off-road capability of your 4X4, or simply want to give it a tougher look, larger, more aggressive tires are most likely on your wish list. However, in order to fit ridiculously large tires, you will likely need to add a lift kit. As you can see from the illustration, adding a larger tire will increase the overall height of this vehicle, but there clearly isn't enough room in the fender well for the tire. To accommodate the larger tire, we'll need to increase the height between the vehicle frame and the axles with a lift kit.
The first question to ask when considering a lift kit is: How big do I want to go? The higher you want to go, in most cases, will determine the amount of modifications you'll need to make. Remember that your vehicle was designed to use a certain size tire, so differential gearing, steering, driveline angles, and brakes may need to be modified or upgraded, to account for the larger, heavier tires.
The amount of lift needed will depend greatly on your specific vehicle, and how much room you want in the fenders when completed. The current generation Jeep Wrangler JK has pretty roomy fenders, and can fit 35 inch tires with as little as a 2 inch lift. The previous generation TJ Wrangler would need more than twice that to fit the same size tire. Internet auto forums are a great place to research how much lift you will need to fit a certain size tire on your vehicle. A lot of lift kit manufacturers will provide suggested tire sizes for each of their kits as well.
There are two basic ways to "lift" your vehicle: A body lift, which consists of spacers placed between the frame and the body of the vehicle, and a suspension lift, which replaces most of the factory suspension components with heavier duty parts. A body lift will increase the overall height of the vehicle while using the factory suspension; it's the cheapest way to add ground clearance and room for larger tires, but should be used with caution, as the factory suspension might not be able to handle the weight of larger tires. A suspension lift is usually a safer way to go, as suspension kits will usually account for the weight of the larger tires. Many off-road enthusiasts will use a combination of the two lift types to get just the right height for their vehicle.
To get an idea of how much taller your vehicle will sit with a lift and larger tires; you'll need to add the lift amount + half the increase in tire height.
So, if you have a 30 inch factory tire and you add a 4 inch lift with 37 inch tires, your vehicle will now sit 6.5 inches taller than stock (4 inches of lift, and 2.5 inches of increased tire height). We use half of the tire difference because the height will only increase by the amount of the tire that is under the axle. The larger tire will increase the height of the axle; the suspension or body lift will increase the height of the vehicle over the axle.
Lifting a vehicle can adversely change its feel and handling, due to the heightened center of gravity. Extreme caution should be taken when driving a lifted vehicle, as roll over is much more likely when making sharp turns than with your stock suspension.