This extra-wide aluminum rim was probably originally sold for a wide sand or drag tire, whereas the skinny steel wheel is for one of those little doughnut spare tires.
As much as we love big knobby tires, they won't get you very far without a set of wheels. But wheels are much more than just pretty metal disks spinning in each corner of your truck. Wheels can affect your suspension and braking distances, they can help or hinder your off-roading performance, and they can make it easier or harder to turn.
We realize that 90 percent of the wheels sold are chosen on looks alone, but there is much more to take into account before you get your next set of rims to stuff under your 4x4. Did you know that a lightweight forged-aluminum wheel can be stronger than a heavy cast wheel and can help increase fuel economy? Or that there is a variety of different lug nuts and some are much safer to attach your wheels with? It's little details like this that most wheel buyers ignore, but that readers of 4-Wheel & Off-Road really want to know.
When looking for wheels you need to determine a few specs. The top priority is the wheel stud/lug bolt pattern of your axles. This is the number of lugs and the diameter of the circle they are placed on. For example, many 1-ton trucks run an 8-on-61/2 pattern where there are 8 wheel studs in a circle measuring 61/2 inches in diameter (current Fords are 8-on-170 mm). This is easily measured when you have an even number or studs as you simply measure from center to center of opposite wheel studs. When you measure the bolt pattern of an odd number of wheel studs you need to measure between the far edge of the first wheel stud to the center of the third wheel stud and this will give you a very close estimate.
Next determine your backspacing or mounting offset. Backspacing is the distance from the back edge of the wheel to the wheel mounting surface. Offset is the distance from the centerline of the wheel to the wheel mounting surface. Offset is considered positive if it's towards the outside of the wheel and negative when towards the back side of the wheel. The wheel in the photo has 5 inches of backspacing and is 81/2 inches wide. Because the wheel centerline would be at 41/4 inches of backspacing, this wheel has 3/4-inch of positive offset. The greater the backspacing the more the tire is tucked in under the fenders of your truck. The closer to "0" offset, the more evenly the load is spread over the wheel.
You'll need to know the wheel diameter. This should be measured where the bead of the tire sits, not out at the very edge of the rim. The wheel in the photo is a 16.5 and you can see where 161/2 inches lands on the rim. | 
Along with wheel diameter and backspacing, you must also know the wheel width. The diameter and width are directly related to what size tires will fit your wheels and how they will bulge out over the rim. A wide tire on a narrow rim is harder to mount and can give a slight bulge to the tread depending on the tire, but it also protects the wheel with its bulge. We have found that many tires that recommend a 10-inch-wide rim can be mounted on an 8-inch, while those that recommend a 12-inch rim shouldn't be mounted on anything less than a 10. | 
Also, the less backspacing the more leverage the wheel and tires put on the axle hub bearings. If you can keep the center of the wheel over the center of your wheel hub, the longer your wheel bearings will last. Many newer trucks have a lot of backspacing and zero to positive offset to help their bearings last longer and to help their scrub radius as can be seen in this cutaway of the new Ford F-150. |
Choosing proper backspacing for your wheels involves three major considerations. One, do you like your wheels sticking outside your truck or not? Two, will wheels that are tucked in closer hit the brakes, body, frame, suspension, or steering linkage during articulation and turning? And three, what backspacing is best for your scrub radius? The first two are pretty simple to understand and can usually be answered by your local 4x4 shop or by test-fitting the wheels prior to purchase. The scrub radius question is when you draw an imaginary line through the upper and lower kingpin or ball joint of your steering knuckle. This imaginary line should hit the ground at the center of the tire contact patch for the least amount of tire scrub when turning, and this is all dependent on the angle of the joints, the diameter of the tire, and the backspacing of the wheel. A larger scrub radius puts more leverage on the steering components and forces them to work harder, and when turning you lose a certain amount of traction.
Dualie wheels have an enormous amount of backspacing, but this is due to the fact that both the inner and outer rear wheels mount together on the same mounting surface using the same wheel lugs. This is why the front axle of a dualie axle uses special hubs or spacers to push the wheel mounting surface outwards to make up for the extreme backspacing and to align with the rear wheels. | 
While measuring all these specs on a set of wheels is important, many wheels have the specs stamped or cast into the back of the wheel. This wheel reads 16.5x9.75, meaning it is a 161/2-inch diameter rim with a 93/4-inch width. | 
Many rims have their weight rating and air-pressure specification printed on them, which is good to know especially if you haul. This can also be important when swapping rims onto a trailer where very high tire pressure is usually run. |
When it comes to mounting your wheels, there are different styles of lug nuts and lug-nut mounting surfaces. Most lug nuts are a tapered design like the four on the left. These are used to both align and center the wheel. The two lug nuts on the right are a flanged style that use the narrow center shank of the nut to key into the wheel and help center it while the washer is tightened to the wheel face. The lug nut in the center is the best of both worlds as it has both a flange and a taper, but it is commonly found only on the 1/2-ton and lighter trucks. | 
The lug nuts are commonly what center your wheel on your axle, and this is known as a lug-centric-style wheel. There are hub-centric wheels that are centered by the hub of the axle, but these are not as common on 4x4s. One feature we especially like if you can find it is the steel inserts in an aluminum rim for tapered lug nuts. These Mickey Thompson wheels have them and we like how they protect against galling the aluminum with the steel lug nuts. | 
We didn't touch much on bead locks for this story as we have done that before and they are less common, but we must say that bead locks are great for off-roading since they clamp the bead of the tire to help keep it from coming off the rim, especially at low tire pressure. Although most wheels are not bead locks, many of them have a safety bead built into the rim (arrows). This small lump helps hold the tire beads in place against the outside of the wheel. However, most 161/2-inch wheels do not have this feature. |
Finally, lets talk about oversized wheels such as 20-inch-and-larger. For the most part we don't recommend them. They reduce your sidewall, add more unsprung weight, and hinder braking. Unless you have massive brakes and need the clearance, we would stick to the smaller rim, and 17s seem to be the current all-around size. However, we like to use the half-tire wheel rule where the wheel shouldn't be larger than half the tire diameter. Lucky for us, the 20-inch bead locks on our UAJK are fitted with 42-inch tires.