Short Sidewall Height
On the street, a shorter sidewall height will definitely improve a truck's handling at speed. We noted a vast improvement in our test truck's overall handling when it was equipped with the two 18-inch wheel/tire combinations (as compared to the 15-inch wheel/tire combinations). The overall ride quality suffered a bit, as the shorter sidewall of the 18s was unable to absorb shock as well as the 15s. This passed more shock to the wheels, but it was in no way uncomfortable. It also forced the suspension to work more and we had to readjust our Rancho RS 9000X shocks to compensate for the lack of sidewall. At highway speed, the truck exhibited less wander and a far quicker reaction to steering inputs when shod with the 18-inch combination. As you can see by the test data, airing down from street pressure to 16 psi cost between 1/2 and 1 inch of sidewall height. This may not seem like much, but in smaller-diameter tires, it means a reduced safety margin between wheel and immovable trail object.
Air-Down Tread Width
How much a tire flattens out when aired down depends a lot on its construction. Some tires have very soft sidewalls and others are very stiff. With this in mind, our testing showed that both sizes of Toyo tires were equal in the percentage of tread width increase when aired down to 16 psi from street pressure. We used a percentage basis to measure this because the tires were of two different widths. Once again, it's important to note that the 15- and 18-inch Xterrains are manufactured differently, hence the large difference in percentage of tread width increase.
Cost
Using the street pricing at the time of this writing, there was a $125.01 difference per tire/wheel between the 33x15 and the 33x18 Pro Comp tire/wheel combinations as tested. If you buy five, this translates to a $625.05 difference. The 35x15 and the 35x18 Toyo tire/wheel combinations showed a $523.12 difference per wheel, but bear in mind that was figured using the 18x12 Weld Cheyenne 5 wheel, which has a street price of $545. Figured using this example, five 35x18s would run you $2,615.60 more than the 35x15s. Once again, it's important to note that these figures represent the specific wheel/tire combinations that we tested, so your actual cost could be radically different depending on the wheel/tire combination you choose. Bottom line: Expect to pay more for larger-diameter wheel/tire combinations.
Weighing In
Jimmy Nylund, one of Four Wheeler's experienced tire testers, notes that "less unsprung weight equals performance in every aspect, and especially so in rotating weight." When compared to a 15-inch tire, an 18-inch tire of the same outside diameter will generally tip the scales at a few pounds lighter. Such is the case with the Toyo tires we tested here, as the 18s weighed 3 pounds less than the 15s. However, this isn't always the case, as the Pro Comp 18-inch tires weighed substantially more than the 15s. The reason for this is that even though the Xterrains share the same name, the two sizes are constructed quite a bit differently (see the tire data on the previous page). Another thing to remember is that it takes more wheel to fill an 18-inch hole than a 15-inch hole, so this will add weight to the 18s, often offsetting the lower tire weight. Interestingly, there was only a 6-pound difference between the 15- and 18-inch Toyo wheel/tire combinations.
On the Rocks
In rockcrawling applications, larger-diameter wheels (18-inch and up) are currently found on about 10 percent of all machines. The debate over larger wheels is still being fought on the boulders of rockcrawling competitions like UROC and ProRock. Many drivers use 17-inch wheels, but entertain the thought of larger wheels as availability improves. Troy Myer, driver of the Wasp rock buggy, is one of the drivers who would consider defecting from his 17-inch-diameter wheels. He said that one of the benefits he found by switching from 15- to 17-inch wheels was that it alleviated wheelhop issues due to the shorter sidewall storing less energy. Rockcrawlers praise shorter sidewalls for their reduced sidewall roll when off-camber, but debate several points including the shorter sidewall's ability to get pinched between wheel and rock after going airborne with an aired-down tire, as well as the increased probability of peeling a tire from the wheel.
The Bottom Line
As with many four-wheel-drive modifications, there is no right or wrong answer to the 15- versus 18-inch question. The answer lies in how you use your rig and your own personal preference.
Point and laugh at them if you want, but there's a place in our world for 18-inch wheels. Look, they allow you to bolt on a larger set of brakes, which is something almost every modified rig needs; they improve handling while unladen, laden or while towing; and if you're into bling, they're the hottest new look. On the other hand, 15s are much less expensive; they're easier to buy in the middle-of-nowhere; and they offer arguably better overall off-road performance and a smoother ride compared to 18s. What does our staff run? Half of us run 18s and the other half run 15s.