Harry Wagner
Contributor
There was a time when 15-inch rims ruled the land, regardless of whether you had a Jeep or a truck. Over the last decade there was a push toward larger rim diameters, with 17 inches now the most popular size. Is bigger necessarily better? To find out, we rounded up two sets of 37-inch Pit Bull Rockers and Allied beadlock rims and bolted them onto our Deathproof Bronco.
Is bigger better?
Pit Bull is one of the few manufacturers that makes a 37-inch-tall tire for both a 15-inch rim and a 17-inch rim, and Pit Bull makes them in both bias and radial construction. We mounted up two sets of 37-inch Rocker radials on 8-inch-wide Allied Rock8 beadlock rims. Much like Pit Bull, Allied is one of the few manufacturers offering beadlock rims in both 15- and 17-inch sizes. Allied offers a staggering array of diameters, widths, and backspacing options. We used 31⁄2-inch backspacing on the 15x8 rims and 33⁄4-inch backspacing on the 17x8 rims. This was as close as we could get to minimize variables.
First world problems: Mounting up eight new Pit Bull Rockers on 32-bolt Allied Rock8 beadlock rims took us a full day. We used antiseize on the mounting bolts and tightened them in a crisscross pattern to 8, then 12, then 15 lb-ft.
Speaking of variables, Allied’s Greg Mulkey stressed that there are a number of them to consider when you’re choosing a tire and wheel combination. Vehicle weight, load rating, brake clearance, and intended use are all factors. “I also urge people to think about future upgrades,” Mulkey shared. “If a 35-inch tire is as large as you want to go, then a 15-inch rim is probably fine for you. But if you are already eyeing those 40-inch mud terrains, save yourself some grief and invest in 17-inch rims from the start.”
Factors for Rim Size
• Intended terrain
• Vehicle weight
• Brake clearance
• Tire load rating
• Current tire size/aspect ratio
• Planned future tire size
• Wheel/tire weight
• Wheel/tire cost
• Tire availability
Comparison
15-Inch............17-Inch
Ply Rating 6-ply (Load Range C) ...10-ply (Load Range E)
Load Rating (lb) 2,910 @ 35 psi... 4,080 @ 65 psi
Pit Bull Tire Weight (lb, ea.) 88... 90
Allied Rock8 Rim Weight (lb, ea.) 34... 46
Overall Weight Per Corner (lb) 122... 136
Pit Bull Tire Retail Cost (ea.) $399... $450
Allied Rock8 Retail Cost (ea.) $217... $308
Overall Retail Cost Per Corner $616... $758
Allied offers a spacer between the beadlock ring and the rim. This is useful when running a thick bead bundle like on our Pit Bull Rockers and allows the ring to be clamped tightly with no gaps or risk of breaking bolts.
Both the Pit Bull Tires and the Allied beadlock rims are more expensive in the 17-inch flavor. You could buy four tires and wheels and a fifth matching 15-inch spare tire and wheel for the same price as four 17-inch tires and rims.
Tire size was not the only variable. The 37-inch Rockers for the 15-inch rims were Load Range C, while the tires for the 17-inch rims were Load Range E. So in addition to having an inch less sidewall they also have a stiffer carcass.
The Allied beadlock rims did a good job of holding consistent pressure. We ran both sets of tires at 8 psi to limit the number of variables.
We ran the Deathproof Bronco through its paces on a local trail and then swapped the tires and did the same lines to make back-to-back comparisons regarding how the tires worked. There was not a clear winner; rather, each size offered advantages and compromises.
At 8 psi the 37x12.5R15 Pit Bull Rockers put a lot of rubber on the ground and conformed to the terrain. This would be our tire of choice for a light rig or if you do a lot of wheeling in the sand or snow where floatation is key.
Even at 8 psi the Load Range E 37x12.5R17 Rockers did not produce much of a bulge on our 4,000-pound Bronco. A heavier fullsize rig would likely have better results. The Allied beadlock rims would support lower pressures, but we wanted to run both sets of tires at the same pressure for consistency.
We brought along our DeWalt electric impact gun to make quick work of swapping tires for back-to-back comparisons. Both tire and wheel combinations weigh over 100 pounds, with the 17-inch combo being 14 pounds heavier.
The 15-inch rims offer adequate brake clearance for our Dana 44 and Ford 9-inch axles. If you have 1-ton axles, rim size may be a non-issue, since 15-inch rims will not fit over the brakes without heavily grinding the calipers or swapping in smaller brakes.
When wheel speed was necessary, the 37x12.5R15 Pit Bull tires were more prone to wheelhop while the 37x12.5R17 Rockers would just hook due to their shorter, stiffer sidewalls.
As speeds increased, the tires presented a tradeoff. The lighter carcass and taller sidewall of the 15-inch combination absorbed small rocks and bumps better, but the 17-inch combo was more stable when cornering.
