RAM 1500 TRX Wheel Fitment Guide — 6×139.7, Offset Sweet Spot, and the 702hp Unsprung Mass Problem

The RAM 1500 TRX arrives from the factory wider than a standard RAM 1500 by 200mm, running 37" rubber on a 702hp supercharged V8. It is the most performance-sensitive platform in the full-size truck segment, and wheel choice affects everything from 0–100 times to suspension response at high speed. This guide covers every parameter that matters.

Hub Specs

• Bolt pattern: 6×139.7 (6×5.5")
• Centre bore: 77.8mm hub-centric
• Wheel nut thread: M14×1.5
• OEM wheel: 18×9 at ET+19
• OEM tyre: LT325/65R18 Goodyear Wrangler Territory AT (approx. 34.6" tall, 12.8" wide)

The Lug Nut Trap

This catches almost every TRX owner buying aftermarket wheels for the first time. Standard RAM lug nuts have a large seat diameter that does not fit the smaller lug wells machined into most aftermarket wheels. When aftermarket wheels are ordered, standard RAM hardware will not seat correctly — the nuts either sit proud of the hole or can't engage the thread at the right depth.

For almost all aftermarket TRX wheels, slim conical seat or spline drive lug nuts are required. These have a narrower shank that fits the tighter tolerances of aftermarket bolt wells. Check the seat diameter of the replacement nuts against the wheel manufacturer's specification before fitting.

Why 17" Wheels Won't Fit

The TRX runs 381mm (15") front brake discs with correspondingly large callipers — among the largest on any production truck. A 17" wheel does not clear these callipers. The minimum functional wheel diameter on the RAM 1500 TRX is 18 inches. There is no aftermarket workaround for this; it is a physical clearance limitation.

Offset — The Critical Decision

The TRX's wide body means the hub sits significantly further outboard than a standard RAM 1500. Offset choice determines whether the tyre stays clear of the upper control arm (UCA) and sway bar on the inside, and the fender liner and pinch weld on the outside. The margins are tighter than they appear on paper.

ET+25 and Above — Not for Wide Tyres

At high positive offset, a wider-than-stock tyre (37×12.50 or wider) is pulled inboard at the face. At full steering lock, the inside shoulder of the tyre contacts the upper control arm and sway bar. Running a 9" wide wheel at ET+25 or higher limits usable tyre width to approximately the OEM 325/65R18 — this offset is not suitable for 37" builds.

ET+12 to ET+18 — The Sweet Spot

The most proven fitment on the TRX for 37×12.50 tyres on stock or minimally modified suspension. At ET+12 on a 9" wide wheel, the tyre face moves approximately 7mm outboard of the OEM position — enough to keep the inside shoulder clear of the UCA and sway bar at full lock, while keeping the outside edge within the fender arch under compression.

At this offset with a 37×12.50R18, one minor modification is typically required on stock suspension: the front inner fender liner can contact the tyre at full compression. The fix is the heat gun method — the plastic liner is softened with a heat gun and gently pushed inward, holding its new position as it cools. No cutting, no permanent modification.

ET0 — Leveling Kit Required

ET0 pushes the wheel further outboard, creating the aggressive stance that many TRX owners are building toward. At stock suspension height, a 37×12.50 at ET0 contacts the front fender liner under compression and at full lock. Adding a 1.5" to 2" leveling kit (spring collars) changes the suspension geometry enough to eliminate this contact — the leveling kit also removes the factory nose-down rake, giving the truck a more level and aggressive stance as a secondary benefit.

Negative Offset — Committed Build Only

Negative offset (0mm and below) pushes the wheel well beyond the fender edge. On stock suspension, the tyre contacts the fender liner, the running board, and the pinch weld at full steering lock. Negative offset requires fender liner removal, pinch weld trimming or protection, and a leveling kit as a baseline — larger lifts and body modifications for aggressive fitments. This is show-build territory, not a trail setup.

Tyre Size Roadmap

35×12.50 — No Modifications

35×12.50R18 or 35×12.50R20 on a 9"–10" wide wheel at ET+18 to ET+25 — zero contact at any suspension position or steering angle on stock height. The simplest meaningful upgrade over the 34.6" OEM tyre. Fits without touching the truck.

37×12.50 — The Most Popular Build

37×12.50R18 or 37×12.50R20 on a 9" wheel at ET+12: stock suspension, heat gun on the front liner if needed. On a leveling kit at ET0 to ET+12: clean fitment with no liner contact. This is the configuration most TRX owners land on — it matches the visual scale of the platform and provides genuine off-road capability improvement over stock.

38"+ — Full Lift Required

Beyond 37", a full lift kit is required — not a leveling kit. Control arm geometry changes under larger lifts, and 38"+ tyres need arch space that a leveling kit cannot provide. This territory also requires a more detailed alignment setup to maintain caster and steering response.

Wheel Width and Diameter

Size	Best Use	Offset Range
18×9	Off-road, overland — maximum sidewall height for trail use	ET+12 to ET+18
20×9 / 20×10	Most versatile — fills arch well, better on-road handling	ET+12 to ET+20
22×10 / 22×12	Street / show — requires leveling or lift, fender work	ET0 to ET+12 with lift

The Unsprung Mass Problem

This matters more on the TRX than on almost any other truck, and it is the most frequently ignored aspect of wheel selection on this platform.

The TRX uses Bilstein Black Hawk e2 electronically adaptive dampers — a system that reads wheel speed and body motion hundreds of times per second and adjusts damping force accordingly. The system is calibrated around the factory wheel and tyre weight. Heavy cast wheels — particularly large 22×12 cast wheels — increase the rotational mass the dampers are working against. The result is measurable: slower 0–100 times on a 702hp truck, increased heat in the dampers under trail use, and reduced high-speed stability over rough ground because the heavier wheel takes longer to respond to damper inputs.

On a standard truck, this difference is academic. On a 702hp supercharged V8 with a $90,000+ price tag and purpose-built adaptive suspension, it is a genuine performance consideration. Forged wheels — which run 30–40% lighter than comparable cast units — preserve the performance the platform was engineered to deliver. Flow-formed (rotary-forged) wheels are the minimum weight-reduction step worth considering on the TRX.

Tyre Recommendations

All-Terrain / Hybrid

• Nitto Ridge Grappler — the single most popular tyre choice on the TRX platform. Combines M/T sidewall knobs with an A/T centre tread: aggressive enough for rock and mud, quiet enough for highway commuting, and proven to handle the TRX's 500+ Nm of wheel torque without spinning off the bead.
• Toyo Open Country A/T III / R/T Trail — in 37×12.50R18 one of the lightest tyres in the category, which directly helps preserve the TRX's acceleration advantage. Popular for builds where performance balance matters more than visual aggression.
• Goodyear Wrangler Territory AT — the OEM choice. Well-matched to the platform from the factory, particularly for owners staying close to stock configuration.

A Note on BFGoodrich KO2 / KO3

BFGoodrich tyres in this size category run slightly undersize — a 37×12.50 KO2 or KO3 typically measures approximately 36.5" actual diameter. This is not a defect; it means the same tyre creates marginally less clearance pressure against the arch. For TRX owners trying to fit 37" rubber on stock suspension without any modification, BFGoodrich is the most forgiving option in terms of physical clearance.