Load index is the number that carries the trailer

Tongue weight lands almost entirely on the rear axle, and the load index is the only figure on the tire that speaks to whether the casing can carry it. Tread pattern contributes nothing.

The scale is not linear and the gaps are large: 121 is 3,197 lb, 123 is 3,417 lb, 124 is 3,527 lb, 126 is 3,748 lb. A 121 and a 126 can look identical on the shelf and differ by 551 lb per tire — over 1,100 lb across an axle.

On an LT tire you will see two numbers, like 123/120. The first applies to a single-wheel rear axle, the second when the tire is used in a dual application — where capacity is derated by 9% to account for road crown.

Never step down to a P-metric tire

When a passenger-car tire is fitted to a light truck, its load capacity is divided by 1.10 under federal rules. BFGoodrich states the practical rule bluntly: never replace original LT tires with P-metric tires, because you lose critical load capacity and durability.

The replacement standard, as Toyo and Tire Rack both put it, is that a replacement tire must have equal or greater load-carrying capacity by inflation than the original specified on the vehicle placard.

The speed rating catches people out

Most mud-terrains are rated Q, which is 160 km/h or about 99 mph. But not all of them. Several very common Toyo Open Country M/T and Nitto Trail Grappler sizes are rated P — 150 km/h, about 93 mph. In LT285/70R17 the Toyo’s Load Range E is a P while its Load Range C is a Q, so choosing the heavier-duty casing actually lowers the speed rating.

What the rating means matters as much as its value. BFGoodrich defines it as the maximum speed at which the tire can carry its rated load under specified conditions — not a recommendation to drive there. USTMA is more pointed: speed ratings do not imply a vehicle can be safely driven at that speed, particularly in adverse conditions.

And Tire Rack adds the clause that matters most when towing: speed ratings only apply to tires that have not been damaged, altered, or driven under-inflated or while overloaded. A badly set-up tow rig creates exactly those conditions, and voids the rating it was relying on.

Pressure: weigh it, do not guess it

The published procedure is placard first, then the load and inflation table if the load has changed. USTMA is explicit that the sidewall number is the maximum the tire allows irrespective of the vehicle, and that you follow the placard.

Michelin’s guidance for heavy loads is the most specific we found: weigh each axle end separately, use the heaviest end to determine that axle’s cold pressure, keep pressures equal across an axle, and never exceed the maximum stamped on the wheel.

No tire manufacturer publishes an “add X psi when towing” figure, and anyone who gives you one is guessing. Weigh the loaded axle, look up the pressure that supports that load for your exact size and load range, and apply it across the axle.

Heat is the failure mode

Towing loads a tire closer to its rated capacity, which is where heat generation is highest. USTMA links under-inflation to heat build-up and internal structural damage; NHTSA links sustained high temperature to blowouts and tread separation. A mud-terrain is heavier and has more tread mass to heat than an all-terrain, and a Q-rated tire has less thermal headroom than the H-rated highway tire it replaced.

What we could not verify

Two things people will tell you confidently. First, that mud-terrain tread squirm makes trailer sway worse. The mechanism is sound — tall, widely-spaced blocks deflect laterally, so the rear axle responds later to a sway input — but we found no manufacturer, SAE paper or standards body publishing it.

Second, that mud-terrains brake substantially worse on pavement. Almost certainly true; we could not find a single head-to-head mud-terrain versus all-terrain pavement braking test with published numbers. Les Schwab will say only that highway use on mud-terrains results in less-than-ideal performance. If you want the published trade-offs, our comparison with all-terrains covers what is actually documented.