When a Tire Shop Sold Him the Wrong Load Rating for His Truck

The call came in on a Tuesday. A guy named Dave, someone I knew from a local Ford truck forum, had just spent $1,200 on a new set of tires for his F-150. He was driving from Detroit to Traverse City to pick up a boat when the rear driver’s side tire blew out on I-75 near Flint. He managed to get to the shoulder without crashing. He called AAA. The tow truck took him to a tire shop in Flint. That shop told him they couldn’t warranty the tire because it was overloaded. Dave was confused. He wasn’t towing anything at the time – just the empty truck and a couple of suitcases. He sent me a photo of the blown tire. It was a set of four P-metric tires he’d bought from a discount chain. The sidewall showed “P275/60R20 114T.” That’s a light truck tire in name only. The “P” at the front stands for passenger car tire. The “114” is the load index – that means each tire can carry about 2,600 pounds. On an F-150, that’s borderline. With a full tank of gas, two passengers, and some luggage, the rear axle was probably near its limit. Add the weight of the boat trailer tongue load? Forget it. I asked Dave what the door jamb sticker recommended. He read it to me: “275/55R20 115T.” That’s an LT-metric tire – light truck construction – with a load index of 115, which is about 2,700 pounds per tire. Not a huge difference on paper. But LT tires have stiffer sidewalls and higher load capacity margins. They’re designed for the constant weight and towing demands of a pickup truck. The tire shop had sold him P-metric tires because they were cheaper and they had them in stock. Dave didn’t know the difference. He just wanted tires that fit his wheels and didn’t cost a fortune. He didn’t know to look for the “LT” prefix. He didn’t know about load index charts. He didn’t know that a P-metric tire on a heavy truck is a failure waiting to happen. A couple of months before Dave’s blowout, I’d written a post about load index and why it matters. Someone on the forum called me “alarmist.” Said they’d been running P-metric tires on their 2500 series truck for years with no issues. I don’t know if that person ever had a failure. I hope not. But Dave did. The Tire Load Index Calculator is one of those tools that seems boring until you need it.

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Tire Load Index Calculator

Enter your tire size and load index — see maximum weight per tire and recommended load for your vehicle.

All data stays in your browser — we never see it.

I sent Dave the link. He plugged in his truck’s gross vehicle weight rating (GVWR) and the tire’s load index. The tool told him that the P‑metric tires were at 98% of their maximum capacity with just the truck, a full tank, and two passengers. Add the boat tongue weight – about 500 pounds – and they were overloaded by about 8%. That’s not a safety margin. That’s a ticking clock. The tire didn’t blow because of a defect. It blew because it was asked to do something it wasn’t designed to do. The sidewall overheated. The internal plies separated. Then the air found its way out. I asked Dave if the tire shop had asked him about his towing habits before selling him the tires. He said “they asked if I wanted the road hazard warranty.” That was it. No load index check. No GVWR calculation. No discussion of LT vs. P‑metric. Just “do you want to pay an extra $30 for flat tire protection?” This is the part that makes me angry. Not at Dave. At the shops that should know better. Selling a P‑metric tire to an F‑150 owner who tows a boat is like selling summer tires to someone in Buffalo in December. It’s not illegal. But it’s irresponsible. A lot of tire shops have salespeople who don’t know the difference between load indexes. They punch the vehicle into a computer, the computer spits out a few compatible sizes, and they sell whatever is in stock. They’re not looking at the fine print. They’re not asking “do you tow?” They’re just moving rubber. The Tire Speed Rating Guide is another one of those things that gets ignored.

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Tire Speed Rating Guide

Enter your tire speed rating — see maximum safe speed and performance characteristics.

All data stays in your browser — we never see it.

Dave’s tires had a T speed rating, which is good for 118 mph. That’s fine for highway driving. But speed ratings also affect heat dissipation. A lower speed rating at high load creates more heat. More heat increases the risk of failure. Dave wasn’t speeding when the tire blew. He was doing 65. But the load was already so high that any additional heat – from speed, from hot pavement, from underinflation – could push the tire over the edge. When the tow truck arrived, the driver looked at the blown tire and said “I see this all the time. People put passenger tires on trucks because they’re quiet. Then they tow something and boom.” Boom is an exaggeration. It didn’t explode. It just … let go. The tread separated from the carcass. The truck pulled hard to the left. Dave held the wheel straight. He was lucky it wasn’t a front tire. The Tire Load Index Calculator would have flagged this problem before Dave ever left the shop. He could have walked into the tire store knowing exactly what load index he needed. He could have said “I need LT tires with a load index of at least 115.” He could have walked out if they tried to sell him P‑metrics. But he didn’t know any of that. And the salesperson didn’t volunteer it. After the blowout, Dave bought a set of LT275/55R20 tires with a load index of 121. They cost him $1,600 – $400 more than the first set. He also bought a portable tire pressure monitoring system because he didn’t trust the truck’s factory system after the incident. He told me he’d never buy tires online or from a discount chain again. He only goes to a commercial truck tire shop now. They ask him what he tows, how much weight, and how far. They check his GVWR. They explain the load index. That’s what a good tire shop does. The discount chain just sells tires. I asked Dave if he ever went back to the original shop to complain. He said “no. What’s the point? They’d say I overloaded it. They’d say it’s not their fault. I just want to forget about it.” I don’t blame him. But I do think about all the other truck owners who are driving around on P‑metric tires, towing boats and campers and utility trailers, with no idea that they’re one hot day away from a blowout. If you drive a truck – any truck – check your tire sidewall. Look for the “LT” prefix. Look at the load index number. Compare it to your door jamb sticker. If the numbers don’t match, or if the load index is lower than the truck manufacturer’s recommendation, don’t drive another mile without fixing it. Use the load index calculator. Know your numbers. Don’t trust the tire shop to do the right thing.