Run-Flat Tires: How They Work, Limits and Repairs

What a run-flat tire does

A regular tire carries the car on air. Lose the air and the sidewalls fold, the wheel drops toward the road, and you're stopping on the shoulder.

A run-flat is built to keep carrying the car for a limited distance, at a limited speed, after it loses pressure. Bridgestone puts the purpose simply: you can keep driving after a puncture long enough to reach a shop or a level, out-of-the-way spot to sort it out, instead of changing a tire on the side of I-95.

That's the whole promise. A run-flat isn't puncture-proof, and it isn't meant to be driven flat indefinitely.

Two ways to build one

Bridgestone describes two main designs.

Self-supporting. The common type on passenger cars. The sidewalls are reinforced, usually with extra rubber inserts, so they can hold the car up with little or no air inside. Pirelli's self-supporting run-flats work the same way, with reinforcements built into the sidewall structure.

Support ring. A ring of firm rubber or another structure sits inside the tire, under the tread, and carries the weight when pressure drops.

On the sidewall, makers mark run-flats with their own abbreviations, such as RFT, ZP, SSR or ROF, or simply "Run Flat". If you're not sure what's on your car, check the sidewall and the tire's spec table.

How far and how fast

There's no single federal number for run-flat distance. Each tire maker rates its own products, and the limits depend on load, speed, temperature and how the car is driven.

Two examples, as the makers publish them:

Those are the makers' figures for their own tires, not a rule for every run-flat. Your owner's manual and the spec page for your exact tire are the final word.

A few practical points:

• Distance is "up to", not "at least". A heavily loaded car on a hot day may not get the full figure.
• Slow down when the warning comes on. The rated speed is a ceiling for driving flat.
• Head for a shop, not your destination. The point is to get off the road and have the tire inspected.

Why run-flats need TPMS

Here's the catch with a tire that holds the car up without air: you may not be able to tell it's flat. The car can sit level and drive almost normally.

That's why Bridgestone says a car on run-flat tires needs a tire pressure monitoring system, and why Michelin says its zero-pressure benefits depend on proper wheels and an operational low-pressure warning system.

Most cars already have one. The federal standard, FMVSS No. 138, has required TPMS on passenger cars, SUVs, trucks and buses of 10,000 lb or less built from September 1, 2007. What matters for a run-flat is that it works. If your TPMS light flashes at startup and then stays on, the system has a fault, and you could be driving a flat run-flat without knowing. Our guide to the TPMS light explains what each pattern means.

Can a run-flat be repaired?

Sometimes, and it's the tire maker's call, not the shop's.

USTMA's puncture repair procedures say run-flat punctures may be repaired under certain restrictions and specifically prescribed procedures, and that the tire manufacturer must be consulted for its repair policy. Bridgestone says the repairability of its run-flats depends on:

• the damage itself,
• how much pressure was lost,
• and how the car was driven while low or flat: speed, load and maneuvering.

The general USTMA rules apply on top of that: the tire comes off the wheel for a full inspection from the inside, the repair is limited to the tread area, and it's a combination of a stem or plug to fill the injury and a patch to seal the inner liner. Our guide to patch vs plug repairs covers those rules.

The honest expectation: Bridgestone warns that driving on an underinflated tire risks hidden damage, which is why the makers tie repair to how the tire was driven. Be ready for the answer to be a replacement. Try the repairability check to see where your puncture falls, then have the tire inspected.

Replacing run-flats

With run-flats again

This is the like-for-like route: same size, load index and speed rating as the placard, and run-flat construction. The Tire Size Finder reads your size off the door-jamb sticker.

Keep in mind that some cars on run-flats are also staggered, with different front and rear sizes. Our guide to staggered tires explains buying two of each.

With regular tires

On many cars, it's possible. Before you do it, check:

1. What the car carries for a flat. Firestone Complete Auto Care points out that cars sold on run-flats often have no spare tire and no jack, because the run-flats were the plan. With regular tires, you'll want a spare, a repair kit or roadside help lined up.
2. The owner's manual. Some vehicle makers say whether non-run-flat tires are approved for the car.
3. The load and speed ratings. The replacement still has to match or exceed the placard. Our guide to XL vs SL tires covers load markings.
4. Ride and handling. The car may ride and handle differently on regular tires. Ask your installer what to expect on your model.

Don't mix run-flats and regular tires on the same axle. Michelin recommends four matching tires, and matching pairs on each axle at a minimum.

Before you order

• Confirm the size and whether your car came on run-flats: the placard, owner's manual and current sidewall tell you.
• Confirm TPMS is working: no flashing light at startup.
• Decide your plan for a flat if you switch to regular tires.

Then shop tires by size. Extreme Tires installs at the Sunrise shop or by mobile van in Miami-Dade, Broward and Palm Beach counties; elsewhere in the 48 states and DC, your order ships to your address for a fitter you trust. See how installation works, or call (954) 773-1896 to talk through run-flats on your car.