What does 100W mean on a tire?
| Load per tire800 kg — index 100 on the ISO scale | 1,764lb |
|---|---|
| All four tiresthe ceiling for vehicle plus everything in it, not a target | 7,056lb |
| Speed rating W270 km/h — exotic and high-performance cars | 168mph |
| Next index up (101)55 lb more per tire | 1,819lb |
| Next index down (99)55 lb less per tire — do not fit this if your door label says 100 | 1,709lb |
Notes
- **100W means two separate things.** 100 is the load index: this tire carries **1,764 lb** on its own, at the pressure printed on the sidewall. W is the speed rating: the tire is built to hold together at **168 mph**. The two numbers are unrelated — a high load index says nothing about speed, and vice versa.
- **The load figure is per tire, not per vehicle.** Four 100-rated tires carry 7,056 lb between them, and that has to cover the car, the fuel, the passengers and whatever is in the boot. Compare it against the gross vehicle weight rating on the door jamb, not against the kerb weight in the brochure.
- **Never fit a lower index than the door label.** The placard inside the driver's door gives the minimum load index the manufacturer signed off on. Going below it is the one substitution in tire fitment that has no upside — it saves a few dollars and removes margin from the only four patches of rubber holding the car down.
- **A speed rating is a ceiling, not a recommendation.** W at 168 mph assumes correct pressure, correct load and a tire in good condition. Under-inflation is what actually destroys tires at speed, and it does it well below the rated number.
- Both markings sit at the end of the size, after the diameter: a tire marked 225/45R17 100W is a 225-wide, 45-series, 17-inch tire rated 1,764 lb and 168 mph.
100W is two answers, not one
100 is the load index: this tire carries 1,764 lb on its own. W is the speed rating: the tire is built to hold together at 168 mph.
The two are unrelated. A high load index says nothing about speed, and a fast tire is not automatically a strong one.
The load figure is per tire
Four 100-rated tires carry 7,056 lb between them — and that has to cover the car, the fuel, the passengers and everything in the boot.
Compare it against the gross vehicle weight rating on the door jamb, not the kerb weight in the brochure. The two numbers differ by several hundred pounds on most cars, and the brochure is the one that flatters.
Never fit a lower index than the door label
The placard inside the driver's door gives the minimum load index the manufacturer signed off on.
Going below it is the one substitution in tire fitment with no upside: it saves a few dollars and takes margin out of the only four patches of rubber holding the car on the road.
A speed rating is a ceiling, not a recommendation
W = 168 mph (270 km/h) assumes correct pressure, correct load and a tire in good condition.
Under-inflation is what actually destroys tires at speed, and it does it well below the rated number.
Where to find it on the sidewall
Both markings sit at the end of the size, after the diameter — a tire marked 225/45R17 100W is a 225-wide, 45-series, 17-inch tire rated 1,764 lb and 168 mph.
Nearby sizes
Read more
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