Tire size: starts with a P, or not?

[ChapmanF]

So, what's different between a P175/65R14 tire and a 175/65R14 tire?

A lot.

If it's got a P, you look in the Tire and Rim Association yearbook to find its load capacity versus inflation tables. These aren't published online, but they're in the stacks at the engineering library across town.

You read down in the table to the tire size, and then across for inflation pressure, and you find the load capacity. For each tire size there will be two possible load-index values; for example, a P175/65R14 can have load index 81 or 84. The higher of the two is the "reinforced" or "XL" flavor for that tire size, and can be inflated above 35 psi for additional load capacity. For the non-XL flavor, the table row stops at 35 psi. You may inflate it higher (up to the max pressure labeled on the tire) but it doesn't give you any more load capacity than at 35. Up to 35 psi, the standard and XL flavors of the same P-metric tire size have the same capacities at the same pressures.

How many of those facts are also true for Euro-metric (without the P) tires?

Not a one.

Euro-metric load/inflation tables are put out by the European Tire and Rim Technical Organisation (ETRTO). Couldn't get those online from the horse's mouth either, but Toyo Tire has reprinted them here. Page A11 for standard load tires, A12 for XL.

The nice thing here is that the load capacities aren't specific to a tire size. You don't enter the table by looking up a size; instead you read down to a load-index value, read across to a pressure, and there's your load capacity.

But you'd better make sure you're in the right table, because the tables for standard load and XL are completely different. Where with P-metric tires knowing the load index is the same as knowing whether the tire is standard-load or XL, for a Euro-metric tire you need to separately know whether the tire is XL or not, because that makes the same load-index number mean completely different things.

Remember how for P-metric tires of one size, the standard and XL load capacities are the same for all pressures up to 35 psi? Not so for Euro-metric. In fact, for two Euro-metric tires of the same load index, the XL's capacity is lower if you compare at the same pressure.

(Hey, was that a typo? Did Chap get that backwards?) Check it out in the table. A standard-load Euro-metric tire with load index 82 carries 1014 pounds at 35 psi. An XL Euro-metric with index 82 carries only 904 pounds at the same pressure (but will carry 1047 at 42). A Euro-metric XL would need to have load index 87 in order to have adequate capacity at 35 psi.

So what does that mean for selecting a Prius tire? The label in my glove box specifies a P-metric, load index 84, inflated to 35 psi front, 33 rear. From the TRA tables, that's 1019 lbs per tire front and 981 rear (using the 32 psi figure because 33 isn't in the table). A P-metric tire with load index 81 also has the same capacities at the same pressures.

To get the same load capacity in a Euro-metric tire, you need at least a non-XL 82 load index inflated to 35/33, or an XL 82 load index inflated to 41/39, or an XL 87 load index inflated to 35/33.

[rant]Now, knowing how much rides on the difference between a P-metric and Euro-metric tire spec, why on earth does TireRack.com's otherwise wonderful search feature leave the P off all the sizes it displays, so you get back P-metric products from Bridgestone and Dunlop mixed right in with Euro-metrics from Sumitomo and General, and all shown the same way with no P? To tell which is which you either have to work backwards from the load index and capacity on the specs page, or view the "1000 x 1000 SUPERVIEW" and try to spot a P on the sidewall.

And the TireRack folks do know there's a difference - they wrote a technical article about P-metric vs. Euro-metric, which just punts at the end by only saying "slight differences in their load capacity calculations" and "considered equivalent and interchangeable if used in axle pairs and sets of four." It's nice that they think so, but for customers with a vehicle that demands such close attention to capacities as for the front axle of a G1 Prius, it would really be helpful if they'd list in plain view which standard their tires are sized by. [/rant]

-Chap