Subaru Forester Owners Forum banner
1 - 1 of 1 Posts

·
Premium Member
2015 Highlander AWD XLE 6AT
Joined
·
4,255 Posts
Discussion Starter · #1 · (Edited)
Over the past several years, I’ve seen more and more passenger vehicle tires carrying maximum load specifications based on 44 psi (3.0 bar, 300 kPa) or 51 psi (3.5 bar, 350 kPa), rather then the “old” 35 psi spec to which I had become accustomed. The meaning and purpose of these increasing values has confused me, so I decided to investigate.

For most everyday purposes, it turns out that there’s no direct correlation between a Euro-metric (ISO) “standard load” (“SL”) tire’s (i.e., 225/55R17, no “P” prefix) maximum load capacity and the maximum inflation pressure. The SL tire’s maximum load capacity is based on an inflation pressure of 36 psi (2.5 bar, 250 kPa) according to a standard set by the European Tyre and Rim Technical Organization (ETRTO).

In other words, for virtually all applications, a Euro-metric SL tire’s maximum load capacity is reached when the tire is inflated to 36 psi cold (2.5 bar, 250 kPa). Inflating the tire to a higher pressure doesn’t increase the tire’s load capacity.

So, why the higher inflation values?

The higher values allow for special applications, the most common being high-speed operation. For example, Michelin recommends increasing pressure in an “H” speed-rated tire by 5 psi (0.34 bar, 34 kPa) above normal when operating at 130 mph (210 kph). If “normal” is 36 psi cold (2.5 bar, 250 kPa), adding 5 psi (0.34 bar, 34 kPa) for the high-speed application would increase recommended pressure to 41 psi (2.84 bar, 284 kPa). This would be OK if the tire’s maximum pressure specification is 44 psi (3.0 bar, 300 kPa).

Another special application is to attempt to improve fuel economy by using tire pressure higher than that specified by the vehicle manufacturer.

How does all of this affect us?

At a practical everyday level, there’s basically no effect on those of us here in the U.S. For our public road applications, maximum tire capacity is achieved at 36 psi (2.5 bar, 250 kPa), and there’s no adjustment required to vehicle manufacturer tire pressure specifications if changing from OEM size tires with a 44 psi (3.0 bar, 300 kPa) maximum pressure specification to ones with a 51 psi (3.5 bar, 350 kPa) maximum pressure spec, or vice-versa.

I’m happy to have sorted this out, and I hope this explanation is useful to others.

Now, about you guys and gals running around on those German Autobahns… :icon_wink:

Regards,
Jim / crewzer

References:

1) http://en.wikipedia.org/wiki/Tire_code#Load_index
2) http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=698083
3) http://marktg.toyotires.com/file/loadinflationtable.pdf
4) http://www.michelinman.com/tire-sel...performance-touring/primacy-mxv4/tire-details
5) Tire Specs Explained: Maximum Inflation Pressure
6) Tire Specs Explained: Maximum Load
7) North American Load and Pressure Markings
8) Tire Replacement Manual
 
1 - 1 of 1 Posts
Top