Tire Pressures for Different Tires Using Load Index

[Zissou]

I was looking into how one should calculate what their tire pressure should be when they change tires. In my instance this was for my winter set, Pirelli P4 195/60r15 88H. As we're moving into a colder season, this should be timely.

That "88" is the important designation as that is the load index. 88 being the load index, H being the speed rating.

Our OEM Bridgestone RE050a 205/40r17 84W are a 84 load index. This means that for our rears at 36 psi the standard load is 1102 lbs per this table from Toyo. While our front 39 psi is not on the table, I extrapolated the data available to determine it's about 1180 lbs.
https://static1.1.sqspcdn.com/static/f/589830/23447320/1378330097907/Discount+Tire+inflation.pdf

So for my Pirelli's with an 88 load index we look for the load rating and what pressure that stacks up to. Finding the closest and rounding up to the nearest psi I have determined the 88 load index should have 32 psi in the rear and 35 psi in the front.

Here's some further reading on this, from Toyo again.
https://toyo-arhxo0vh6d1oh9i0c.stac...ication_of_load_inflation_tables_20170203.pdf

Hopefully this is helpful for keeping your tires inflated correctly!

 

[Blindeye_03]

Hmm.. SO for the bfg g-force comp-2 a/s the load for 205/50-16 is 87... 36 front and 33 rear? Ill have to read more about this later but for now Im running stock pressures and noticed that the ride is just as stiff with these tires as were the RE050As.

edit: So on the second page of the document from tirerack it gives pressures for XL tires which would be the RE050A right?

31f/28r pressure seems awfully low for a 205/50-16 tire...

 

[Blindeye_03]

bump for curiosity.

 

[Zissou]

Hmm.. SO for the bfg g-force comp-2 a/s the load for 205/50-16 is 87... 36 front and 33 rear? Ill have to read more about this later but for now Im running stock pressures and noticed that the ride is just as stiff with these tires as were the RE050As.

edit: So on the second page of the document from tirerack it gives pressures for XL tires which would be the RE050A right?

31f/28r pressure seems awfully low for a 205/50-16 tire...

Yeah so to match that 1180 lbs load in the front and 1102 lbs load in the rear you'd want to pump up a 87 load index tire to 36 psi front and 33 rear assuming they're both standard load (SL).

But you're right, our OEM tires are XL load range which would mean pressuring way down keeping things equal on the chart. I initially did not notice this.

I found this blog post from tirerack
https://blog.tirerack.com/blog/miles-of-smiles/xl-vs-standard-load-tires-what-should-i-do
Basically it says XL have more reinforced side walls. For the OEM tire size it's likely due to how narrow that side wall is because most of the RE050a's available in all sizes are sold as SLs, plus the performance benefit they mention.

What I'm going to do is go with the initial recommendation and pump tires up to what the corresponding SL index is if the OEM tires were SL as well. Or vice versa (treating both as XL) as the results are essentially the same.

 

[Joker328]

Interesting. I got about 38 and 35 for my X-Ice's at 195/55R15, so not much different from stock. I think I ran them around there or slightly softer last winter. I wonder if the fact that this is based on the load rating of the tire vs. the actual load on the tire on our vehicle matters though. Like this approach seems to be designed to ensure the tire has sufficient pressure to handle the max load, but our cars aren't anywhere near the load capacity of the tire. I'm sure when the actual manufacturers are developing the recommended PSI, they are considering traction, safety, comfort, fuel economy, etc. at gross weight, not at the load capacity of the tires (which is much higher for obvious reasons). In any case, it's probably a good approximation for people to work from. Thanks for digging up this info.