New tire size, new tire pressure?

[Melectric]

The stock tire size I have is 205/55R16 and the door jam is marked with 36 PSI for all four tires.

For winter, I go down to a set of x-ice3's in size 195/65R15.

I never really thought about it but should I adjust the tire pressure up from 36psi?

 

[AZjeff]

Question for CapriRacer. He helped me out when I upsized.

 

[SirTanon]

Call me crazy, but I consider tire pressure tire pressure, regardless of the tire size. If you think about it, a larger tire requires more air to bring it up to pressure, and a smaller tire requires less. CapriRacer would be a more definitive source, though.

 

[Linctex]

36 psi for max fuel economy, but traction on ice will suffer.

Drop down to 30-32 PSI for a little better traction on icy surfaces

 

[LotI]

Originally Posted by Melectric
The stock tire size I have is 205/55R16 and the door jam is marked with 36 PSI for all four tires.

For winter, I go down to a set of x-ice3's in size 195/65R15.

I never really thought about it but should I adjust the tire pressure up from 36psi?

The answer is in the load and inflation tables.

If your pressure is 36psi you most likely have XL (or RF) tires. 94 is the load index. Your snow tires are XL with a load index of 95. You could drop your tire pressure about 1psi but your owners manual may advise running higher pressure with winter tires.

 

[FordBroncoVWJeta]

2017 Jetta 205-55-16 = 36PSI
2012 Jetta 195-65-15 = 30PSI (Put in 33)

 

[CharBaby]

I did the same thing on an '06 mazda3

For 3 seasons...205-55-16 Pirelli P7 Cinturato A/S+...Door jamb place card F:32/R:32

For winter only...195-65-15 General AltiMAX Arctic......?________________F:32/R:32

 

[CapriRacer]

The principle is to match the load carrying capacity. Then if you can, match the spring rate of the tire by using the same inflation pressure so the suspension reacts the same. (Yes, in spite of the fact that an uninflated tire may feel stiffer, the inflation pressure stiffens the tire way, way more - to the point where you can ignore it - except for ride quality.)

Usually winter tires are sized such that they have the same load carrying capacity as the "summer" tire, so the same inflation pressure would be called for. Let's see if that is the case here:

A P205/55R16 SL has a Load Index of 89, and a P195/65R15 has a Load Index of 89 - the same. So the inflation pressure ought to be the same. Sharp eyed readers will notice I specified the TRA (Tire and Rim Association - the US standardizing organization) sizing system (the letter "P"), and not the European or Japanese system (without the letters). For practical purposes, the systems are the same, and it was just easier to stay within the same system. Switching back and forth doesn't really change the way the tire is built, just the way its load carrying capacity is expressed, and there can be some minor - but confusing - differences.

Please note: When up sizing or down sizing (changing the wheel diameter), 2 things ought to happen: The overall tire diameter shouldn't change much, and the Load Index shouldn't change much either. The net effect is the inflation pressure shouldn't change much. EXCEPTION: Sometimes, upsizing results in a tire size without enough load carrying capacity - and in that case, usually the tire needs to go from SL (Standard Load) to XL (Extra Load) and you have to add 6 psi.

 

[Kira]

Great, succinct thread. A directly related notice made me laugh yesterday.

205-55-16 = my car's stock size
215-45-17 = my car's sportier version's stock size
195-65-15 = my car's earlier vintage stock size, likely dropped for reasons of style

Yesterday I looked up my size 205-55-16 on Tirerack. NOTE: It's a very popular size.

A big notice came up: "DANGER, this is not the stock size. Accurate changes of pressure are involved" (or words to that effect).

Gotta laugh.

 

[Melectric]

Thanks everyone! Tire pressure is such a trivial thing until you venture away from the OEM tire size and ratings.

So I found one of those mythical tire pressure and load charts. It turns out my summers are 91 and my winters are 95 load indices. So I could drop down to 30 PSI from 36 for the same load capacity.

Maybe I'll try the lower pressure since they kinda feel mushy at the moment at 36 PSI.

 

[CapriRacer]

Originally Posted by Melectric
Thanks everyone! Tire pressure is such a trivial thing until you venture away from the OEM tire size and ratings.

So I found one of those mythical tire pressure and load charts. It turns out my summers are 91 and my winters are 95 load indices. So I could drop down to 30 PSI from 36 for the same load capacity.

Maybe I'll try the lower pressure since they kinda feel mushy at the moment at 36 PSI.



Ah ….. Mmmmm ……. I think there is something you are not aware of.

Your winter tires are XL (Extra Load) tires, and that means that the 95 Load Index occurs at 41 psi, not 36 psi like your SL (Standard Load) summer tires.

That also means that at 36 psi, they both have a 91 LI. So the drop in pressure you are considering isn't justified.

Further, if you use the word "mushy" the same way I do, then you'll want a HIGHER pressure. not a lower one. On the other hand, this could just be a function of the tire itself as how rapidly the tire responds to input is one of theose things that can vary from make/model to make/model.

 

[Melectric]

Mushy may have been a bit off. More like less stable which just be the taller sidewalls and narrower tire width.

I found an app note from Toyo and it seems at least from the charts that the XL rating only kicks in above 36PSI and yes is measured at 42PSI.

Below 36PSi, the impression that I have is that the load carrying capacities of the tire are equivalent between standard and extra load. Maybe I'm offbase and got lost in marketing material?