PSI adjustment for larger tires

[930.engineering]

.
32 psi is ok
34 psi is ok
36 psi is ok

Huh? So if your 215/45-18 recommends 36 psi, what should my 165/65-14 Honda Insight tires be inflated to? Like 12 psi? No, the manual says 38/35 psi front/rear.

Volume, not section! More exactly, volume provides load capability.
That's why heavier cars run bigger tires (taller and wider). However,
there's way more to suspension adjustment, tire choice and tire
pressures to any manufacturer. This task takes many many weeks if
not months.

The GTI calls for 39 psi and I round up to a nice even 40.

33 - 35 psi provides more traction to your GTI with two (one driver and
one passenger), in particular on wet surfaces. And better ride as a bonus.
.

 

[GaryMX5]

The margin of error in a tire gauge may be more than that 2 psi.

I agree there are a lot of inaccurate gauges out there but I have a Longacre racing digital and a intercomp racing analog (from my autocross days). I bought them for their accuracy. There's less than a half psi difference between the two.

 

[Kantaro]

I go by the stock tire's load (based on its load rating and specified PSI) and match that with the new tire's load rating:

https://www.toyotires.ca/sites/default/files/loadinflationtable.pdf

 

[GaryMX5]

Earlier yesterday I emailed Tirerack asking them if they had a recommendation for tire pressure for my new Pirelli's. I gave them info about my car and the old tires as well but it seems they already had more than enough info to make the comparison (duh). Had a reply in my mailbox this morning (they must have bored employees manning the computers and phones on the weekend). Their recommendation based on Mazda and Pirelli info they had was 34 PSI. I think I'll stick with that for awhile.

 

[Astro14]

There is a school of thought that says you should match the original pressure to keep the same spring rate in the tire so the suspension reacts the same.

That’s what I was going to suggest.

The difference between the two, a slightly increased width and slightly increased diameter, is so small, that a reduction in pressure won’t make a material difference. So, no need to adjust.

 

[GaryMX5]

That’s what I was going to suggest.

The difference between the two, a slightly increased width and slightly increased diameter, is so small, that a reduction in pressure won’t make a material difference. So, no need to adjust.

I had to look up how a "tires spring rate" is calculated.... Wow, it appears to be even more complicated than psi calcs and not for mortals!!