I've just came across an article talking about changing tyre pressures to suit a change in tyre (wheel) size. The thinking being that a reduction in tyre sidewall size increases its stiffness, and therefore increases its load capacity, thus less air is required to maintain the same load capacity.

The proposal is that one should look up the load capacity of the original tyre by its required pressure and reference this against the load capacity of the new tyre size where the required pressue would be listed.

For my car, I've moved from the original 205/50/16 to 235/40/17 and so I'd need to look up the load capacity of the 205/50/16. According to the website, this is 1277lbs as the stock pressure is listed as 39psi.

Cross referencing this 1277lbs load capacity against the pressure required by a 235/40/17 gives me 35psi

Given that this calculation doesn't factor in the differing tyre walls across different tyre brands and models, this could easily acount for the bone shattering ride some owners have complained of over the years when running larger diameter wheels... Throw in heavier wheel and tyre combinations, stiffer sidewalls and stiffer suspension and its only going to get worse.

This is interesting Ryan, thanks for sharing that.

I was advised several years ago with my 225/45/17s to let a few lbs out for exactly the same reason of crashiness and it's proved to be effective with no visible detriment to the wear.

I’ve been running with 35 psi for some time now makes for a more comfortable ride. I certainly will not be doing anything like 155 mph so 35 psi will be quite safe.

You don't want your insurance going up, Jim

yes, I run with 235-40-17 with 2.55 Bar at the front, and 2.45 Bar at the rear.

watch this

https://www.fiatcoupeclub.org/forum/ubbthreads.php/topics/1640951/re-tyres-pressures#Post1640951

IMO 39 psi is too high even on the standard wheels, 33-36 is more like it depending on the quality of roads you drive on.

That's quite high, do you carry a lot in the boot?

no, i don't carry much, i'll lower the pressure tomorrow, thanks for the tip.

I think back in the 90s, tyres had much softer sidewalls.

Can absolutely relate to Graham,

See my subtle attempts to reduce "crashiness" in previous posts.

The number one best factor was Joe (Country Cruising) reducing the pressure all around after a shakedown test drive. Have driven them at lower pressure ever since- no issues of lost grip. No issues of advance tyre wear. But noticeably less bouncy.
The number two factor was replacing my Toyo Proxes for the "newer" Toyo design (I'm on the plus alloy wheel design size wheel so limited choice) and this has had a felt difference.

Additional factor - quite some time gently tightening up old trim screws in the cabin, and pads to the doors and handles where they meet plastic on plastic. You'd be surprised of the improvement!

Regards all,,

MM