Tire pressure question and request for advice

[rluciano]

​Why does the P85+ get lower tire pressure than the S85 or the P85? Ignore the stagger for a moment. Let's focus on the front tires. Same width, similar diameter. Bigger rim. So a lower volume of air with a lower pressure (maybe a stiffer sidewall) but still it seems to imply much lower protection for the rim. It probably reduces noise and improves ride comfort but at what cost?

Other manufacturers (Porsche and BMW for example) seem to increase recommended tire pressure as rim diameter increases (particularly if width is constant).

My daughter borrowed the Tesla the other night. (The Prius was in for service.) A landscaping truck in front of her lost a few rocks. She started to swerve around them, but an oncoming car was just onto my daughter's side of the double yellow line. So faced with the choice of hit the oncoming car or hit the rocks. She chose the rocks. Net result: one trashed tire and two bent rims.

I am trying to decide if I should replace the rims and run 42-45 psi to give them some protection or buy a staggered set of 19s from TireRack. I was thinking of the BBS RX-Rs in 19*8.5 and 19*10 with 245/45 19 and 275/40 19s. Any advice or other suggestions?

 

[jerry33]

I suspect the pressures are about handling, which is what you purchase the P85+ for. The higher pressures on the other versions are likely for energy efficiency.

One of the reasons, besides cost, that I didn't go for the P85 was that at the time I purchased they came with 21" wheels. I'm not thrilled about 19" wheels, but the brakes won't allow for a sensible tire size so 19" is the best you can do.

 

[lloyds]

Yup, that's correct. The + is designed for performance and handling. Lower tire pressure helps improve handling.

 

[rluciano]

I went ahead and ordered the BBS 19*8.5 and 19*10 wheels. So in a week or two, I'll be able to experiment with different pressures with 19" rims. I know lower pressure usually improves handling. But lowering both pressure and volume of air dramatically decreases wheel protection. I know the service center says the 21s are necessary to get that last bit of performance on P85+. I say possibly true but irrelevant. I have a 2003 Ruf with 19" wheels. It flys around Texas World Speedway. The limiting factor is the nut behind the wheel, not the 19" wheels.

 

[jerry33]

I know the service center says the 21s are necessary to get that last bit of performance on P85+.

That is only true if the tires are exactly the same, and you're talking about smooth (and I mean smooooth) pavement. Add either a stickier compound 19" tire and/or some uneven surfaces (like real roads) and the difference will be lost in the noise.

 

[yobigd20]

​I am trying to decide if I should replace the rims and run 42-45 psi to give them some protection or buy a staggered set of 19s from TireRack. I was thinking of the BBS RX-Rs in 19*8.5 and 19*10 with 245/45 19 and 275/40 19s. Any advice or other suggestions?

I'm leaning towards the following staggered setup:


255/35ZR20 front on 20x9" rims. (sidewall height: 89.25mm, diameter ~27.0)
285/35ZR20 rear on 20x10" rims. (sidewall height: 99.75mm, diameter ~27.84)

Choices for me limited as I want Michelin Pilot Sport A/S 3 Ultra High Performance All-Season tires. I'd prefer to have 245/40/ZR20s on the front which would bring the sidewall to 95mm and diameter to 27.7" which is much closer to OEM, but those specific Michelin's are not made in those size. I haven't figured out details about offset and rim selection yet. I am not an expert here and will need to consult with someone next year (I have a ways to go until my 19s are done) before I make any real decision.