[SoftShoe]

Originally Posted by aerobod

Depends on other factors, too, such as high speed tuning option, partial vs full load, tyre manufacturer recommended pressures and likely in the US legal implications of under inflated tyres.

The EU spec is more consistent over the years by the looks of it and more aligned with performance driving with a less understeer related pressure spec.

Yeah, I posted a similar “the pressures may be car-specific” guess, earlier.

But, as you suggest, it may be more accurate to say:
car+country+regionally-regulatory-specific. Though that doesn’t bring us close to an equation.

Also, do you think they print less than full-load pressures on the door without stating so? … and would they print the high speed tuning pressures on the door if they apply to regionally illegal speeds?

I don’t know, but I’m starting to suspect that the exact mathematical equation they use may include words.

[Sportstick]

Originally Posted by SoftShoe

Yeah, I posted a similar “the pressures may be car-specific” guess, earlier.

But, as you suggest, it may be more accurate to say:
car+country+regionally-regulatory-specific. Though that doesn’t bring us close to an equation.

Also, do you think they print less than full-load pressures on the door without stating so? … and would they print the high speed tuning pressures on the door if they apply to regionally illegal speeds?

I don’t know, but I’m starting to suspect that the exact mathematical equation they use may include words.

In the US, the door label is a NHTSA regulatory requirement based on the vehicle GVWR.

[SoftShoe]

Originally Posted by Sportstick

In the US, the door label is a NHTSA regulatory requirement based on the vehicle GVWR.

GVWR includes occupants and baggage - adjustable variables that should put us all at the same ultimate GVWR limit, yes?

(I’m imagining that lighter cars on the same tires can carry more weight in occupants and baggage before reaching the same ultimate load limit. In that case we’d all (those of us with the same oem tires) be at the same GVWR and, therefore, the same psi … which we don’t have.)

[aerobod]

For the 2017 M240i with a manual transmission and a maximum permissible weight in Canada (similar to GVWR) of 1980kg / 4365lbs, the following are the tyre pressures (bar/psi) for different tyre sizes and speed conditions:

18” 225/40-18 runflat all-season tyres all around:
Below 160km/h - 2.4/35F, 2.7/39R
Above 160km/h without high speed synchronization option - 2.6/38F, 3.0/44R

18” 225/40-18F, 245/35-18R summer tyres
Below 160km/h - 2.4/35F, 2.7/39R
Above 160km/h without high speed synchronization option - 2.4/35F, 2.7/39R
Above 160km/h with high speed synchronization option - 2.6/38F, 3.0/44R

[peteyrx7]

I'm intrigued by this thread. I literally just picked up my new car less than a week ago. It's a week 25 build, M240I Coupe with a 6MT and staggered PSS tires. recommended pressure is 35 up front and 41 in the rear.

[SoftShoe]

Originally Posted by aerobod

For the 2017 M240i with a manual transmission and a maximum permissible weight in Canada (similar to GVWR) of 1980kg / 4365lbs, the following are the tyre pressures (bar/psi) for different tyre sizes and speed conditions:

18” 225/40-18 runflat all-season tyres all around:
Below 160km/h - 2.4/35F, 2.7/39R
Above 160km/h without high speed synchronization option - 2.6/38F, 3.0/44R

18” 225/40-18F, 245/35-18R summer tyres
Below 160km/h - 2.4/35F, 2.7/39R
Above 160km/h without high speed synchronization option - 2.4/35F, 2.7/39R
Above 160km/h with high speed synchronization option - 2.6/38F, 3.0/44R

Does this present the theory that - for a ‘17 MT coupe in Canada - the door says either:

• 35 / 39
Or
• 38 / 44

?

[aerobod]

Originally Posted by SoftShoe

Does this present the theory that - for a ‘17 MT coupe in Canada - the door says either:

• 35 / 39
Or
• 38 / 44

?

It says 2.4/35, 2.7/39, probably as there is no speed limit above 120km/h across the country, so the "over 160km/h" tyre pressure would not be valid on public roads.

[Sportstick]

Originally Posted by SoftShoe

GVWR includes occupants and baggage - adjustable variables that should put us all at the same ultimate GVWR limit, yes?

(I’m imagining that lighter cars on the same tires can carry more weight in occupants and baggage before reaching the same ultimate load limit. In that case we’d all (those of us with the same oem tires) be at the same GVWR and, therefore, the same psi … which we don’t have.)

GVWR includes the the vehicle and fuel plus maximum load. I do not think you will be able to uncover all the factors that lead to any variations. For example, consider complexity reduction at the plant for labels such that they may work with a lower common denominator to be able to use the same label, perhaps underrating one vehicle's capacity. The FMVSS regs do not preclude that. They just cannot overstate the safe operation limit. Most OEMs have approved more than one source for each specific tire application to protect vehicle assembly against supply hiccups. There may be variation from a Continental to a Pirelli specification for the same size based on sidewall design. How each behaves at load in a handling and braking test may affect the recommendation. Each vehicle's combination of options affects the total weight. There are more variables than are apparent to "civilians". As long as drivers recognize and utilize the GVWR information, they should be able to safely carry that load.

[Kinesthesia]

TN car, mine has 240/40 front and rear but 32 psi front, 38 psi rear

[SoftShoe]

Originally Posted by Sportstick

GVWR includes the the vehicle and fuel plus maximum load [Â…] Each vehicle's combination of options affects the total weight. There are more variables than are apparent to "civilians". As long as drivers recognize and utilize the GVWR information, they should be able to safely carry that load.

I think we may be talking past each other a bit here, while mostly saying the same thing.

Regardless, my car lists 38 and 45.
With an occupants/baggage never exceed weight of 785.

We all seem to agree that the door psi’s are for max load (although it seems some cars list partial load numbers along with max).

So my psi’s are likely for about 605 pounds more than my car ever carries when I drive it hard.

And because my door is 1psi off of the high speed tuning psi’s listed in the book, it seems my door may be advising for a load and speed scenario I never encounter.

SHORT VERSION:
Unless I start frequently carrying three friends and bags at 110 mph, it seems wise to target lower pressures for better ride and handling (and tread wear).

…Maybe significantly (up to 15%) lower.

While the dealer, and many shops, continue to pump them all the way up for legal CYA