[Fun]

Tires original 245/45-20 95V run flat. Are 99V or 99Y ok? I’m going to a non run flat. Can I change speed ratings? I’m thinking 99V (stick with V)

[arpad_m]

Higher ratings are always better and acceptable, but with tires, you can pretty much do whatever you want, you just need to abide by the load and speed ratings you choose.

95V means the gross load carrying capacity is 1542 pounds per corner (95) at absolute maximum tire pressure (typically 50-51 psi), and the max allowed speed is 149 mph (V); 99Y stands for 1709 pounds per corner (99) and a max speed of 186 mph (Y).

Again, these are only the limits imposed by the tires, they don't mean other components will support the same (or higher) limits.

[WhiteMale]

Originally Posted by Fun

Tires original 245/45-20 95V run flat. Are 99V or 99Y ok? I’m going to a non run flat. Can I change speed ratings? I’m thinking 99V (stick with V)

That's a load rating not speed.

[Fun]

Originally Posted by arpad_m

Higher ratings are always better and acceptable, but with tires, you can pretty much do whatever you want, you just need to abide by the load and speed ratings you choose.

95V means the gross load carrying capacity is 1542 pounds per corner (95) at absolute maximum tire pressure (typically 50-51 psi), and the max allowed speed is 149 mph (V); 99Y stands for 1709 pounds per corner (99) and a max speed of 186 mph (Y).

Again, these are only the limits imposed by the tires, they don't mean other components will support the same (or higher) limits.

So it’s load and speed. Also I like the Pirellis P Zero AS Plus 3 a lot. Discount tire says they are more of a performance tire. They will corner better etc. They also say. Thanks

[jadatis]

If size is the same, the 95V is a standard load personscartire wich carries maxload AT 36 psi upto 160kmph/99mph, and the 99Y is an XL/reinforced / extraload wich caries maxload AT 42 psi, officially upto 220kmph/ 137 mph, but to laws of nature also upto 99mph.

So yes you can use them, but they need a tiny bit higher pressure for the same load on tire.

Here a paint picture I made wich gives the official system of ETRTO uses to highen up the reference-pressure for higher speed used then 99mph.

The 50 or 51 psi given on sidewall is the maximum allowed cold pressure. Sometimes only 44psi, and an ocacional XL// 60 psi.

But I now use an easyer and better system looked off from trucktires. Is for every 10 kmph/6.2mph different then referencespeed, 1 loadindex step different.
Higher speed/lower loadindex
Lower speed/ higher loadindex.
And I use also for W and Y 160kmph/99mph referencespeed.

For reference-speed and - pressure, maxload is given in sidewall as loadindex. so in fact loadindex-maxload is also a reference.

Then sometimes in same size, there is V and Y speedcode in same loadindex, wich can never be rigt to laws of nature, because then the Y is allowed to ride 220kmph with maxload on tire and 42 psi in tire, without overheating any part of tire-material.
And the V speedrate only 99mph with same load and 42 psi.

[jadatis]

Officially rimms also have a maxload.
If that is 95 for yours, officially you should change rimms to if you go to 99 loadindex.

Luckyly rimms have large reserves, so in practice no problem.

[jadatis]

Officially rimms also have a maxload.
If that is 95 for yours, officially you should change rimms to if you go to 99 loadindex.

Luckyly rimms have large reserves, so in practice no problem.

[arpad_m]

Thanks jadatis, this graph is indeed much more helpful as it accounts for the extra cold pressure and load reduction recommended at different speeds.


I only use (Y) tires, also marked as Y+ in some places that support 186+ mph speeds with a load index of at least 103-104, because I don't want to worry about any of these changes up to 150 mph. My wheels are OEM SS 1LE fully forged wheels, they should be good. Still looking for a safe, isolated 3-4 mile stretch to see if I can reach 200 mph with the extra power my car has.

[jadatis]

I gave the graphics to show how comlicated they made it.
My rule of tumb system is safer and easyer to use.
For every 10kmph/ 6.2 mph higer speed, 1 loadindex step lower is about 2.5% lower maxload.
Then official formula is a bit different, but then calculating lineair is not that bad.

Your 200mph for more then a minute then 100mph higher then 99mph.
/ 6.2. Easyer 200mph is about 320kmph is 160 kmph higher then 160kmph is 16 loadindex steps lower to use in pressure formula 99LI becomes 83 loadindex.

Its all about heatproduction, so tire-material wont reach a temperature above 120 degr C/250degr F

2x speed needs half the deflection to give same heatproduction, gives about root half is 0.71 times as much surface on ground so maxload roughly at same pressure (42 psi) is about 29% less make it 30 %
16 loadindexsteps gives 63% maxload so rule of tumb even more reduction, so never unsave.


In Europe above speedcode Y, so higher then 300kmph.is given on sidewall as (Y) and always ZR instead of R.
Had mysrlf told by man of Dutch VACO that nowadays ZR in sizes is used if tiremaker dit something offstandard in design, so has amongs other things, give system for determining pressure himself.
But mostly they also put W or Y on it, and then you can follow that system.

That offstandard could be other treathwidth then rule, to make it more ideal for rimmwidth, wich always in steps of 0.5 inch.
Ideal is when rimwidth= treathwidth.

HW division/ aspect ratio. 80 / treathwidth standard 71% of section width.
Every 5% lower treathwidth 2% more.

Can make you a cold pressure /axleloadcapacity list for your tires, if you give data.
With chosen build in reserve and for speed chosen.
Then you " ONLY" have to determine axleloads in your use 99% acurate, tge most tricky part, and your respocibility.

[arpad_m]

Yeah, these linear calcs make sense, and the offer is very kind of you, thanks. I don't worry about R vs ZR designations, because as you said, it's hard to know why each manufacturer add the Z.

If I may ask you to create two lists, one for my current tires and one for the set I'm planning to switch to soon, because my rear tires are close to the wear bars and they have a 2019 date code, slowly getting long in the tooth. Also, the set I now have was from a great wheel+tire combo deal, but they're tall, the fronts are rubbing a bit at full left lock, and the rears are very, very close to the wheel well liner, not touching, but almost, so I'm planning to change things up a bit next time.

My car weighs 3950 pounds (1777 kg) with me in it, weight distribution front to back is approx. 54:46.

Current set
Front: Michelin Pilot Sport 4S 285/35/20 (104Y), load range XL, max cold pressure 50 psi, made in USA
Rear: Michelin Pilot Sport 4S 305/35/20 (104Y), load range SL, max cold pressure 50 psi, made in USA

Planned set
Front: Michelin Pilot Sport 4S 275/35/20 (102Y), load range XL, max cold pressure 50 psi, made in USA
Rear: Michelin Pilot Sport 4S 315/30/20 (104Y), load range XL, max cold pressure 50 psi, made in USA

Thank you so much in advance


EDIT: fixed the front:rear weight distribution that I messed up, this ain't a C8 of course

[jadatis]

Then I assumed psi and lbs wanted, an made list for max 99 mph and gives 90% of calculated axleloadcapacity for 99mph. Deter mined this to give maximum reserve, at wich comfort and gripp is still acceptable. If you want it different write it, I will wait on your reaction before I make for planned set.

Here first for curent tires, in next post for planned set
In front of cold psi for front, so used LI 104/ 1984 lbs maxload AT 42 psi.
Behind cold psi for behind LI 104/ 1984 lbs AT 36 psi. 50 psi is maxcold, but reference is standardised 36 for SL and 42 for XL

Calculated your axleloads from 46/54 from 3950 lbs total, but how did you determine that that acurate.

Axleload Fr/cold psi/ Axleload rear
1681 lbs/ 18 psi / 1934 lbs/1817 rear
1761 lbs/ 19 psi / 2027 lbs
1842 lbs/ 20 psi / 2119 lbs
1922 lbs/ 21 psi / 2211 lbs
2002 lbs/ 22 psi / 2303 lbs
2082 lbs/ 23 psi / 2395 lbs
2161 lbs/ 24 psi / 2486 lbs/2133 front
2241 lbs/ 25 psi / 2577 lbs
2320 lbs/ 26 psi / 2668 lbs
2399 lbs/ 27 psi / 2759 lbs
2478 lbs/ 28 psi / 2850 lbs
2557 lbs/ 29 psi / 2941 lbs
2636 lbs/ 30 psi / 3031 lbs
2714 lbs/ 31 psi / 3121 lbs
2793 lbs/ 32 psi / 3212 lbs
2871 lbs/ 33 psi / 3302 lbs
2949 lbs/ 34 psi / 3391 lbs
3027 lbs/ 35 psi / 3481 lbs
3105 lbs/ 36 psi / 3571 lbs/ referencepress rear
3183 lbs/ 37 psi / 3660 lbs
3261 lbs/ 38 psi / 3750 lbs
3338 lbs/ 39 psi / 3839 lbs
3416 lbs/ 40 psi / 3928 lbs
3493 lbs/ 41 psi / 4017 lbs
3571 lbs/ 42 psi / 4106 lbs/ reffpress front
3648 lbs/ 43 psi / 4195 lbs
3725 lbs/ 44 psi / 4283 lbs
3802 lbs/ 45 psi / 4372 lbs
3879 lbs/ 46 psi / 4460 lbs
3956 lbs/ 47 psi / 4549 lbs
4033 lbs/ 48 psi / 4637 lbs
4110 lbs/ 49 psi / 4725 lbs
4186 lbs/ 50 psi / 4813 lbs

[arpad_m]

Thanks a lot for this info. I accidentally flipped the weight ratio, of course the front is heavier, sorry about that (edited my post in the meantime). In general, I'd like to use the lowest safe cold pressure for a max speed run so as not to go too far above 50 psi hot, I don't like riding the fringes with any parameter.

The ratio and the overall weight were calculated and thus approximate, but I'll get a 4-corner measurement next time I'm in a shop that has these precision scales.

I'm very curious now what max axle loads will look like with my planned new tire set.

[jadatis]

Then I also edited and swiched around front /rear in the list. Then Fr 24 psi and rear 18 psi.
ETRTO advices not to go lower then 22 psi upto 99mph and above that 26 psi.
Us TRA only gives minimum 26 psi.
But I suspect you want list for higher maximum speed used. So give that before I make list for wanted set.
But at certain speed you loose comfort.
When weighed loaded as used, and per wheel, we can do with less reserve, so at higher speed still acceptable comfort.

In the RV world weighin is often done.
They have the advantage that weight and division staysvpretty constant in the years.
So one time weighing can do for years.
For normql street cars weights can vary more, but you can anticipate on it, by using lower speed, if for instance incidental heavy loaded.
Dont forget the persons also weigh.

So dont try 200 mph with 4 persons and load in car., 75mph is also enaugh then.

[jadatis]

If you agree, I make the lists for constant speed used of 250kmph /155mph.
Thwn that once you try out 200mph, it will be for 10 seconds, and tires did not have the time to overheat.
Then for using 95% of calculated loadcapacity, so a lesser reserve.
Then still anticipating needed if not alone in car, so those 4 persons with heavy truncks to the airport for instance.

For that I then will sustract 10 loadindex steps from the 104 and 102, so 94 and 92.
Still then way below 50 psi cold pressure and warm.