^^ Dave, I think that's good advice and something I considered when selecting replacement tires for the Z (or any car for that matter).

--Spike

 

Not quite… In fact… not at all.

When you go with a tire that has a lower load carrying capacity than your original equipment tire, your only option is running the tire at a higher psi than recommended for your car. That means you jack-up the psi (looks to be 42 psi in your case with the tire you selected).

Good luck with that. It will ride poorly, wear unevenly, and if you hit a pot hole… well, can you say “blow-out?”

You should ask your tire dealer to provide a better solution.

--Spike

 

STILL not understanding the concept of load ratings.

 

^^ Well…I admit I’m old…getting a little feeble and sometimes confused… and maybe a little slow.

But on my worst day, I know enough not to buy the wrong tire that requires a “jacked” psi to make it work marginally on my car.

--Spike

PS: Notice the high number of readers on dada’s thread. This thread is living longer than it should. I hope we are not boring people. I’d love to hear more input and opinions from the readers.

 

UPDATE: Discount Tire is sending me new tires for the front, what a great place. So again I asked what is the correct PSI, and some engineer sent me a response that with my new tires it is 42psi!!!

Spike sorry your are getting too old. Lets race

 

^^ You would win.

Unless of course you blew a tire.

--Spike

 

Interesting thread. I just got a set of used wheels and tires. The wheels are wrapped in
Pirelli PZero Rosso.

* Front: 255/35ZR19
Asimmetrico
Sidewall Style: Blackwall
Serv. Desc: (96Y)Load Index 96 = 1565lbs (710kg) per tire
Speed Rating "(Y)" = 186+mph (300kph)
Load Range: XL
UTQG: Treadwear: 220
Traction: AA
Temperature: A
220 AA A

# Rear: 275/35ZR19
Asimmetrico
Sidewall Style: Blackwall
Serv. Desc: (100Y)Load Index 100 = 1764lbs (800kg) per tire
Speed Rating "(Y)" = 186+mph (300kph)
Load Range: XL
UTQG: Treadwear: 220
Traction: AA
Temperature: A
220 AA A

Right now, I put 37 psi all around. Is this the correct psi for these tires?

 

Here he is that dude from Holland.

With the formula I use in my spreadsheet, you can calculate the new advice-pressures the same way the European car-manufacturers did when they made the plackard with the advice-pressures for the original tires.
Became wiser since I translated it and placed it on several english fora.
I calculate back in the first part, with use of the maximum permisable axle loads and the advice pressures normal front and rear and the maximum advice-pressure front. That is different then someone discribed here about using the original tires, wich would bemore secure for normal use advice, but you dont know them at all times. With my system you dont kneed to know them.

That first part can not be used at all times.
1 when the plackard is made for diagonal tires
2 When made with the American formula of the TRA
3 when pressures on the plancard that are on the under border that may be used for the ETRTO (European ) formula. That is 150kPa( 21 psi) though some dared to go to 140kPa( 20psi).
4 when the normal advices are not given. When there are 2 advices it could also be the advices for what I call in the spreadsheet heavy load and vacation load. If you then think it is for normal and heavy, the first part calculates wrong becouse it uses the wrong data.

But when you can determine the load on each tire to the Kilo and can fill the tires accurately, the pressures calculated are the right ones, you may not go lower for the conditions they are for.
And here lies the problem, you can not determine them accurately, even a scale has his deveation and a manometer to.
That is why you have to add some extra for savety and reserve.

For cars with diferent sises on front and back, as Dada has, you have to make 2 spreadsheets, in wich you fill in the data of both axles at the front , so dont use the rear then. This is becaus otherwise the sheet calculates for the rear axle with the heavy load old advice of the front tires.
Did it that way because the rear tires nowadays are held to the maximum of the tire for most cars to cover overloading wich only hapens at rear.
A car that has 2 diferent sises of tires mostly doesnt carry cargo so uses the maximum axle-loads for the heavy load.... I hope.

So the first part can give many exeptional errors.
The advice-pressures for Heavy load and Vacation load with Towbar load can always be trusted ( If you fill in the right data), though they can be higher with the original tires then with the TRA formule. And because the TRA formule stops at 180 kPa ( 26 psi) where the ETRTO stops at 150 kPa( 21psi) sometimes lower on the front tires. Then this advice can also be trusted, though it looks a bit scarry. Never go lower then this advice in the conditions they are given for, you must see it as a minimum.

For a stiffer ride and less fuel-consumption, you can go higher , but dont overdoo it, You can add to the actual load on the tires about 20% as maximum, if you do more the fillings come out of your teeth and you have less gripp on the road. can use this though for creating under-or over stear.
less grip on rear gives over-steer ( spinning) and less grip on front creates understear.

Sorry, I am not so good in keeping things short, so I stop for now with the direct link to my spreadsheet.
http://cid-a526e0eee092e6dc.skydrive...0tyre-pressure
In this map always take the newest version, and there are examples placed.
From there you can navigate through my public map to other spreadsheets.
One is made to see what the temperature in the tire does to the pressure.
Was an experiment for races. Idea was to put cold that pressure in so that warm the tires get the same pressure as for normal conditions.

 

For calculating with my spreadsheet you now have enaugh from the tires.
As I described in my former reaction, you have to make 2 spreadsheets because you have different tires on front and back.

From the car you need to know the maximum speed ( manual or estimate or use the maximum speed you yourself think you will not go over).
And the maximun permissable axle-loads front and rear.
If you know the alignment of the car the camber-angle ( wheels placed like this /-\) . When under 2 degrees not needed.
The plankard-advice pressures you can not use because they are probably made with the TRA formula. But you can fill in your own import.
For this I can fill in my spreadsheet to calculate the axle-loads yourselves.
Have not translated that to English, because there is a lot of Dutch text in it.
What I need for that is the distance between the fron-wheel centre and back-wheel-centre ( Dutch is wielbasis) and the empty weight of the car, and where the motor is placed. Also the distance from front wheel centre to front ( and back ??) seat to determine the place of the gravity-point of the pasengers. You may look at it yourselves, and see if you can figure it out.
If you fill in in LBS the answers wil be in LBS to.
http://cid-a526e0eee092e6dc.skydrive...andendruk-spel

 

Hi there,

So, I've read through this thread and everything else I can find on determining correct psi, and I'm still more than a little confused. Here's the deal:

I've got an '07 Z Enthusiast on stock rims (F: 18x8.0, R: 18x8.5) and am purchasing 4 new tires. The tires I've selected are Potenza RE-11s (F: 225/45/18, R: 245/45/18). The load rating on the rear tires is 100 compared to the rating on the stock tires of 96. The new front tires will have the same load rating as stock (91).

Will one of you kind folks please assist me in determining what psi I should be running? The tires are not yet on my car, but I'm hoping to get them installed within the next few days.

thanks very much,

jeff

 

Hi Jeff, I consider this about the worst thread I’ve seen on my350z.com. It’s loaded with inaccuracies, confusing information and stupid chart-values, references to tires that are inappropriate for the Z, and lots of ego-stuff going on (of which I admit guilt as well).

I suspect those who were advising a psi past what Nissan recommends for the Z will go silent realizing that there are potential liabilities for careless statements.

The bit on this thread about European load ratings is useless since it has nothing to do with how Americans drive cars. Think about driving across Texas in the summer on the expressway at high speed. Doing this with under-inflated or over-inflated tires can prove fatal, based upon historical data and accident reports.

The important thing for normal street driving is buying the correct tires that match the car's specifications. Of course track tires and setup is a different issue.

The tires you are purchasing will run well with the psi values Nissan recommends for the Z. Load ‘em with 35-37 psi in the front and 36-38 psi in the rear.

--Spike

 

I followed this thread with great interest when it was active. I too was pretty confused. Since then, I've discovered a few things. Here is what I think I know.

Load rating is determined by the tire size, build, and speed rating.

Your stock front tires have a load index of 91, which translates to a load rating of 1356 lbs. @ 35 psi. (provided you don't exceed the speed rating of the tire)

35 psi is the standard assigned maximum load pressure for Standard Load tires under the Tire and Rim Association specifications (North America).

Load carrying capacity is proportional to the pressure in the tire in an almost linear fashion. So if you run a lower pressure than 35, you reduce the load carrying capacity proportionally. However, with standard load tires, increasing the inflation pressure doesn't (by definition) increase the load carrying capacity of the tire. If you saw a graph of the inflation pressure vs. load carrying capacity, the load carrying capacity would rise steadily with increase inflation pressure until the load carrying leveled out at 35 psi. Increase tire pressure, and the load carrying won't increase.

Anyway, your new front tires have the same load index as the stock tires. That means for your Re11s to give you the same load carrying capacity as your stock tires, you need to run them at least at 35 psi.

The rear tires are different. Your new tires have a load index of 100, where your old tires have an index of 96. So at 35 psi, your new tires will have a load carrying capacity of 1764 lbs vs the 1565 lbs of the stock tire. This means that you have lots of reserve capacity and could actually run the rear tires at a lower pressure than 35 and still have the same load carrying capacity as the OE tires. However, you wouldn't want to run them at lower than 35 as it likely would affect handling and tire wear.

The real question that started this thread had to do with putting XL tires on a Z. With XL tires, the load carrying capacity of the tires increases with inflation pressure up to 42 psi. Since the OP had tires with a lower load index in the front, he has to run his pressures higher than 35 to bring the tires load carrying capacity up to that of the OE tires. Interestingly, a standard load tire and an XL tire with the same load index have the same load carrying capacity at 35 psi. The only difference is that you can increase the load carrying capacity in an XL tire by increasing inflation pressure up to 42 psi if you need to.

What does this mean for you? With your new tires, since the load rating is the same as, or exceeds the stock tires, run your new tires at Nissan's recommended pressure.

 

Stick with 35 psi and you'll be fine.

 

I have to make a few corrections to the Spike's incorrect information.

 

Although Spike has been very knowledgable when it comes to stuff like this and has given very valuable information in the past.

I'd have to agree with bdblackz since once you change your tires (size, manufacturer, etc.) you'd have to rely on the tire manufacturer's specs instead of Nissan's. Especially since it's the tire pressure labels on the door jam are probably provided by the OE tire manfacturer's themselves.

PS ~ Ross get back to work!

 

I think making sure people know how to use their tires in the safest, correct way is part of my job

 

I appreciate all the responses very much. Thanks, guys. I didn't mean to reignite an old argument, but it looks like I've done exactly that.

After reading the first few replies, I was all set on sticking with OE psi recommendations, but then in comes bdblackz...

bd: Sorry, but I've looked at so many charts, tables and excel spreadsheets, and I've been utterly confused by all of them - would you mind taking a look at the table you're referring to and giving me a recommendation? That is, 245/45/18 96 load rating @ 35psi switching to same size with a 100 load rating (not XL, if that makes any difference)?

Thanks again,

jeff

 

Jeff, that actually is an XL tire but 35psi is the correct pressure. Thanks for going about it the right way, and hope you like the RE11s.

 

Sounds good. Thanks!

 

WT_? Isn’t that pretty much what I said?

--Spike