Hey I started this thread and I have been following this advice anyway and you even agreed with my original logic but you and I failed to figure out the wrong tire size per load.

Now get me some free Bridgestone snow tires so I can save some money please

 

^^ Dream on...

Just kidding of course.

--Spike

 

OK… so now I’m getting at least some of this.

It’s obvious that some tire shops (and even some tire manufacturers) are recommending running tires at a higher psi than they should.

So…why recommend a higher psi than is optimum?

• Drivers typically do not maintain proper tire pressures since they just do not take the time to monitor this important aspect of a tire.
  
  
• A slightly over-inflated tire is safer to drive upon than an under-inflated tire.
  
  
• If you run a slightly over-inflated tire, you biggest liability is poor wear followed by a harsh ride and less than optimum handling.
  
  
• An under-inflated tire can be very dangerous. An example is the Ford Explorer situation (a few years ago when owners were reporting/experiencing major problems with under-inflated tires). Many owners sued Ford, and Ford sued their OEM tire supplier. It was ugly.

The same width tire (for example a 275mm wide tire) on a 20” wheel has less cubic volume than the same width tire on an 18” wheel. That means the owner of the 20” wheel must be more vigilant maintaining the tires’ psi than the owner of an 18” wheel.

Now you see why a tire manufacturer or a tire shop may want to inflate the tire on a 20” wheel more they would on an 18” wheel.

--Spike

 

"Higher than they should?" If you are referring to the 42psi (or whatever the table said), that is not higher than it should be. That is the correct pressure...

Again, if you're talking about that 42psi, that is optimum for his case. Higher than optimum in this case would be 44psi, and I didn't see anyone recommending that?

Yes - way too many do not.

Yes

Mostly. Handling is usually better with a little more pressure. But don't get that 42psi (or whatever the table said) confused with "over-inflated". That is the correct inflation in this case. Anything less is "under-inflated".

Exactly.

That's not necessarily true - you're forgetting about aspect ratios. And regardless, it is equally important to monitor pressures no matter what size/type/brand tire.

Still not sure what you're talking about... tire manufacturers/shops base the inflations on that table (at least they should be), they don't just say it should be higher because it's a 20".

 

^^ ...not understanding everything you say here... but respecting your credentials as an engineer in the tire industry.

I still think 42 psi is too high for the Z. It will feel like "wooden-wheels."

Question: Do you recommend 42 psi for 20" wheels as well as 18" wheels?

--Spike

 

Oh dear God. Beating the dead horse here again...OK, the whole point of the rating system is that different load rated tires are built differently, so as I said before 42psi in his tire does not feel like 42psi would in the stock tire. Different load ratings mean that his new tire, at 42psi, would feel like the stock one at 35psi, not like wooden-wheels. Can't say it any simpler than that.

 

^^ So, in fact, you are staying with the 42 psi as being OK on a 350Z?

--Spike

 

Yes in my case. OEM tires can handle a load of X, my tires must be filled to a tire pressure to handle the same load or more. THe correct tire pressure on my tires to get to X is 42 psi.

 

So… What is the value of X?

--Spike

 

X=The Load Index a number used to represent the maximum weight a tire can support. The index number corresponds to the actual load carrying capacity.

OEM X for Front = 1356 lbs at 35 PSI
OEM X for Rear = 1565 ibs at 35 PSI

My 20 " rims (albeit the front tires are wrong size)

X for Front = 1323 at 42 PSI
X for Rear = 1609 at 42 PSI

 

So… Do you really want to pump-up your tires’ psi to the maximum weight the tire can support? (Maybe if you’re carrying a couple bags of concrete in the rear… dunno)

Also… Your “X values” seem wrong. The Z’s weight distribution F/R % is 53/47. If you want to be precise on the “X value,” it should reflect this difference.

--Spike

 

Dada - just ignore him... he's just making up complete garbage now because he realizes he was wrong and doesn't want to admit it.

 

Its ok, I dont mind

Load Index for our 350z front is 91 (1356 lbs) and rear is 96 (1565 lbs) which has as I have indicated corresponding load in pounds.

Your tire's load index lets you know how much weight the tire was designed to carry safely. By using a load index chart, you can match the rating up with the number of pounds the tire can support when it is inflated to its maximum air pressure. For example, if your load rating is 81, you can then determine that the tire was engineered to carry 1,019 pounds when fully inflated. Multiply this weight by four to find out the maximum carrying capacity for your vehicle. When buying aftermarket or replacement tires, it's important to find a model that matches the load index that was on the original equipment.

See here for the chart:

http://www.discounttire.com/dtcs/infoLoadIndex.dos

ON ANOTHER NOTE, HELP: I HAVE FOUND OUT THAT DISCOUNT TIRE DIRECT SOLD ME A TIRE FOR THE FRONT OF THE CAR WHICH WAS LOWER THAN THE LOAD INDEX OF THE OEM, WHICH THEY THEMSELVES SAY ON THEIR WEBSITE IS A BAD IDEA. I HAVE SENT THEM CONFIRMATION FROM FALKEN IN WRITING THAT THEY SOLD ME THE WRONG SIZE TIRE, AND EVEN FROM HANKOOK. IF THEY DONT REPLACE MY TIRES, WHAT CAN I DO? I WAS SUPPOSED TO GET 245/35/20 THEY SOLD ME 245/30/20.

 

Wow, I come back from vacation and I see the insanity continues.

Dada - what I don't understand is why you're using 42 psi instead of 41. According to the chart in the RTSM, your 275/30/20 tire has the load rating of 1609 at 42psi, at 41psi it's rated 1565 which is identical to the OEM tire.

Personally, I think you bought the wrong tires if the recommended inflation pressure varies so greatly from the normally accepted values. Either that's the case or there are flukes and glitches in the table when moving from some tires to others.

 

Your right 41 psi is the right pressure but Falken said it is hard for me to remember to fill that in the rear and the other amount in the front so they said just fill it with 42 all around it is better to fill a little more than a little less

The front tire is the wrong size I should have gotten 245/35/20 but Discount Tire screwed up.

 

Well… I’m sometimes wrong , but never in doubt .

And... Why so serious? Take it easy man…relax… lighten-up a little.

dada is going with your recommendation. Let’s hope when running that psi he doesn’t hit a pothole.

--Spike

 

Dave, as usual you have cut through the garbage and provide the correct answer. You said (and I took the liberty of bolding your most pertinent statement).
And… This is Post #55 on dada’s thread (again, I’ve taken the liberty of bolding the most pertinent statement in this quote).
In summary you (DaveZ#3) said: “Personally, I think you bought the wrong tires if the recommended inflation pressure varies so greatly from the normally accepted values.”

Your statement agrees with what the Technical & Service Team at Hankook Tire America says (quoted from dada’s message #55 on this thread): “What I did notice with your sizing is that your fronts are now a lower load carrying capacity than your original equipment tire. This is not what you want to do. We never recommend that you lower the load rating of a tire from the original equipment. I would recommend having your front tires replaced with a tire that will have an equal or higher load rating.”

Well crap, dada obviously bought the wrong tire (and even the wrong size). And notice the engineer from Hankook recommends starting with 36 psi for the Z.

I suggest that dada complains to his tire shop and ask for a replacement tire. At least he should get the right sized front tire.

--Spike

 

Spike despite the wrong tire it wont make that much of a difference we are talking about 245/30/20 vs the correct tire size of 245/35/20. Whether 41 psi or 42 psi, as long as the correct tire pressure is slightly more than less we are observing the equality with the oem load index.

We are misunderstanding the issue of vehicle weight, when we make a statement about 6,000 pounds. The tire and pressure choice is not based on the static curb weight of the vehicle alone. The car maker knows the dynamic forces on the vehicle, such as load shift during cornering and braking. Such dynamic forces will create unequal and transient loads on different wheel positions during maneuvering. The maker then chooses a size and pressure to handle that dynamic loading.

Simply taking the static curb weight of the car and choosing a pressure is not correct. You must consider the dynamics.

However, no tire maker knows all the considerations upon which the makers have made pressure decisions, and so choose a replacement new tire size and new pressure(s) to compare with the load capacities of the original maker's size and pressure levels. This is the standard.

Hope this clears it up-it cannot be put more eloquently.

 

UPDATE, got a response from Continental Tire Company:

[Web Inquiry: 091001-000036] Thank you for contacting Customer Relations at Continental Tire North America, Inc. After discussing your inquiry with a product service engineer, we have concluded that based on your information the 42 PSI would be the recommended inflation for the your model of the Nissan 350Z Touring.

 

Spike - my though on the "wrong tire" was coming from the idea that the new tire should have had inflation pressures in the same range as the old one. If this tire is so different that it requires such significantly higher pressures, then maybe it's not the best choice.

Some time ago when bdblackz pointed me to the RTSG, as an exercise I ran the numbers for both of my sets of tires (street and track/autocross) and both came out within 2 psi of our usual 35psi setting. It demonstrated the plausibility of using the guide. But I believe it is not flawless, there appear to be cases where the replacement is not realistic. Rather than use crazy pressures, I would prefer to pick another size or rating that would be closer to what we would expect to use (34-36 psi).