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Discussion Starter #1
PLEASE NOTE: THE FOLLOWING INFORMATION WAS OBTAINED FROM TOYO'S ELECTRONIC FITMENT GUIDE. IT MAY BE POSSIBLE TO RUN LOWER PRESSURES THAN STATED BELOW, HOWEVER, PLEASE EXERCISE CAUTION AS IT IS GENERALLY PREFERABLE TO RUN A LITTLE HIGHER PRESSURE THAN TO RUN UNDERINFLATED. IF YOU HAVE SPECIFIC QUESTIONS REGARDING YOUR VEHICLE, PLEASE PM ME. THANKS!

I haven't seen any threads on this, so if I'm repeating info already discussed, I apologize, but it's information worth repeating anyway. I want to bring to everyone's attention the importance of correct air pressure, especially in plus applications (meaning if you have 18 or 19 inch wheels).

When you go to a larger rim diameter, you are making the air chamber smaller, thus requiring additional air in the tire to carry the same load. For example, if your Mazda 6 came with the P215/50R17 tires, your minimum air pressure stated on the door placard is 32 psi. Same is true with the 16 inch tires.

Now, let's say you've upgraded to 235/40R18s. Hopefully, you're using a reinforced construction tire like the Toyo Proxes 4 (load index 95). You'll now need to have at least 33 psi to maintain the same load. Not bad, not a big difference.

But let's say 235s are too wide for you because your wheel offset makes that size rub, so you're using 225/40R18s. If you're using a reinforced size (load index 92), you'll now have to run 38 psi to maintain the same load! Many owners continue to run the original 32 psi and wonder why their tires wear out quickly. Running low air pressure creates heat which accelerates wear on the tire.

If you do not have tires with a load index of at least 91, your tires may not properly carry the load of your car!

Perhaps you've got 19s on your beloved 6. Hopefully, you've got at least a 235/35R19 91V (reinforced). You'll have to run 39 psi to maintain the same load as the oe tire at 32 psi!

Even if you've simply gone to a wider tire on the oe wheels, you need to make sure you've got the right air pressure. A 225/45R17 (94 load index) requires a minimum of 35 psi.

If you've gone to anything smaller than the above listed sizes (like 215/40R18 or 225/35R19) you should probably inflate your tires to the max air pressure listed on the sidewall. Chances are, though, you have a tire that won't carry the load of your vehicle properly.

Also, check your air pressure at least once a month as air will seep out at the rate of 1 psi per month. Your air pressure will also change with the ambient air temperature. For every 10 degrees of ambient air change, your tires will lose/gain 1 psi. If the last time you checked your air was in June and the average temperature was 80 degrees and now its October and now its 60 degrees, you've lost 6 psi of air (4 months + 20 degrees ambient air change)! Make sure you check the air when the tires are cold. Do yourself a favor and get a quality gauge. Thanks for looking!
 

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Thanks for the info, Toyoguy, that was helpful.

I've got a question for you, though, about Toyo tires. You suggested using the Proxes 4 in a different thread, but I was looking around at tires a few days ago, and noticed that Toyo doesn't make a 215/50R17, which is what Mazda specifies for the tire. So, does this mean we can't or shouldn't use the Proxes 4 on the Mazda 6? Or use a different size. I'm new to tires, so any advice is helpful.

Also, if you do recommend a different size, what are the drawbacks or advantages of the different size?

Thanks,
Drew
 

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Discussion Starter #3
Drew, actually Toyo does make a 215/50R17. We make it in the Proxes TPT, which is a touring all-season tire similar to the oe Michelin. That tire has a 95 load index. We also make that size in the Proxes T1-S ultra-high performance with a 91 load index.

However, if you want the Proxes 4, you'll have to go to 225/45R17. That's what I did. It is a 94 load index tire. On the spec sheet, the 225 is .4 inch smaller in overall diameter, but when I had the tires mounted, I placed the Toyo next to the Michelin and there was no noticeable difference in overall diameter (both were unmounted).

The advantages of going to a wider tire mean more contact patch on the ground which should translate to more grip. Of course, that is going to be dependent on tread design and tread/void ratio (how much rubber is actually on the ground vs. tread grooves).

The disadvantage could be a slight penalty in fuel mileage, as wider tires create more friction/drag. You also might get more road noise. I haven't noticed any more road noise from the Proxes 4s vs. the Michelins. Again, this will be dependent on tread design.
 

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Very informative post Toyoguy. Experience has proven that you are on the right track, but I will respectfully disagree with one point you made. I'm currently running 91 load index 225/45/17 tires on a 7.5" wide wheel. At first I was running 38 psi front and 35 psi rear. I rotate my tires every 7500 miles and had noticed that both the front and rears were showing overinflation wear (center more worn than the shoulders). I dropped the pressures down to 35 psi front and 32 psi rear and have been getting much more even wear. In case anyone cares, this is on a set of Goodyear Eagle RS-A's (got a great deal on them when I bought my wheels). They are wearing really well. Almost 35K miles and they look like that will make it to at least 60K miles. They should though, since the don't offer much grip to speak of and they almost all the miles have been accumulated on the freeway.
 

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Discussion Starter #6
I'm going to forward the question to our technical department, but my initial thought is that tread compounding has more effect on grip than load ratings. If you're looking to increase grip, you should look for a tire with stickier tread compounds and larger tread blocks.

Reinforced load tires have thicker body plies, but I'm not sure if that affects cornering. I'll post after I get an answer from tech. Great question!
 

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For reference, since it wasn't written in the original post, the stock 17" tires have a load rating of 93. So why would the 235/40/18's in your original post (load rating 95) need a higher tire pressure?

Toyoguy, in practical purposes- what disadvantage is there to reducing the tire's rated load capicity? Obviously I've speculated on one with my previous question.

Is it a matter of safety, longevity, or something else?

You wrote:
Many owners continue to run the original 32 psi and wonder why their tires wear out quickly. Running low air pressure creates heat which accelerates wear on the tire.
It is just a longevity thing? I know running at 38psi, as you suggested, would lead to uneven tire wear on a lot of tires.

One thing that worries me about my competition tires, which are deliberately small, is their very low load rating. Would that make them more prone to a blowout? While I race with them at 44psi front, 32psi rear, I still drive to and from events on them at "only" 36psi front.

I find it interesting that the recommended tire pressure for a Toyota Camry is a very low 28psi front and rear. The tires have a load rating of 92. Knowing that it is worse to underinflate a tire by 2psi than to overinflate a tire by 4psi, their low 28psi recommendation always worried me.

Is the load rating or recommended tire pressure based upon the volume of air it takes to fill a tire? I know Porsche uses hollow-spoked wheels to increse the volume of air in their tires while keeping a low profile.
 

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Discussion Starter #8
Whew! Lot to digest! Okay, let me try to answer each question individually.

For reference, since it wasn't written in the original post, the stock 17" tires have a load rating of 93. So why would the 235/40/18's in your original post (load rating 95) need a higher tire pressure?
When you go to a lower profile tire, you change the shape of the air chamber. In the case of going from 17 to 18, you are also making it smaller. Therefore, you generally have to squeeze a lot more air in there to carry the same load. I was quoting information from Toyo's Fitment Guide and I hate to contradict that information as it is researched quite thoroughly. In checking load and inflation tables, it would appear that you may be able to run lower air pressures than I originally stated. For example:

Size--Load Rating--Load @ 32 psi

P205/60R16--91--1224
P215/50R17--93--1146
235/40R18--95--1224

However, I would tend to err on the side of caution and run a little higher than oe. Uneven wear would likely appear only on grossly overinflated tires, but my original point was to remind everyone the importance of maintaining proper air pressure.

Toyoguy, in practical purposes- what disadvantage is there to reducing the tire's rated load capicity? Is it a matter of safety, longevity, or something else?
Yes to all of those. You don't want to overload the tires as the heat buildup could cause catostrophic tire failure (worst case, depending on how overloaded the tire is). Heat will also cause the tire to wear quickly. You want the tire to be able to meet or exceed the highest stated GAWR (Gross Axle Weight Rating). I have a 6i 5-door. The front axles have a GAWR of 2370 lbs. This is the most amount of weight that the springs/shocks can handle. You can find the max load rating on the sidewall of the tire. The oe Michelins can carry 1433 lbs. @ 41 psi. Typically, we reduce the load rating by 10% as a safety margin. That gives you 1289 per tire x 2 = 2578 lbs per axle, which exceeds the GAWR of 2370. I run the 225/45R17 94W Proxes 4s which max out at 1477 lbs @ 50 psi.

I don't know what the 6 cylinder models state on the door placard. Perhaps someone could pm me. As you can guess, there are tires that are smaller than the oe tire that meet or exceed the GAWR, but then we strongly recommend that you do not go smaller than 2% of the original overall diameter to avoid negative effects to ABS and engine management systems, speedo, odo, etc.

One thing that worries me about my competition tires, which are deliberately small, is their very low load rating. Would that make them more prone to a blowout? While I race with them at 44psi front, 32psi rear, I still drive to and from events on them at "only" 36psi front.
I think you are running 215/45R16s? If they have a load index of 86, you will need to have at least 36 psi in them. At that inflation, they max out at 1168 lbs. That's less than the oe 16s carry at 32 psi and barely above the oe 17s. Those are considered the minimums required by Mazda. I hope you aren't carrying much other than yourself! You would really do better to have a larger size on there. They are 2 inches shorter than oe! Are you really benefitting that much from better gearing?

Is the load rating or recommended tire pressure based upon the volume of air it takes to fill a tire?
Yes and no. A tire's load rating depends on how much air the casing can hold. A standard load tire may max out at 44 psi, but actually does not carry any more load after 36 psi (there are slight differences with P-metric and Euro metric tires). A reinforced tire may carry up to 51 psi. If you can fit more air in the tire, it can carry more load.
 

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Hey Toyo...great info. I just put Proxes 4 215/45ZR17's on my 6 and they are inflated to 32 psi. Where should I be?
 

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When you go to a lower profile tire, you change the shape of the air chamber. In the case of going from 17 to 18, you are also making it smaller.
One of the many reasons I recommend people go with wider wheels- to keep the size of the chamber similar. It's a seldom-followed rule of plus-sizing. Of course, wider tires have benefits from seeing less sidewall deformation as well. Got any neat "stats" on that?

Size--Load Rating--Load @ 32 psi

P205/60R16--91--1224
P215/50R17--93--1146
235/40R18--95--1224
How is this calculated? Is there a table on Toyo's site I can take a look at?

The front axles have a GAWR of 2370 lbs. This is the most amount of weight that the springs/shocks can handle.
Does that mean adding a firmer suspension effectively increases the GAWR? (Assuming the suspension links are strong enough.) I wondered about the effects of a firmer suspension on tires. I'll reference my wife's Toyota Camry again. The car weighs the same and has tires using a similar load index, but recommends only 28psi. Because of the softer suspension?

I'm running much firmer than stock spring rates, so I can imagine they'd put some punishment on tires- especially small tires, which don't roll over bumps as easily.

They are 2 inches shorter than oe! Are you really benefitting that much from better gearing?
In short, yes. I drive a 6i MTX without any options that is probably around 3000lbs, as I've removed roughly 50lbs from the stock weight. It's as light as a Mazda6 can come, which admittedly is still pretty heavy. But the lower gear ratios are like adding a cylinder to the engine. It usually does not necessitate added shifting, since 2nd gear ends "only" four mph sooner- not usually a big deal, and well worth the 8% improvement in wheel torque. It boosts the power to weight ratio (up until the lower shift point) closer to that of the V6. A secondary benefit is lowering the car without changing the suspension geometry, which is in many ways a preferred method of lowering the center of gravity height. For autocrossing, a low center of gravity is very, very useful!

I'll post after I get an answer from tech. Great question!
Still eagerly waiting to hear the answer on my first question! :)

Thanks for your help so far!
 

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Discussion Starter #11
Hey Toyo...great info. I just put Proxes 4 215/45ZR17's on my 6 and they are inflated to 32 psi. Where should I be?
You should carry no less than 37 psi.

Of course, wider tires have benefits from seeing less sidewall deformation as well. Got any neat "stats" on that?
I'll see what I can come up with.

How is this calculated? Is there a table on Toyo's site I can take a look at?
Information is obtained from ETRTO's Standard & Reinforced Load & Inflation Tables. It is a dealer/manufacturer resource only and is simply too dangerous in the layman's hands. :laugh: Just kidding! You may be able to get this information from a local tire dealer, but it's not on our website.

Does that mean adding a firmer suspension effectively increases the GAWR?
I'm not sure. I have a friend at H&R. I'll ask him.

I wondered about the effects of a firmer suspension on tires. I'll reference my wife's Toyota Camry again. The car weighs the same and has tires using a similar load index, but recommends only 28psi. Because of the softer suspension?
I'm not sure I understand your question correctly. Curb weight and GAWR/GVWR aren't the same. A car manufacturer will determine minimum air pressure based on several factors, including weight, weight transfer and, among other things, ride comfort. So, I suppose suspension calibration is taken into account. Can you get me the original tire size, GAWR and GVWR?

Still eagerly waiting to hear the answer on my first question!
Still haven't heard from tech. I emailed them last week, but haven't gotten a response. I'll call them on Monday.
 

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Curb weight and GAWR/GVWR aren't the same. A car manufacturer will determine minimum air pressure based on several factors, including weight, weight transfer and, among other things, ride comfort. So, I suppose suspension calibration is taken into account. Can you get me the original tire size, GAWR and GVWR?
Well, I guess I was hypothesizing that the softer suspension would give it lower GAWR/GVWR ratings, hence the lower tire pressure ratings. To get the actual readings, I'd have to go outside with a flashlight, which I'm too lazy to do right now. The original tire size is 205/65/15, however.
 

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Discussion Starter #13
Does that mean adding a firmer suspension effectively increases the GAWR?
Finally got a response from H&R. Changing the springs alone does not raise the GAWR/GVWR. The other side to this answer is if you are able somehow to raise the GVWR (why would you want to?), then you may have to go to larger tires or higher load index tires. The only practical application I can think of for wanting to raise the GVWR is for trucks that need extra carrying/towing capacity.
 

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Man I've been wondering about this for a while now.

I've been running 40 lbs at all 4 corners with 225/40/18s just because it made the car feel (handling, and whatnot) better. I also get significantly better gas mileage at the higher psi. So does anyone have any formulas for how to calculate what tire pressure you should be running?

Thanks
 

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Originally posted by wheelwatcher@Oct 17 2004, 03:00 AM
Man I've been wondering about this for a while now.

I've been running 40 lbs at all 4 corners with 225/40/18s just because it made the car feel (handling, and whatnot) better.  I also get significantly better gas mileage at the higher psi.  So does anyone have any formulas for how to calculate what tire pressure you should be running?

Thanks
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That is based on feel, not calculation. For competition, it's not uncommon to run 45psi in your front tires- but you shouldn't need more than 35psi in the rear.

You might get some uneven wear doing this, but the car will turn better. For drag racing, you'll want to lower the tire pressure.
 

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Discussion Starter #16
I've been running 40 lbs at all 4 corners with 225/40/18s just because it made the car feel (handling, and whatnot) better. I also get significantly better gas mileage at the higher psi. So does anyone have any formulas for how to calculate what tire pressure you should be running?
I am going to assume that you are asking about daily-use street pressures and not competition since you mentioned gas mileage.

There are two types of 225/40R18 that I could find from various manufacturers. Standard and reinforced load. The standard load tires have load index of 88 and the reinforced tires have a load index of 92. Hopefully yours have a 92 (this is found by looking at the size, for example 225/40R18 92V). I'll explain why.

The original tires on the Mazda 6 are the P205/60R16 91H size. Mazda calls for 32 psi minimum. Using load/inflation tables, a 91 load index tire carries 1224 pounds at 32 psi. Using the same tables, I have found that both 88 standard load and 92 reinforced load tires require 36 psi to meet or exceed the 1224 pounds of the oe tire minimum. At 36 psi, they both carry 1235 pounds. Here's the problem if you have the 88 load index tires: 1235 is the max they can carry, even if you put more air. The 92 reinforced tires can go up to 1389 @ 42 psi.

The 17 inch option uses a reinforced size with a 93 load index (P215/50R17 93V). Curiously, this higher load index tire actually carries less than the 91 load tire at 32 psi (1146 pounds). Reinforced tires tend to carry less than their standard load counterparts at the same inflation pressure, but can exceed 36 psi and ultimately carry more total load than standard load tires. I use the higher load number of the 91 tire (1224) simply as a safety margin. Better to run a slightly higher pressure and be safe than sorry.

Why is this important? Because your tires need to be able to exceed the heaviest Gross Axle Weight Rating (GAWR) by at least 10%. If you take the 88 load tires and multiply the 1235 times two you get 2470. While this exceeds the 2370 GAWR for the front axle by 100 pounds, after you deduct 10%, you are left with a realistic load capacity of 2223 pounds, thus making it possible to overload your tires if you have passengers/cargo, etc. even with max pressure in the tire.

Anyways, the answer again to your question is at least 36 psi. all around. Although there are no formulas available to calculate load, most tire professionals will have load/inflation tables available to them to help you determine which air pressure is right for you, especially if you've plus-sized your wheels and tires.
 

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I think you're giving the OEs a little too much credit with the detailed calculations you're using.

GAWR is not necessarily a function of the limit of the suspension load. It is often a function of other things, such as the engines ability to pull the load without overheating, the handling performance of the vehicle at that load or the weight class that the vehicle is tested at for fuel economy ratings.

Tire size and GAWR are also adjusted based on the full line of vehicles. For example, a rental Mazda 6 in New Zealand that I had used 205/55-16 tires. This is because you can't get a V6 there, so the heaviest car will be fine on the 205/55. In the US, the 16 has to be sized for the heaviest V6, so even though a 205/55 would be fine on a US I4, Mazda would want to avoid having multiple tires in Flat Rock, so everyone gets a 205/60.

OEs set the tire pressures based on many, many factors. Only one of the factors is load, obviously you can't go below the minimum required to support the load of the vehicle. But the OEs will adjust up for fuel economy, steering feel, durability or wear and might adjust down for ride or wear. There is a commonization factor again, it is usually simpler to set just one pressure across all vehicles in a line, even if it is not optimal for each particular vehicle.

Bottom line is there are so many variables, you will never come up with a "formula" to determine what pressure to run. Figure out what the minimum pressure is for load capacity and adjust upwards from there based on your individual preferences.
 

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Resurrecting an old thread!!

I had a set of new Proxes 4 (225/40/18) mounted last week. I've been wondering about the tire pressure question since. With a quick search I found this thread.

I was running my tires at 32psi until this morning when I blew them up to 38psi as suggested by ToyoGuy. The car went from slugish to perky again; I couldn't believe the difference it made. I'd just like to confirm that this prescribed pressure (38psi) hasn't changed since the thread was last active.

At this pressure, is the tire at it's optimal contact profile? I'm worried that it might be over-inflated leading to premature wear on the center band.
 

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Discussion Starter #20
It is truly amazing the difference air pressure makes. Even I forget that, sometimes. If you have a contact at your local tire shop, ask them to measure the tread depth in both the shoulder area and the center. Then come back in 5,000 miles, when it's time to rotate and ask them to measure again. If the tire is overinflated at 38 psi (it shouldn't be), then you'll see more wear in the center. If that's the case, then drop them a pound or two, but 38 should be good. That is what I'm running in my T1-Rs (225/40R18).
 
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