Diligent inflation maintenance is strongly recommended for preventing 'underinflation' and increasing safety. The aim for inflation maintenance is preventing tire damage caused by 'underinflation'. Proper inflation also improves various tire performance, such as even wear, increased traction, handling, lower rolling resistance, and ride comfort.

1. Tire Information Placard on a Vehicle

Above photographs are examples of the 'Tire Information placard' on a vehicle. The 'Tire Information Placard' can be located in various places such as the door jam or glove box. This is the best place to get information regarding 'Proper inflation'. There is various information as to the OE tires printed on the placard. Most importantly, the vehicle manufacture's recommended inflation pressures can be found on this.

2. Tire Load-Pressure Table (provided by standards such as TRA, ETRTO, etc.,)

The table above is provided in the 2009 TRA Year Book. It shows the designed maximum load capacity calculated at each inflation pressure, but not the recommended pressure.

3. Maximum Inflation Information on Tire Sidewall

There are the maximum load and pressure stamped on the sidewall of a tire. Many drivers misunderstand this as the correct inflation pressure for the tire. It is the maximum cold inflation pressure, not the 'proper inflation' pressure. There is a lot of information regarding tire pressures, but some do not express 'proper inflation' for a vehicle, which sometimes leads customers' confusion. Hence, it is one of the most important duties to provide the information as to 'proper inflation' for customers.

Back to top

"Inflation Pressure" is one of most important issues for tire maintenance. In this section, the inflation pressure of extra load (reinforced) tire is described.

*In the past, “REINFORCED” had been commonly used but nowadays it has been replaced by “EXTRA LOAD”.

Reinforced tire is standardized in ETRTO (European Tyre and Rim Technical Organization). Recently, the installation of reinforced tires has been increasing not only in replacement tires but also in OE-fitment tires.

The below graph shows inflation pressure vs. load capacity of standard load tire and extra load (reinforced) tire in the same size (205/40ZR17).

The load capacity of a extra load (reinforced) tire is higher than standard load tire. At the same time, the inflation pressure is also different between a extra load (reinforced) tire and a standard load tire. Therefore, it must be careful about the inflation pressure of extra load (reinforced) tires when standard load tires are replaced with extra load (reinforced) ones or vice versa, even if both tires are the same size.

205/40ZR17 (Standard) L.I. 80 = 420kg at 230kPa

205/40ZR17 (Extra load / Reinforced) L.I. 84 = 415kg at 230kPa

Therefore, you have to inflate "240kPa" in this extra load (reinforced) tire to support 420kg.

Standard Load Tire

Extra load (reinforced) tire

*This refers to a speed of 160km/h and to camber angles up to 2ﾟ. (The table above is provided in the 2008 ETRTO Year Book)

Additionally, inflation pressure in a tire decreases naturally, about 5-10% (10-20kPa). Hence, it is recommended to check the inflation pressure of tires at least once per month.

Back to top

1. Introduction

Most tires are 75,70,65 or 60 aspect ratio but lower profile tires of 55,50,45 even less than 40 aspect ratio are becoming popular and 'plus sizing' is one of hottest trend in the market. This is why Lower Profile Tires are popular:

(1) They provide very good vehicle handling qualities, however, ride comfort may not be as good as higher profile tires.

(2) They allow for a larger rim diameter that allows the vehicle to be equipped with larger diameter disk brakes for improved breaking performance.

(3) They enhance the appearance of the vehicle and are in fashion.

(4) More and more new cars are coming equipped with lower aspect ratio tires at the O.E. level.

However the flexible region (shoulder - side area) of those tires is narrower than compared with standard tires. Thus, it is very difficult to determine whether tires are correctly inflated just by a visual inspection. See photos below:

	SIZE:225/40ZR18 92W reinforced proper inflation pressure 220kPa (31 psi)
	underinflation pressure 1 150kPa (21 psi)
	underinflation pressure 2 100kPa (14 psi)

It is important to note that inflation pressure is one of the most important factors. Now, the recommended way for plus sizing and providing the correct inflation pressure will be discussed.

2. General Rules of Plus Sizing

Plus Zero fitment, is to increase section width by 10mm and drop aspect ratio by 5 points. The rim diameter stays the same.

Plus 1 fitment requires an increase in section width of 10mm, a reduction in aspect ratio of 10points and an increase in rim diameter of one inch. (+10/-10/+1)

Plus 2 calls for a 20mm jump in section width, a 20-point drop in aspect ratio and a two-inch an increase in rim diameter. (+20 -20/+2)

Plus sizing requires close attention.

(1)	Do not change overall tire diameter by more than plus 2 percent or minus 3 percent except for tires that are installed on full size pick-ups. In case of Full size Pick ups, do not change more than plus 6 percent or minus 3 percent. This is to prevent problem with the ABS breaking system, as well as to prevent an error to the speedometer or odometer.
(2)	Never fit tires with load-carrying capacity lower than the vehicle manufacture specifies. Example: factory installed size = 205/55ZR15 91W. Would size 215/45ZR17 87W be suitable 'plus 1' fitment? No. Although the tire diameter is OK, the load capacity is too low. However size 215/45ZR17 91W XL (Extra Load) is suitable.
(3)	Never use an inflation pressure lower than the recommendation of the vehicle manufacturer. The 'Tire Information Placard' can be located in various places; such as the door jam or glove box.
(4)	Take into consideration other factors such as; wheel offset, clearance of the vehicle body and suspension components.

3. Inflation Pressure at Plus Sizing

When carrying out any plus sizing, the inflation pressure recommended by the vehicle manufacture must be considered, because the load capability of the replacement tire must be equal or greater than the original factory tire. However, extra load (reinforced) tire will be required 30-40kPa (5-6psi) higher inflation pressure.

For example; the O.E standard size, 195/65R15 91H, has load-carrying capacity of 515 kg at 200kPa(28psi) that recommended by the vehicle manufacture.

<The table above is provided in the 2008 ETRTO Year Book.>

4. CONCLUSION

When deciding upon replacement tires, it is important to check load-carrying capacity. Therefore, it is recommended that a tire that has load-carrying-capacity that is equal to or greater than the factory tire, and be capable of maintaining the recommended air pressure. Attention must be paid to the air pressure when extra load (reinforced) tire are fit because that tire may be required higher inflation pressure.

Toyo Tires, as a tire manufacturer, cannot recommend any down grade of the recommended load index. It is also our recommendation that the tire pressure be checked at least once per month and the air pressure must be checked BEFORE the start of a long journey. Likewise, it is important to check the tire pressure while they are COLD.

Back to top

Tire can only display maximum performance when they are inflated to the proper pressure. Too low or too high pressure will make the tires more susceptible to damage. It may also adversely affect tire life and fuel economy, leading to higher operating cost. The load on the tire determines proper inflation pressure. Therefore, it is very important to know the load per tire.

Calculated is almost equal to Measured.

Actually, it will be difficult to measure the wheel base, the distance between the kingpin and drive axle and so on by oneself. Therefore it is recommended to calculate it by below tables. For example; When the G.V.W is 45 ton, the front axle weight is 6 tons, the drive axle weight is 18 tons and the trailer axle weight is 22 tons. (Per tire: 3 (t) on front, 2.25 (t) on drive, 2.75 (t) on trailer)

Calculated is almost equal to Measured.

Back to top