The 4. This 4. The “4. In this case, the tire is 4. The next digit represents the wheel size the tire will fit. This tire is meant for trailer wheels that are 8 inches in diameter.Tire diameter can vary slightly for each tire model. The listed diameters are from calculations based on the tire size. When changing tire sizes, we recommend staying within 3% of the diameter/height of the original tire.This indicates the tyre’s width in inches. In this case, the tyre has a width of 4. This represents the tyre’s load index, indicating its maximum load capacity. A higher number indicates a higher load capacity. This denotes the tyre’s rim diameter in inches.As a general rule, you want replacement tires that are within 3 percent of the diameter (height) measurement of the tires originally on your vehicle.The proper amount of pressure in a 4. B tire would be 60 psi. This should be listed somewhere on the sidewall of the tire. Unlike passenger car tires, trailer tires should be kept inflated to the maximum pressure as stated on the tire itself. Most passenger cars’ psi requirement will be between 30 to 35 psi, but several vehicles fall outside of that range and every vehicle will have specific requirements.What psi Should My Tires Be? An average passenger tire will come with a recommended tire pressure of 30-35 psi. This greatly depends on the size and type of tires run on the vehicle. However, you can find the manufacturer’s recommended tire pressure on the tire’s sidewall and on the vehicle’s driver-side door jamb.No, for most cars a tire pressure of 40 psi is too high. You might find 40 psi suitable for some vehicles, especially if they’re carrying heavier loads or if the manufacturer’s recommendation is close to that figure. However, it’s generally too high for most passenger cars, which usually recommend between 30-35 psi.