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The Effects of Changing From 8R19.5 to 225/70 19.5 Tires

by Tim Plaehn

The 19.5-inch wheel size is used on medium-duty trucks and motor homes. The 8R19.5 tire size is a tall profile tire that would probably be mounted on an older vehicle. The 225/70 R19.5 is a modern, low-profile tire for the 19.5 inch truck rim. Changing to the newer size from 8R19.5 tires would make finding new tires a much simpler process. The Goodyear RV tire website notes this is a popular conversion for the owners of older motor homes.

Wheels

To convert from 8R19.5 to 225/70 R19.5 tires may require the purchase of new wheels. The 8R19.5 size can fit on a 5.25-inch-wide wheels, and the low-profile 19.5 tires require 6-inch-wide wheels. Besides a better fit of tire to rim, the wider wheels will provide enough dual-wheel spacing for 225/70 R19.5 tires. Dual-mounted tires should never touch, and the 5.25-inch rim is too narrow to give proper dual-wheel spacing for the newer tire.

Tire Diameter

Tires sized 8R19.5 have a diameter of 33.1 inches, and 225/70 R19.5 tires are 32 inches in diameter. Changing tire sizes will result in a speedometer that indicates faster than the vehicle is actually moving. 8R19.5 tires will rotate 628 times in a mile, and 225/70R19.5 tires rotate 644 times. The result is a 2.5 percent speedometer error with the new tires. The Goodyear website states that no gearing change is required for this amount of error.

Load Capacity

Both the 8R19.5 and 225/70 R19.5 tires have load range F capacity ratings. This makes the capacity for the two tires sizes similar but not exact. The low-profile 225/70 R19.5 tire has a load capacity of 40 to 140 lbs. higher depending on the vehicle location of the tire. A set of six 225/70 R19.5 tires mounted on a truck, replacing 8.R19.5 tires, would increase the tire set load capacity by 440 lbs.

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