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Alphanumeric tire sizes such as F78-14 were used in the 1970s before the widespread use of radial tires. The typical tire from that era had a tall, narrow profile, and replacement tires should have similar dimensions.
Dimensions
An F78-15 tire has a nominal diameter of 27.2 inches. The 78 aspect ratio indicates the tire would have a maximum width of 7.75 inches. The tire would fit on a rim that was 15 inches in diameter and 5.5 inches wide.
Replacement
The metric tire sizes that are closest to F78-15 are 195/80-15 or 205/75-15. A 205/75 tire has a diameter of 27.1 inches and a width of 8 inches. The 80 series tire sizes are not commonly found.
Considerations
F78-15 tires were of bias-ply construction. Tires in a 205/75-15 size will be easier to find as radial tires. Radial and bias ply tires should not be mixed on the same vehicle. If a wider tire is desired, 215/70R15 or 235/60R15 will provide the same diameter, but wider wheels will be required.
