Tire sizes can be a bit confusing, as the size designations changed a lot over the years. From standard bias ply sizes such as 6.70-15 to Alpha Numeric sizes like F70-15, all the way up to modern P-Metric sizing, such as 205/75R15. Even the most savvy car guys have trouble decoding certain sizes, so take a look at this handy chart, and it should help you figure out the bias ply and radial tire size cross reference!
Bias Ply Before 1964 Bias Ply 1965 To 1972 Bias Ply Alphanumeric 78 Series Bias Ply Alphanumeric 70 Series

Metric Radial 80 Series

P-Metric Radial 75 Series

P-Metric Radial 70 Series

 Bias Look Radial Rec. Rim Width
520-12 - - - 125R12 P125/75R12 - - 4-5
520-13 560-13 - - 155R13 P155/75R13 P155/70R13 - 4-5
590-13 600-13 A78-13 - 165R13 P165/75R13 P175/70R13 - 4-5
640-13 650-13 B78-13 - 175R13 P175/75R13 P185/70R13 650R13 4-6
725-13 700-13 D78-13 - 185R13 P185/75R13 P205/70R13 - 4-6
590-14 645-14 B78-14 - 155R14 P175/75R14 P185/70R14 - 4-6
650-14 695-14 C78-14 - 175R14 P185/75R14 P195/70R14 - 4-6
700-14 735-14 E78-14 E70-14 185R14 P195/75R14 P205/70R14 - 4-6
750-14 775-14 F78-14 F70-14 195R14 P205/75R14 P215/70R14 750R14 4-6
800-14 825-14 G78-14 G70-14 205R14 P215/75R14 P225/70R14 800R14 5-7
850-14 855-14 H78-14 - 215R14 P225/75R14 P235/70R14 - 5-7
550-15 560-15 A78-15 - 155R15 - - 560R15 4-5
590-15 600-15 A78-15 - 165R15 P165/75R15 P175/70R15 - 4-5
640-15 685-15 C78-15 - 175R15 P175/75R15 P185/70R15 - 5-6
650-15 735-15 E78-15 E70-15 185R15 P195/75R15 P205/70R15 - 5-6
670-15 775-15 F78-15 F70-15 195R15 P205/75R15 P215/70R15 670R15 5-7
710-15 825-15 G78-15 G70-15 205R15 P215/75R15 P225/70R15 710R15 6-8
760-15 855-15 H78-15 H70-15 215R15 P225/75R15 P235/70R15 760R15 6-8
800-15 885-15 J78-15 K70-15 225R15 P235/75R15 P235/70R15 800R15 6-8
820-15 900-15 L78-15 L70-15 235R15 P235/75R15 P255/70R15 820R15 6-8