Matt:

Below is an edit from the 1955-60 GMC Master parts book showing the applications for 8 stud 19.5" diameter wheels.

The "year series" decode as X = 56, T = 57, S = 58/9 and N = 60.

Model 15 = 3/4 ton, 25 = 1 ton. Add in the P makes it a delivery van and PM is a delivery van with Hydra-Matic transmission.

The rear being a "D" indicates a "dually" with a total of 4 wheels on the rear axle.

BUT (big but) some of these same models were also available with 16", 17" and 18" wheels.
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Last edited by Bill Hanlon; 08/27/2026 1:41 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.