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1978 Mack RD770SX Tractor

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X doesn't have a meaning as many assume (I need to scan this when time allows).

In the "Mack Highway Vehicle Unit Symbol Definition" page of the salesman's manual (dated 6-65):

S - Truck, six-wheel [Note: truck, not tractor]

LS -  Truck, six wheel, weight-reduced version

SX - Truck, six-wheel, off-the-road dumper or mixer

X - Truck, four-wheel, dumper or mixer

Remember, the first digit indicated gross vehicle weight.

For example, R487ST, the 4 indicates a GVW of 30,500 to 56,000 pounds, and a GCW of 53,000 pounds.

For example, R685ST, the 6 indicates a GVW of 30,500 to 76,000 pounds and a GCW of 65,000 to 76,800 pounds.

For example, DM863SX, the 8 indicates a GVW of 31,000 to 100,000 pounds.

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