Posted by Paul Cook in Kempner, Texas on Saturday, May 05, 2007 at 12:37:22 :
In Reply to: 7.5X16 vs 9.00X16 posted by ScottW on Saturday, May 05, 2007 at 11:41:18 :
Yes. The gear ratio and related tire size was specified by the GVW.
In the early years of "flat fender" production, they were available in a 7600# GVW with 7.50 x 16, eight ply tires and a 8700# GVW with 9.00 x 16, eight ply tires.
My references show the same spring capacities so apparently the only difference in the GVW's was the capacity of the tires. Note that tire size made 1100# difference.
The greater rolling diameter of the larger tire on the 8700# GVW truck - and the assumed higher loads being carried - called for the 5.83 differential gearing. The reduced rolling diameter of the smaller tire on the 7600# GVW truck - and the assumed lighter loads - called for the 4.89 differential gearing to accomplish the respective jobs.
These tire and gear combinations provided about the same road speed at comparable engine speeds.
According to my available references, availability of the smaller tire and the 4.89 gearing combination went away in 1961. By then, there was a 10 ply option for the 9.00 x 16 tires.