When I started driving a truck in 1976, 10 and 13 speed transmissions were the norm, with a single stick, and air servos for multiple ranges. Old school guys still ran twin stick Macks, or Spicer 4x4s, which featured 16 forward gears. Mack also had a single stick five speed, popular on company fleet trucks. Low power to weight ratios made multiple gears a necessity, and for top speed, the more the better. My last truck had nearly double the horsepower and torque of my old '71 International with an 8V71 Detroit Diesel, yet both pulled the same gross weight.
In the 1970s and early 80s, Mack was still a vertically integrated manufacturer, meaning they made all their own components. They would not install a transmission from a competitor. The company I spent 20 years with bought a 1982 Mack R model that had been reposessed from an owner/operator. The factory five speed had been replaced with a dual range 9-speed Fuller Road Ranger. That truck would cruise at over 80 miles an hour versus 68 for the rest of the fleet, and climb the mountain outside of town at 10 mph faster than any of the five speeds. Fuel mileage averaged over six miles per gallon, compared to five for the other trucks.
Shortly before production of the R model ended in the late 80s, Mack relented, and started offering multiple range transmissions from other vendors, after their own attempt at that configuration proved a bust. I'm not an engineer, but I suspect similar considerations are driving the trend in today's passenger vehicles.
