My next question to Eaton was, "With the large gap [gap in gear ratios?] between direct and overdrive gears, how much of a drop in speed will there be when pulling a mountain in direct vs. overdrive?" As it turns out, there is another overdrive gear in this transmission that is a .86 [0.86:1] and is only to be used for pulling hills (not cruising on the level, as the transmission can overheat). Do not spec "specify", designate the truck to use the .86 overdrive gear as a cruising gear. Use the .73 gear for cruising in level terrain. Eaton does not consider this transmission a double overdrive 18 speed even though there are two overdrive gears.
Here's my guide to the best use of the various gears [discussing Harleys]:
5th is best used for relaxed cruising or high speed riding on motorways and so on. As a cruising gear, 5th is best used for speeds of 50mph and upwards. Remember also that a lower gear will normally result in faster and safer overtaking manoeuvres.
4th is best used for low speed cruising or making fast progress along interesting country roads. Most European specification Harleys are a bit overgeared in top, as this helps them to get through the very stringent German noise regulations. . . .
Four forward speeds and reverse, with syncromesh engagement for 2nd, 3rd and top gears. A central direct gear change lever is designed to leave the front floor of the car unobstructed.
Overdrive is fitted as standard equipment but cars without overdrive can be supplied if specially required.
Gear ratios: Overdrive 0.778 : 1; Top, 1.0 : 1; 3rd, 1.377 : 1; 2nd, 2.043 : 1; 1st, 3.376 : 1; Reverse 2.968 :1.