Any on-highway truck you spec today will likely employ downspeeding to some degree. Engine downspeeding is enabled through faster axle ratios that allow the engine to maintain a lower RPM—thus saving fuel—while still providing the necessary power to the axle.
Axle ratio is written as a ratio, such as 2.16:1. But what does 2.16:1 mean? The ratio is simply the number of teeth on ring gear divided by the number of teeth on pinion.
For example a 2.16 ratio could have 41 teeth on the gear and 19 teeth on the pinion.
To make it all work, the transmission needs to be tuned to handle the engine speeds and an axle needs to be ready to accept it; faster ratios enable the engine to maintain the 200 HP needed at the wheel end despite a lower engine RPM.
There are two types of downspeeding: aggressive with direct drive transmission (ratios from 2.15-2.47/2.64) and slightly faster ratios with overdrive transmissions. Direct drive is aimed specifically at long-haul fleets that spend 80 to 90% of the time in the top two gears and travel at least 30 miles between stops.
Overdrive transmissions is versatile for applications such as linehaul, regional and city delivery with ratios ranging from 2.64 to 4.11.