quando a relacao de macha é 1 pra 1 (1:1) o virabrequim vira 1x para virar 1x o drive shaft ( as rodas) , e quando esta em overdrive , o motor gira menos que 1x para a roda girar 1x , ( 0,7:1) por ex...
entao comparando qquer rotacao especifica da marcha 1:1 com overdrive , em overdrive vai ter velocidade maior ,mas o carro nao vai ter forca pra desenvolver essa velocidade se a rotacao for alta.
isso é usado para reducao de gasto de combustivel , nao para performance... como o passos disse , vc usa isso pra andar em velocidade de cruzeiro em uma rotacao mais baixa que o 1:1... a roda vai estar girando mais que o motor , que vai estar so mantendo a velocidade...
Overdrive is NOTHING more than any gear in your transmission that turns the drive shaft (output) faster than the engine (input). Therefore, when in this gear (usually at highway speeds), your engine turns at lower revolutions per minute (rpm), saving you gas, wear on your engine, and reducing the amount of engine noise into the passenger compartment. An overdrive gear can be had on a manual or automatic transmission. In the good old days, an automatic had 3 speeds, with third gear a direct 1:1 ratio. By the late 1970's manufacturers started adding a 4th gear which happened to be an overdrive gear, usually around 0.7:1. This meant your engine now required 30% fewer revolutions at highway speeds. The mystical OD button that everyone is referring to is just a defeat switch which keeps the transmission from going into the overdrive gear. This is added to give the driver some convenient control when in situations where the transmission "hunts" for the right gear. When climbing a hill the automatic transmission will drop a gear as more torque is needed to maintain speed. Once the speed is achieved the transmission moves to the higher gear, torque lost, speed drops and back and forth the transmission goes. All the OD button does is remove the highest (and lowest torque producing) gear from the equation. This was far more necessary in the 1980's with the weaker engines of the time. Today, automatic transmissions with 5 to 8 gears are the norm. In the Lexus 8 speed automatic, the top two gears are overdrive gears. Unless you live in a mountainous area with an older car let your automatic transmission do its job. If your driving with a manual transmission, this is all moot....there is no button, and you know when to shift.