Over drive is any gear (or additional gearbox as was common 20+ years ago) that multiplies input speed to output. It used to be common to have an external gearbox, but with so many speeds available (5,6, and even 8 now) it is redundant. Electronic controlled transmissions will automatically engage overdrive when you are cruising.
And almost every single car with an automatic transmission nowadays has an overdrive gear built in.
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