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You should leave overdrive on all the time unless you're hauling large loads (which I wouldn't advise with a corolla). Overdrive is a gear ratio that is almost 1:1 and causes the engine to run at extra low speeds, when the overdrive is disabled (on 4 speed automatics) the transmission will only use the first 3 speeds, which will result higher engine speed, and more gas consumption. Even if it is the city, I'm pretty sure the corolla's overdrive kicks in at or around 35mph
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