I just bought a 1991 Lesus ES 250 and want to know if the overdrive should be on or off for normal driving.

Hello, I highly recommend purchasing an owner's manual. There are several current auctions on ebay for owner's manuals for this vehicle which are under $15. Here is a link to them: http://search.ebay.com/search/search.dll?sofocus=bs&sbrftog=1&from=R10&submitsearch=Search&satitle=1991+lexus+es+manual&sacat=-1%26catref%3DC6&sargn=-1%26saslc%3D2&sadis=200&fpos=ZIP%2FPostal&ftrt=1&ftrv=1&saprclo=&saprchi=&fpos=ZIP%2FPostal&lsot=&fsop=1%26fsoo%3D1&coaction=compare&copagenum=1&coentrypage=search&fpos=ZIP%2FPostal Best of luck! CarGrrl

You can leave it on, it will save gas and hurt nothing. An automatic tranny will use the proper gear all by itself. Occasionally when driving a very long uphill road you may turn it off if the tranny seems to shift itself a lot.