In an earlier contribution to this discussion,
VladicRavich@gmail.com <VladicRavich@gmail.com> wrote:

> I just bought a 1989 Volvo 740 GL and it has that button for the
> overdrive. What does it do? and when should I use it? I know this
> is probably a stupid question, but I would still much appreciate an
> answer. I'm sure whatever it is, it's only half as endearing as the
> hand cranked sunroof.

It's an additional 2-speed gearbox behind the main gearbox. When it's Off,
the main gearbox output drives straight through to the axle. When the
overdrive is engaged, it provides a higher gear than normal top gear -
useful for quieter cruising and improved fuel economy. I don't know anything
about the overdrive used by Volvo but overdrives in general can usually only
be engaged when the main gearbox is in top gear (plus maybe the next one
down).
--
Cheers,
Roger
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