Does the time difference between São Paulo and Singapore change during the year?

No. The gap stays at 11h all year.

When is the best time to call Singapore from São Paulo?

There is no overlap between standard working days, so one side has to call early or late.

São Paulo is 11h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.

São Paulo

Brazil

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Singapore

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A 9-to-5 working day in São Paulo does not overlap one in Singapore at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
São Paulo · Brazil	America/Sao_Paulo	UTC−3
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between São Paulo and Singapore?: São Paulo is 11h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between São Paulo and Singapore change during the year?: No. The gap stays at 11h all year.
When is the best time to call Singapore from São Paulo?: There is no overlap between standard working days, so one side has to call early or late.