When watching your best lap replay in the GT Theater in Gran Turismo 5, you won't be able to view your last sector time, because the replay will restart as soon as you cross the finish line.
This simple sector time calculator will calculate your last sector time or split time. Fill in your final lap time and the first sectors.
Type your known splits in the usual Gran Turismo time formats. e.g 1:12.345 or 0'56.789
To get the sector times in GT watch the best lap replay after the race or in the replay theater in GT mode.
The splits can be found in the bottom-left corner at the Live Timing Monitor.
Be aware that the correct splits will not turn up before you cross the split time line. If you are watching the replay just after the race, sometimes gray splits will be available. But only when each split time is highlighted, They change form gray to white.
To get the correct final lap time, take the time from the replay theater. Not the best lap time form the replay itself. This time refers to the best lap time in the stint the best lap replay is taken from.
The Gran Turismo time calculator is a project in progress. I'll update by request or when I run into new challenges in my "online" racing.
Contact me for wishes to future updates. Either at the boards or in this post about the GT split time calculator.