Lower gears are not ideal for doing any logging, I would find a long straight road (
Private Runway) and do a run in 4th gear from virtually 1500RPM right upto the redline that way you get a full reading across the entire rev range.
In the lower gears because the revs whip round so quickly generally the readings are never spot on, You need a longer gear where the engine is under load and you tend to get more stable readings.
When me and a friend do Mapping/Logging on his Saab powered project its all done in 4th gear