[tristancliffe]

Load two laps (from same or separate sessions).

Put data on a chart as normal. Speed, RPM etc.
In a section at the bottom put laptime channel.
Edit the properties of the chart (I forget the exact terms now) and select the Diff checkbox on one of the laptimes.

Bother. That's a rubbish explanation I think. Send me your email address and I'll send you a saved worksheet I use for laptime deltas.