Lap delta is the running time difference between your current lap and a reference lap, usually your best. A negative number, often shown in green, means you are ahead of the reference at that point of the lap. A positive number, often red, means you are behind. It answers one question while you drive: am I faster or slower than my best lap, right here?
How it is computed
A lap time only arrives at the finish line, so a live delta needs more than the clock. The usual method uses distance:
- Record a reference. During the reference lap, the app stores the elapsed time at many points of the lap, indexed by lap distance, the metres travelled since the line.
- Look up the same point. On your current lap, at each moment it reads your lap distance and finds the reference lap’s time at that same distance.
- Subtract. Delta = your elapsed time − the reference time at that distance.
Comparing at the same point is what makes the number fair. If you only subtracted a full lap time from a running clock, the figure would start far below zero and climb to the final gap as the lap ended, which says little about where you gain or lose.
The method needs lap distance, or a position that can be turned into one, from the game. The same numbers give a predicted lap time: the reference lap time plus the current delta, the time you would set by holding the same gap to the line.
Reading it while you drive
- Watch the trend, not the digits. A delta that shrinks through a corner means you are gaining there. One that grows on the straight points to a worse exit.
- Expect noise. It is jumpy for the first seconds of a lap, when a small gap is a large fraction of the distance.
- Check the reference. Best lap, last lap or a lap you saved are different questions. A delta against your best is a target; against the last lap it shows if you are improving.
- Compare by sector. The delta at a sector boundary is a quick way to find which part of the track costs you time.
- Do not chase it in a corner. It is a glance-and-adjust number. Fuel load, tyre wear and traffic change it from lap to lap, so a lap that starts behind is not wasted.
How Horizon Labs shows it
Horizon Labs has a Lap Delta widget with rows for BEST and LAST, each with a lap time and a delta, plus MAX and AVG speed for the lap. You can switch any of these rows on or off in the layout editor, and the widget comes in more than one visual style. It also exists in the Designer as a monospace readout like +0.000.
It has two delta modes:
- Live
- Compares the running lap time with the best and the last lap time, so you see a number while you drive.
- End of lap
- Updates when you cross the line: the last lap against the best lap, and the last lap against the one before it.
In the alternative styles, the live delta is matched by position: Horizon Labs records how long each lap took to reach each point of the track, using the lap distance the game sends, and compares your running lap with your best and last lap at the same point. The classic style compares the running lap time with the best and last lap times, so read its live figure as a pace gauge and the end-of-lap figure as the real gap. The widget only shows timing during a timed session, and it clears when a session restarts.
Which games provide the data
The widget needs the current, last and best lap times from the game. Horizon Labs gets them from:
- Forza Horizon and Forza Motorsport, with race laps (not free roam).
- EA SPORTS F1, which also sends lap distance.
- DiRT and GRID, which send lap distance and sector.
- Assetto Corsa and Assetto Corsa Competizione, iRacing, Le Mans Ultimate, rFactor and rFactor 2, Automobilista, Project CARS and RaceRoom.
Games that do not send lap timing, such as BeamNG.drive, Live for Speed, the truck games and EA SPORTS WRC, leave the chrono rows empty. Gran Turismo sends last and best lap but no current lap time, so only the end-of-lap mode has something to work with. The terms are in the telemetry glossary.
Horizon Labs only reads the telemetry the game sends out. Which fields exist is each game’s choice.