How to read your lap data
Start with the speed trace: it shows where you brake, how slow you go in each corner and how early you get back to full speed. Overlay it with your best lap and look for the biggest gaps, then check throttle and g-force to see why. Make one change at a time and confirm it with the data.
- Plot speed against distance, not time, so laps line up corner by corner.
- Braking point, minimum speed and the point you get back to throttle explain most lap-time differences.
- The g-g diagram shows how much of the tires’ grip you’re using, and where you’re leaving gaps.
- Compare against your own best lap, change one thing, and check that it actually helped.
Start with the speed trace
The speed trace is speed on the vertical axis and distance around the lap on the horizontal. Straights show up as rising lines, braking zones as steep drops, and corners as valleys.
Each valley has three things to read. Where the line starts to fall is your braking point. The bottom is your minimum corner speed. The point where it starts climbing again is where you got back to acceleration.
Overlay two laps and the differences jump out. A later drop means you braked later. A higher valley floor means you carried more speed. A line that climbs earlier and stays above the other all the way down the straight is the most valuable gain of all.
Braking, minimum speed and throttle
Braking: compare where braking starts and how steep the drop is. A steeper drop means harder braking and peak deceleration. A drop that starts early and is shallow usually means you’re braking gently, not too late.
Minimum speed: small differences matter. One mph more at the bottom of a corner before a long straight can be worth more than braking later.
Throttle: if you have a throttle channel, look for hesitation. A gap between the brake release and the start of throttle is coasting. Several small stabs on exit suggest the car isn’t settled or you got on it too early.
The g-g diagram
The g-g diagram plots longitudinal g (braking and acceleration) against lateral g (cornering) as dots. Together they draw the shape of the grip you used.
A driver at the limit fills out the edge, with dots tracing round from braking into cornering and back to acceleration. A cross or plus shape, with an empty space between the arms, means you brake, then turn, then accelerate, with little overlap. That empty space is the room trail braking and a smoother throttle pick-up can use.
Don’t chase the edge everywhere. Bumps, kerbs and cambers spread the dots out, and grip varies with tire temperature.
Each dot is one moment of a lap: cornering (sideways) against braking and acceleration (up and down). Novices draw a cross; experienced drivers fill the circle by blending inputs. Illustrative data.
Comparing laps the right way
Changing several things at once makes it impossible to know what worked. One change, one session, one comparison.
- 1Choose a clean reference: your best lap, without traffic or yellow flags.
- 2Add a time-difference (delta) trace. Where it slopes up, you’re losing time; where it slopes down, gaining.
- 3Find the two or three corners with the steepest loss. Ignore small noise.
- 4For each one, read the speed trace to see whether it’s braking, minimum speed or exit.
- 5Pick one change for the next session, write it down, and check the data afterwards.
Delta compares where you are now with where you were on your reference lap at the same point of the track. Negative (green) means you're ahead.
Know what your data can and can’t tell you
GPS-based data is great for speed, position and lap time but limited in tight sections at low sample rates. Braking is often inferred from deceleration rather than measured from the pedal. OBD-II engine data depends on the car and arrives more slowly than a dedicated logger.
Use data to confirm what you felt and to find where to look next, not to override what the car and your instructor are telling you.
FAQ
What should beginners look at first?
The speed trace and the time-difference trace. Find the corner where you lose the most time and check whether it’s braking, minimum speed or exit.
Why plot by distance instead of time?
Two laps reach the same corner at different times. Plotting by distance lines them up at the same spot on track.
Do I need a brake-pressure sensor?
It helps you see exactly how you use the pedal, but you can learn a lot from deceleration, which most GPS systems can calculate.
- Delta time explainedDelta time shows if you’re ahead of or behind a reference lap right now. Predictive vs reference delta, reading the bar, distance alignment and limits.
- Lap timers and GPS accuracy: 1 Hz vs 25 HzHow GPS lap timers work, why sample rate matters, how interpolation at the timing line improves accuracy, and when an external 25 Hz GNSS receiver is worth it.
- What is HPDE?HPDE is coached, non-competitive driving on a real race track in your own car. How run groups, instructors and classroom work, and how it differs from racing.