Lap Timing Apps Compared — RaceChrono, Harry's LapTimer, TrackAddict

Three apps own this space: RaceChrono, Harry’s LapTimer, and TrackAddict. All three turn a phone into a lap timer, take an external GPS and an OBD-II feed, and burn a data overlay onto GoPro footage. They differ in how reliably they hold those connections together for a 25-minute session, and in price. On an ND MX-5 the inputs are the same regardless of which you pick: the phone’s GPS, an optional external GPS puck, and the car’s OBD-II port.

The inputs, and why they matter

A phone’s built-in GPS updates at roughly 1 Hz — one fix per second. At 60 mph that’s a position sample every 88 feet. Good enough to know which lap was quickest; useless for comparing two laps through a single corner, because the trace between samples is interpolated, not measured. Every serious lap-timing setup adds an external GPS that updates at 10 Hz or higher.

Common external receivers, all of which pair over Bluetooth:

RaceBox Mini / Mini S — 25 Hz GPS with a built-in IMU (accelerometer + gyro). The Mini S adds standalone logging so it keeps recording if the phone connection drops. Widely regarded as more accurate than older dedicated units; the GPS trace lands within a few feet of the real racing line.
Qstarz BL-1000GT — 10 Hz, one of the long-standing iOS-compatible pucks.
Various 10 Hz Bluetooth GPS units that all do the same job to varying degrees of reliability.

The OBD-II feed is separate. Plug a Bluetooth OBD adapter into the ND’s port under the dash and the app can log RPM, throttle position, coolant temp, and other engine PIDs alongside the GPS data — so the overlay shows what your right foot was doing through the corner, not just where the car was. The adapter to get is an OBDLink MX+: it reads at a high rate (up to ~50 Hz on supported PIDs) and, importantly, the ”+” model is the one that works on iOS. Cheaper ELM327 clones are slower and flakier, and many don’t talk to iPhones at all. For more on adapter selection see /nd-miata/obd-adapters-apps/.

You can run external GPS and OBD at the same time. That’s where the apps start to separate.

The three apps

RaceChrono (Pro)

The free tier times laps with the phone GPS. RaceChrono Pro (a one-time in-app unlock, in the ~$18–20 range last we checked) adds external GPS and OBD support, sector timing, a predictive lap timer, a track library of 2,600+ circuits, and hardware-accelerated video export with a configurable data overlay. Available on iOS and Android.

Its reputation is for keeping the GPS-plus-OBD sync intact across a full session. Drivers who got tired of OBD data dropping in and out of the overlay tend to land here. If you want one recommendation and don’t want to think about it, this is it.

Harry’s LapTimer

The most feature-dense of the three and the oldest. The top “Grand Prix” edition is the priciest single app of the bunch (~$28). It does everything — multi-camera, deep analysis, broad hardware support.

The recurring complaint is reliability. Owners report it dropping the OBD or GoPro connection mid-session, missing the start/finish line, and worst of all failing to keep external GPS and OBD-II data synced, so the OBD channel flickers in and out on the exported video. When it works it’s excellent; the variance is the problem. iOS-first, with an Android version that historically lags.

TrackAddict

The budget entry. The base app is free and a modest Pro unlock turns on the features that matter. Video overlay is handled by its sibling tool RaceRender — free with limits (3-minute clips, a watermark) or paid to remove them. Several drivers who bounced off Harry’s report TrackAddict simply working. It’s less polished and the analysis is thinner, but for “log my laps, overlay my GoPro, don’t fight me,” it earns its place.

Quick comparison

	RaceChrono	Harry’s LapTimer	TrackAddict
Platforms	iOS, Android	iOS, Android (iOS-first)	iOS, Android
Cost	Free + ~$18–20 Pro unlock	Free + paid tiers up to ~$28	Free + small Pro unlock
External GPS	Yes (Pro)	Yes	Yes
OBD-II	Yes (Pro)	Yes	Yes
Video overlay	Built in (Pro)	Built in	Via RaceRender (separate)
Reputation	Stable GPS+OBD sync	Powerful but connection-flaky	Reliable, less polished

Prices and tier names shift between platforms and over time — confirm in the store before buying.

Where a dedicated logger pulls ahead

A phone app is the cheapest way to get real data, and for most HPDE and track-day drivers it’s genuinely enough. The limits show up at the edges:

A phone is a fragile recorder. It overheats in a windshield mount in summer, the screen times out, the Bluetooth stack drops a connection, and a phone call or notification can interrupt a session. A standalone unit like a RaceBox Mini S or an AiM Solo 2 keeps logging on its own.
Sample rate and sensor fusion. A 25 Hz GPS with a fused IMU resolves a corner far better than a 1 Hz phone fix, and the IMU fills the gaps between satellite fixes. Notably, a modern external GPS such as the RaceBox can be more accurate than an older dedicated dash logger despite both quoting 25 Hz — the receiver hardware, not just the number, drives accuracy.
CAN / direct ECU logging. OBD-II polling is fine for RPM and throttle but it shares the bus and rate-limits. A logger wired to read the vehicle CAN bus or a dedicated dash (AiM and similar) pulls more channels, faster, without the OBD round-trip.

The honest split: if you’re chasing your own lap times and want video, a phone app plus an OBDLink MX+ and a RaceBox is a strong, affordable rig. If you’re coaching, comparing drivers, or want bombproof unattended logging, a standalone unit is worth it. The two aren’t mutually exclusive — plenty of people run a RaceBox into RaceChrono and get the best of both.

For a side-by-side of the standalone hardware, see /nd-miata/data-loggers-compared/ and the broader /nd-miata/track-day-data-logging/ overview.

ND-specific notes

The ND’s OBD-II port is in the usual spot, under the dash to the left of the steering column. A Bluetooth adapter lives there permanently with no ill effect, though pull it for long storage to avoid a small parasitic draw.
On a track day you’re logging engine PIDs, GPS, and IMU — none of which require touching the infotainment. If you’re also running telemetry through the car’s Gen 6 Mazda Connect screen, that’s the territory ScreenTune’s upcoming track edition is aimed at; for app-and-GPS lap timing as described here, nothing on the head unit is involved.
Mount the phone or logger low and out of direct sun. The ND’s small dash top bakes in summer, and a thermally throttled phone stops recording before you notice.