Here is the hardware and software I recommend using to log and analyze telemetry data.
What to buy...
1. Phone
You can use your current phone or tablet (iOS or Android). I use my phone because since it‘s large enough and I can look at the data whenever convenient.
2. GPS Data Logging Device
RaceBox Mini is a device which streams GPS data to your phone/tablet via Bluetooth at a high, 25 Hz refresh rate. Modern phones only stream data at 1 Hz, which makes having an external GPS device critical.
Do not let this device get wet. Place a bag over it if you need to use it in the rain.
RaceBox Mini $200
3. Data Analysis Software
RaceChrono Pro is data analysis software for iOS and Android. It is easy to use with an intuitive user interface. It works on the racetrack and autocross among other functions. It has the ability to import and export sessions including VBOX files (.vbo) and other formats so you can share with your friends. The analysis view is overlaid on a satellite image which is helpful to get a sense of the scale.
A key feature of RCP is how it is always recording data. Once you "Start" a session, it is always recording, even if you are not on track. This gives you the ability to edit your "Start" and "Finish" trips after you have already completed a lap or two. Furthermore, you could not set the start and finish at all until after the event and all of your lap data would be recorded.
On race day, I find this app to be extremely helpful to quickly find time between runs or between sessions. The app can also live/predictive timing if you mount your phone inside the car.
RaceChrono Pro: https://racechrono.com/
4. OBD2 Data Logger
OBDLink MX+ is a OBD2 data logging device. This device is not required, but it's nice to have. This device plugs into your cars OBD2 port and streams data from your vehicle to your device (phone or tablet) via Bluetooth. The most important data from this device is throttle and brake pressure. RPM is usually not valuable as a driver aid. The fewer channels you stream, the higher refresh rate. It also supports CAN data at a much higher refresh rate.
OBDLink MX+: https://www.obdlink.com/products/obdlink-mxp/
Here are some video tutorials...
Below are a few video tutorials I made. I'm not a pro video creator, so I apologize in advance for the lack of narration and graphics. Maybe I'll do something like that in the future, but for now, these tutorials show the buttons you need to click to get started.
One more thing...
Data Analysis Tips:
Analyze data primarily using SPEED and DELTA-T.
Set up "Sectors". This will help analyze data quickly at events.
Each "Sector" starts and ends between speed peaks. For example, you can't brake too late, drop a half second and connect that into a run where you didn't miss your apex -- physically impossible.
Be careful to not set sector splits where the rise in speed isn't very substantial. For example, in a slalom or transitional element, there could be a small speed peak, but that would not be an appropriate position for a sector split.
When analyzing data between runs, look for SPEED differences in particular. Higher speed will almost always result in a faster time in a given section and correspond in a drop in the delta-t trace. For example, if your average speed was higher in a section, you'll likely see a time savings. Between competition runs/sessions, you may not have time to look at things like lateral acceleration, throttle trace, or probably anything else, therefore I recommend leaving those hidden until after the event.
If speed and delta-t are not tracking as expected, you may want to review the video.
Learn to ignore GPS drift.
If you have a co-driver and you are looking at data during the event, it may be useful to only look at YOUR OWN DATA. You have proved that you know how to execute the laps you just ran and most of the time you just need to clean up your own mistakes and string your best driving together in one run.
Looking at someone else's data who might have a different driving style or skill level can mess with you mentally. That's not how you want to start your next run. Maybe you did not ride with them and don't know exactly how they did what they did. It is likely best to save this type of analysis for after the event.