I built this GPS based speedo and lap timer to put on my kids and my karts for a couple of reasons, one was the cost of new 'off the shelf' systems and the other was the ability to record lap data as GPS based coordinates and apply to Google Maps or similar to study at home.
Initially I set up a working model with a cheap GPS patch antenna, an 8 digit 7seg LED display and a couple of Arduino Nano's to kick things off. All up I used about $30 worth of parts. I also used a couple of USB to serial adapters for development work so I could talk to the GPS engine from my PC and design the Arduino code.
I was able to read the Velocity over ground from the GPS strings and display them as speed in kph on the 7seg display. Sitting at my desk wasn't getting me much speed though so using a cigarette lighter USB adapter as a power source I sticky-taped the kit to the dash of my car and used it for a few trips to work and back, just testing and checking reliability of speed etc. It worked great!
Next I designed a housing for the 7seg displays to fit under the front of the naseau panel and printed it out on my 3d printer. The prototype functions well, but needs a bit of adjustment for asthetics.
The GPS position lap data is recorded onto an SD card simply as time and positions and converted to lap data that can be read by google maps by an awesome free utility called GPS Babel.
I designed a small circuit board to take an arduino nano, an SD card, a USB header to upload new firmware to the Arduino as I develop it, some headers to plug the GPS antenna and the 7 seg display into and a LiPo battery.
Currently I have to take the SD card out and put it in my computer or laptop to get the lap data off however I also built in the facility to plug a bluetooth module into the PCB but had trouble getting the code to work reliably and that went in the too-hard basket for now. There are other options too, but for now I'm just going with the removable SD card.

OK, so 3 attachments is the limit
The system works brilliantly, the prototype GPS Antenna could do 10Hz but at that speed the GPS data was very unreliable so I stuck with 1Hz for testing. Good enough for car speed, but not for accurate lines on a track map. I got a better ($25) GPS patch antenna with engine so I can get faster GPS updates at 5Hz. It can do 10Hz if I want but I think 5 plots per second will be enough.