Hi, the handbook gives a description of how to use the speed limiter, pages 117/118 in mine. One of the things to remember is that you will not keep a constant speed when going downhill so keep an eye on your speed. The actual how it works is pure alchemy to me! I hope this helps, enjoy your 500X, I do.
Basically take it up to 70mph or what ever and push the SET (+) or SET (-) button and release. Release the accelerator and it should continue at that speed. It you touch the brakes it will stop. You can press the accelerator to speed up and then it will return to your set speed again.
1. press the limiter button on the middle of the left side steering wheel button cluster.
2. Accelerate to the chosen limit speed.
3. Press "set" on the right side of the steering wheel.
The car will not exceed that speed, no matter how much you press the accelerator.
Press the middle button again to cancel it.
Just to follow up, (and correct a wee mistake) it's the button below the right side cluster on the steering wheel that's the limiter, not the centre button of the main cluster. It's a dial logo with an arrow going round it.