Hi Tony - welcome to the forum!
Your layout sounds fabulous. I think Micro Scalextric is perfect for the scenic treatment - the small size allows for plenty of space and all the OO model railway scenic materials work very well.
As for the speedometer, I guess you mean a lap timer? The older 1995-2018 Micro Scalextric track system does have a mechanical lap counter, but not a timer. The new 2019 track system has neither - at the moment. I heard some rumours that the Scalextric team are developing something - hopefully for 2021.
In the meantime, I find using a lap timing app on a smart phone or tablet works very well. I use 'LapTracker - Auto Timer' on my iPhone and iPad. There are similar apps for Android devices. The apps use the camera to trigger the timer. It is possible to set up a phone in a trackside building to look like a scoreboard - so it can be quite realistic and well as functional.
I hope that helps. Take care and have fun!