My late 2017 S75D originally showed 203 miles rated range at 80% charge. After about 9000 miles this has dropped to 200. Is this normal?

The guestimator has more sample points to approximate mileage based on your driving habits. That 200 miles has some errors around +-10% depending on where you live. If you now moved to a new area where it's always flat and drive 9203 miles, that 80% at 200 miles will be 220 or 230. Then, also the temperature also plays a huge part in the guestimator.

I didn't realise it was that clever since the range it shows is so far off what you can actually achieve. For that reason I long since switched it to show simple percentage charge and then use the energy page for a much more accurate range forecast i.e. based on last 30 mile average. Why it can't show that figure on the dash display (instead of or as well as the Wh/mile figure it does show) I do not know? It would be so much more useful than the ever over-optimistic battery range figure. Having said that, maybe now I have more miles on my car (7k miles) I should try switching it back to show battery range and see if it's more realistic for my lead footed driving style!

This is correct. Don't listen to the Service Center personnel in this case. They will tell you it is based on driving habits, thus putting the blame on the owner instead of the car. You could drive another 10,000 miles at 25 mph and I promise your rated range would not miraculously jump up to a higher level even though your Wh/mi was at 120 (although I'm not volunteering to test this). For another data point, my 80% is at 193 miles with 45,000 miles.

So, you're saying if I take a 3.3 volt battery and a 3.3 volt battery from another area (both batteries rate 3300 mah), it will hold the same capacity? Wonderingi if that is true in reality. I propose a bet.

I can't bet, as I have no idea on what you are asking me to wager. Nor what the voltage and capacity of a battery has to do with my driving habits.

waste of time looking at that number. Maybe it can give you an idea of battery degradation over time, ie you started out with 200 miles at 80% charge, but now it says you get 192 miles at same charge level. This will happen to all. I drive exactly 16 miles to and fro work each day and take the same route each time. I never end up with the same amount of miles remaining when i get home. some days it is close to 18 miles gone, some days it is 30 miles gone and a couple times even more. weather and my lead foot are the two factors i have noticed to influence it the most. on a really hot day, the car will work harder to keep the batteries cooled and my 16 mile trip will result in 30+ miles of battery used.

As long as I can make it from supercharger to supercharger when travelling, I have all the battery I need.

I had this epiphany also, and realized that Superchargers were getting closer together faster than my battery degraded, so no more concern.

My 75D with ~18k miles on it has an 80% of around 199mi... My pack's capacity (currently) is about 70.9kWh, about a 3% degradation from its capacity when new. Here's some more fun data from Teslafi: