Let me explain:

When my model Y was new, it showed 316 miles at full charge.

It now has 6,500 miles on it. Today I upgraded from version 40.3 to version 40.8. Knowing that this involved an efficiency improvement, I looked at some numbers before and after.

Prior to ver. 40.8: car showed 79% charge or 229 miles.

Extrapolating this out: 229 / 79 x 100 = 290 miles at full charge.

After ver. 40.8: car showed 79% charge or 235 miles

Extrapolating this out: 235 / 79 x 100 = 297 miles at full charge.

The car has a 75Kw battery, but people on this board have said only 73Kw is usable.

When the car is new, it does not have a history to know how may Wh/mile you use since this is different for all drivers and configurations. They derive 316 miles rated range by assuming a certain speed (probably 50 mph) on level ground with no A/C, no lights, and no wind. If you do the math, 73,000 W / 316 miles = 231 Wh/mile. I don't know anyone who averages that low consumption.

As you drive your car, it keeps track of your consumption. At 6,500 miles, my lifetime average is 252 Wh/mile.

Let's do some more math:

73,000 W battery / 252 Wh/mile (my historical average) = 290 miles at full charge (same as my calculation prior to installing version 40.8.

75,000 W battery / 252 Wh/mile (my historical average) = 297 miles at full charge (same as my calculation after installing version 40.8.

It seems likely to me that version 40.8 software unlocks the extra 2Kw of your battery to allow full use of the 75Kw battery pack.

As an experiment, I charged to 100% after installing version 40.8 and the car showed a range of 297 miles. This seems to confirm my logic above.

Conclusion #1: It appears the full 75Kw of the battery is now usable.

Conclusion #2: The decreased range we experience at 100% charge is due to our average Wh/mile (configuration, weather, terrain, driving habits), not battery degradation or charging habits.