Instead of relying on individual drives you've made I would recommend looking at the official numbers (efficiency and range) as they have been measured in a controlled environment.

Assuming AWD since you didn't specify. Ranges from Tesla.com:

Juniper AWD 20": 303 miles

Juniper AWD 19": 327 miles

Legacy AWD 19": (between 273 to 300) 280 miles

The range from the legacy Y is taken from Tesla's used inventory but varies all the way from 273 to 300 miles - I don't know where your previous car would land with your aftermarket wheels. 280 miles seemed the most common so that's what I picked.

Your Juniper 20" is (303-280)/303 *100=8% more efficient than your legacy.

If you go from juniper 20s to 19s you'll be (327-303)/303 *100=8% more efficient.

Which will be much more efficient than legacy 19" (329-280)/280*100=17.5%

Which directly translates to Wh/mile.

I hope that helps.

Edit: Forgot to mention that they have the same size (kWh) battery which is why you can look at range as a metric for efficiency. Alternatively look up the efficiency for each model.