I have the same problem. 7/2018 Model 3 LR RWD 20000 miles. At early 2018, the mileage range was pumped up from 310 to 320 by a software update. Then it started to drop about 0.5-1% every month. Right now it's 288 miles for 100%, or 259 miles for 90%. I have contacted the service several times...