The Tesla mileage display is a joke. I started with 295 miles, drove 190 miles and stopped for lunch with 50 miles left on the display. An hour later, I got in my car and the display showed 22 miles and I was only able to drive 12 miles before the battery ran dead. Tesla was going to charge me $130 for a tow. Then they canceled my service appt to fix the problem, claiming the mileage display worked perfectly. What a joke.
The display does work correctly. It measures the amount of energy in the pack, uses a
reference value for how much energy is required to go a mile, and then spits out how many miles are left. The thing is, you used energy while the car was parked; likely sentry mode or possibly Smart Summon if you have the FSD package. Possibly even 3rd party data loggers like Teslafi if you didn't set it up correctly to let the car enter a deep sleep.
This is unlike other EV's which will constantly change the number of miles left by basing it off your recent driving. If you drive down a mountain road at 35 mph you might have 9999 miles left or something, if you drive up the mountain road at 75mph you might have 75 miles left with a full battery. Tesla has decided NOT to use that method. The method Tesla uses is fairly accurate (maybe 80 to 90%) in low speed driving (35 to maybe 50mph), it's less accurate at higher speeds, in very cold weather, and driving through elevation changes.
There's no perfect way to do it other then to learn the car and adjust a little. Most owners will "spot check" the computer by looking at their usage on a trip, it's displayed in wh/mi. Then they set the display to show as energy percent rather than miles. They know that a full battery is roughly 75kWh (75,000 wh) Owners now have the tools to spot check for themselves.
If your recent trip was 300 wh/mi and you started at 90% and now have 40% left, you used half the battery (50%) or 37,500 wh. You know you have 37,500 wh left in the battery before 0% and you know your recent conditions meant 300 wh/mi (this is real world and accounts for if you were heating the cabin, the wind, the speed and route, etc) so you can do some quick math to figure out 37,500 wh mile will give you 125 more miles at 300 wh/mi.
Now to fix your battery usage while parked. Make sure Smart Summon is turned off if you have FSD. Make sure you set the climate overhead protection to off or fan only. Finally if you're really in a pinch make sure you turn off Sentry Mode. With those things off, your car should be able to enter a deep sleep mode within 10 miles of parking and closing the doors. In this deep sleep mode you should see maybe 1kWh (1,000 wh) of battery loss over like 6 to 8 hours, maybe even less.
Gas cars don't lose range while parked, but they do lose range when driving at highway speeds, up hills, in the cold, etc. It's just less noticeable because they have huge gas tanks compared to an EV. A Long Range Tesla like ours carries about the same amount of energy as 2 to 2.5 gallons of gas. Fill a gas car with 2.5 gallons of gas and you'll certainly notice a difference in range if you normally get 30 mpg and instead get 26 mph because you're on the highway going uphill (almost 14% less range then you expect).
I know it's frustrating right out the gate, but get to know the car a little bit first. Try to plan your first few months of road trips to always have 20 to 30% battery buffer left, even if that means having to charge a little extra by making another stop or a bit longer. Once you get some miles under your belt and start to get a feel for the conditions (hills, highways, etc) it'll become like second nature and you can push it deeper without stressing out.
teslamotorsclub.com
