Why are Car Nav % Remaining Predictions so Wrong?

[Greg29]

Just returned from 2,000 mile trip in my S75 and found the ABRP charging recommendations to be spot on with the right inputs. I navigated to the next SC with each charge and thank goodness I didn't rely on car nav predictions for remaining battery %'s as they were consistently way off on the low side. I was shooting for 14% remaining at each stop and car nav would predict 5% or so each time and slowly climb during the trip to match the 14% ABRP predicted.
My driving was very consistent on highways the entire time and weather did not change. Why is car nav so inaccurately conservative?

 

[F@kegman]

of course the onboard system is giving you a conservative estimate just like an ICE car fuel gauge. You can always go past the needle hitting empty or the miles to empty readout saying zero. Call it a little built in reserve as long as its constant you can learn how it really performs. But there are the exceptions, my Son's Honda Accord runs out of gas when it say it's out of gas. He learned the hardware.

 

[Greg29]

of course the onboard system is giving you a conservative estimate just like an ICE car fuel gauge. You can always go past the needle hitting empty or the miles to empty readout saying zero. Call it a little built in reserve as long as its constant you can learn how it really performs. But there are the exceptions, my Son's Honda Accord runs out of gas when it say it's out of gas. He learned the hardware.

It would be helpful if the algorithm were more available. Knowing that it is predicting based on rated range (or 150% of rated range) would be nice to know. Unfortunately it was so far off reality that it is hard to know what it is using for calcs.

 

[Darmie]

I find it most accurate when you start at 10 percent or higher. When we had our first Tesla, S75 I would do my own calculation. 100 mile stretch on the highway multiply by 1.2 and add 30 buffer. Before departing, ensure I was at 150 range miles, then launch. Wouldn't much consider what the computer would say. Now with Waypoints coming, (give or take 5 years Elon time) maybe they will look at updating how the car handles range miles on the highway.