Every EV has one, heck even many ICE cars have one: The “Guess O Meter” (in ICE cars it is called “Distance to Empty” which, unlike EVs, tends to be pretty accurate). I initially heard about the term “Guess O Meter” (GOM) by some Nissan Leaf drivers before I even owned my Focus Electric. The number of miles left in the battery would vary wildly in their cars even, in some cases, within the duration of a single drive!
In short the meter is supposed to give you an accurate representation of how many miles you can go with the charge left in the battery (likewise the ICE version “Distance to Empty” (DTE) is how many miles you can go with the gas left in the tank). On an ICE vehicle the DTE reading is pretty accurate and doesn’t fluctuate much–and rarely, if ever, increases. Not so on an EV. Here is an experiment: Take your EV out on the highway for a few miles and watch the GOM drop precipitously; next, on the same trip, drive slowly around some residential streets for a few minutes and watch the GOM climb and climb.
An extreme case of this: My coworker took his FFE down the Woodward Dream Cruise this past summer where he had to drive it under 10 mph for quite a few miles. His GOM rose and rose and rose topping 200 miles at one point!
Now Green Car Reports has just posted this article about a long range drive with a Tesla Model S and I found this quote rather informative:
I don’t have a lot of faith in the Model S range meter. Its number is a projection based on rule-of-thumb efficiency assumptions, battery temperature, and a safety fudge factor. (New York Times reporter Jonathan Broder famously fell victim to wildly fluctuating range numbers.) I call it the guess-o-meter.
Well look at that! Even the much praised Tesla Model S’s range meter is as wild as my FFE’s!
From my experience the GOM seems to place too much stock in the most recent driving performance and ignores the longer term average. I would suspect that it would be far more accurate if it simply used the rolling average of the power consumption over the past week or two (instead of appearing to only use the past 20 minutes or so!). Using a longer term rolling average would also make it a bit more stable and not change so drastically (I think this is what gets most people: Drive for a few miles and all of the sudden you are up or down 20 miles).