Whrring #3 - GoM
My case for why GoM should be fixed
GoM is referred to as Guess-o-meter. To put it simply, it is the estimated range of the car left in the tank.
Car OEMs usually show it in one of the two ways.
Direct calculation
Previous driving based calculation
I’ll talk about them in detail. But before we get there, here’s a word from my company.
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What impacts range?
Assuming the driver, the car and driving style is constant, EV driving range is impacted by :
Speed of the car
Weather
Elevation gain / loss
Wind direction
HVAC & Traffic
If you drive your car at a high speed, your range will go down. In extreme cold and hot climates, the car has to work hard to keep the battery pack at an optimal temperature, thus impacting range. If you have an elevation incline, then your range will go down. Likewise, an elevation decline will increase your range. If you have a headwind or a sidewind, then the range will go down, because the car has to cut through the wind. HVAC, when kept on, will reduce the range of the car. (Hi Bengaluru readers!)
My experience is limited to real time driving experience of both the 2022 MG ZS EV and the 2023 Tiago EV. MG ZS sticks to the direct linear calculation, whereas the Tiago shows the GoM based on recent / previous driving.
Direct calculation
MG ZS EV has a range of 400 km on a full 100% charge, with AC off on Normal mode. The range goes up or down depending on HVAC and mode selection. For example, with AC off in Eco, the range shown is 420 km. Similarly with AC on in Normal mode, it shows 340 km as the range. You get the idea.
These numbers are shown on the instrument cluster and always stay the same. Let’s pick HVAC off and Normal mode for this article. 400 km range through a 50kWh battery pack. This means that the efficiency of the car is 8 km / kWh.
Let’s say I drove from Point A to Point B with an efficiency of 10 km / kWh and consumed 40% of the energy. The GoM in my car will show 240 km (400 km * 60%) and I would have driven 200 km. Thus, if I drove at 8 km / kWh later on, the total driving distance would be 200 + 240 = 440 km.
In this approach, the car’s baseline range is fixed. Depending on the way you drive, the range will increase or decrease. This is the direct calculation.
Previous driving based calculation
In this mode, the range is calculated based on how you’ve been driving in the past. Tata vehicles, Nexon EV, Tigor EV and Tiago EV tend to do this. They tend to calculate it as driving over the past 50 km or 100 km or 200 km or past one hour etc. This is a good way to know how much range you will get depending on the way you’ve been driving, but that is not always helpful.
Based on your driving, the range number will keep changing. If you’ve driven over a downhill, the range shown will be much higher than what the car can actually achieve, unless you keep going downhill. This is my biggest gripe with this system.
Let’s assume my MG ZS EV uses this method to calculate the range. I’ll take the same example as before. Traveling from Point A to Point B with 10 km / kWh efficiency and consuming 40% of the energy. The GoM in this case will be 300 km (10 km / kWh * 50 kWh * 60%).
This method assumes that I will continue to get 10 km / kWh efficiency over the course of the entire journey which is highly unrealistic. The previous approach considers a more realistic scenario that the car will be driven at 8 km / kWh consistently.
The base line keeps shifting, which makes the learning curve difficult. This approach only works when the driving condition across the entire route is the same. It highly unlikely in majority of the scenarios. It makes it less intuitive for new EV users to adjust to a dynamic range number.
What do you think is a better way to show GoM and what approach do you prefer?
My bias
I have done many inter city road trips in my MG ZS, whereas I’ve driven the Tiago EV mainly in Mumbai. I drove a rental Tigor EV from Mumbai to Pune and back.
Other factors that impact GoM range -
a) Driving styles and who is driving. My wife is a steady driver and always gets better results on fuel consumption when we are alternating our driving on long distance runs. Assuming same thing with the EV also.
b) Tyre pressure when cold. The OE tyres we received were 1-ply rating sidewalls and appear to be softer whilst the replacement tyres for the front that I fitted are 2-ply rating sidewalls and appear to be firmer. I also check tyre air pressure regularly and as winter approaches, this becomes even more important on cold mornings. 35psi after driving is easily 31psi morning cold start. Recommended tyre pressures are always for prior to cold start.
c) I have no idea why wipers consume so much SOC in our Nexon EV. First noticed this when I was at a service station. One clown was running the wipers repeatedly, I asked him why, and he told me it was to empty out the liquid remaining in the reservoir. These are the clowns we have to live with. Or not. But the SOC dropped.
d) Routes with fewer sharp turns also consume SOC more than routes which go straight. Maps here in BLR tend to divert traffic through a vast assortment of Cross Roads and Main Roads instead of pushing us towards the straighter but busier roads.
The GoM in our Tata cars is like an IQ test. Something like the many different apps for public charging, each trip is like a Sudoku game, fit the numbers. It is also to pacify friends who keep looking at the GOM nervously.