[andymc]

As with any vehicle, "official" EV range (fuel efficiency) figures come from WLTP standardised testing. I think we all know this isn't realistic for any vehicle, but it's still an improvement on its predecessor, the NEDC. In fact, the US equivalent test, the EPA, seems to be the most accurate of the lot. Some manufacturers (including MG) have taken to providing their own real-world figures in addition to the WLTP. I had come across a table giving realistic ranges for various EVs some time back - I'll try to dig it out for you later.

Any car's fuel efficiency is affected by wet conditions (more drag and rolling resistance) and strong winds, especially headwinds. In my diesel 75s, I've seen as little as 520 miles from a tank in the depths of winter, when I was mainly doing a combination of short (10 miles) and moderate trips (25 miles) which meant that in addition to encountering poorer driving conditions more regularly, the engine wasn't at peak efficiency for a relatively high proportion of my mileage. That's some drop-off from the 630 or more miles I can expect from a tank in summer, or on long trips in good conditions. But my diesel car's range dropping from 630 miles to 520 miles was very unlikely to ever cause me to run dry before getting where I was going. EVs are affected by these issues as well, but it's more noticeable in the shorter-range ones like the original Leaf, the eGolf, the BMW i3 et al which had smaller batteries and took longer to recharge. The greater the range (and the quicker the recharge times), the less of an issue this becomes.

The batteries in some older EVs can be affected by extreme cold because very low temperatures make the electrolyte fluid inside them more viscous. More recent models have active thermal management systems to address this - although running that will also sap a minor additional amount of energy from the battery, it's certainly better than not having it at all. Whether or not your EV has this feature, if you have a driveway or garage you can plug it in overnight to programme a charge timed to finish shortly before you expect to set off. This means that on cold nights/mornings, you can use mains power to preheat the battery and cabin (like a 75 with an FBH), eliminating cold battery issues and unnecessary battery drain.

The relative efficiency of EVs compared to ICE cars is also a factor - I might come back to that, if you're interested, but I don't want to present you with a wall of text in this post.