I am on my second i3+rex. I love the things to pieces. In the US the rex comes on when the main battery is down to 7% charge. (Europe or re-coding the car allo

I am on my second i3+rex. I love the things to pieces. In the US the rex comes on when the main battery is down to 7% charge. (Europe or re-coding the car allows you to set when it comes on -- up to 75% I believe? I haven't coded either of mine) You can actually outrun the rex engine with the right combination of high speed, climbing grade, or cold weather. If it gets to 0% the car will literally shut down. A serious warning appears at 2% about this. I've never had mine below 2.5% (there is a setting on the driver display that allows you to display this number). It takes uphill driving over 75mph in cold to really draw down those last few % and outrun the range extender. The rex tank is 9L, so it adds about 70-100mi to the total range of the car depending on how you drive it. We add a few gallons a year to ours, as the EV itself is sufficient -- it's great for those surprises life throws you where you need another few miles. $0.02. :)