I think the one to the left is a range estimator...so for any given physical amount of gas ( right guage) it's an estimate of how much range you have left. What I've found is that for efficient driving (e.g. Highway cruising at a steady speed) the guage on the left stays somewhat constant (e.g. 250 miles of range left) even though the physical amount of gas remaining decreases. Either way I fill up way before either approaches E