I find that typically the low fuel light comes on at 1/8 tank. If I fill up then the TCH takes 14 gallons. Normally I go about 40 miles after the low fuel light before reaching E. I frequently will set the B trip and fill up when convenient after reaching E. If your curious, lookup my TCH tank log as I often note how many miles past E I drove along with the MFD mpg, gallons used, and cruising range after filling.

I find the cruising range does go up as the MPG goes up, but it appears to be weighted over a few tanks. I've seen it over 700 miles!