It's the gear ratio you see on the Y-axis: 1. For example the XJ has a 3:55 gear ratio and It is currently equipped with 33's. This chart essentially states it is just barely outside the highest efficiency zones. The top yellow shaded regionl, to my knowledge, is best for wheeling/crawling, the bottom yellow region is best for highway cruising, the green is the best compromise between the two. Hence your XJ crawls extremely well at Uwharrie (even without the lockers), But it has a hard time holding 65 on a slight up hill, lol. It would love a 3.73 or 4.10 gear swap.