Beef cows (n = 100) were ranked for efficiency based on cow rump fat thickness at calving, calving date, and calf weaning weight (% dam BW) over 2 years. The nine most (ME) and least efficient (LE) cows were used to compare feed intake and ruminal fermentation using four 26-day periods with decreasing dietary nutrient density. There were no phenotype × diet interactions for variables of primary interest. Rump fat and calf weaning weight were greater, and the calving date was earlier for ME cows than LE cows (P ≤ 0.032). The ME cows were lighter (P < 0.001) but had similar DMI (P = 0.93) to the LE cows, resulting in greater dry matter intake (DMI) %BW (P < 0.001). Ruminal contraction amplitude height and area (P ≤ 0.015) and ruminal digesta weight were greater for LE than ME cows (P = 0.043). Ruminal ash-free neutral detergent fiber (aNDFom) passage was greater for ME cows than LE cows (P = 0.047), but the rate of aNDFom degradation did not differ (P = 0.69). Total tract digestibility did not differ. Efficient cows had greater rump fat, weaned heavier calves, ate more relative to their BW, had a smaller ruminal digesta mass, and had greater ruminal passage of aNDFom without reducing digestibility.