density of oatmeal (source (although this is with sugar)): 693 kg/m³ = 0.69 kg/L

nutritional value of oatmeal (source): 297 kJ/100g = 2970 kJ/kg

(oatmeal with sugar and stuff: 4600 kJ/kg, same source as the first one)

typical leisurely cycling energy expenditure (source): 80 kJ/km

bicycle oatmeal-efficiency: (2970 kJ/kg * 0.69 kg/L) / (80 kJ / km) ≈ 25.62 km/L ≈ 3.9 L/100km ≈ 60 mpg

bicycle oatmeal-efficiency with sugary oatmeal: (4600 kJ/kg * 0.69 kg/L) / (80 kJ/km) ≈ 39.68 km/L ≈ 2.5 L/100km ≈ 93 mpg

please do correct me if I'm wrong, but it looks like the math pretty much checks out on that one!