Fuel selection patterns during exercise are thought to be conserved among sea-level native mammals when intensity is expressed relative to maximum aerobic capacity (VO2max). However, this claim is based on data from only a few species larger than rats, and has never been tested statistically. Thus, we investigated fuel use in a small mammal (Mus musculus, CD-1 strain), and combined these with published data on rats, dogs, goats, and humans to evaluate the robustness of the mammalian fuel selection model. We found mice rely less on carbohydrates to power moderate intensity exercise at the same % VO2max than larger mammals. We suggest this difference is due to a decline in aerobic scope (O2 available for exercise above resting metabolism) as body size decreases. We propose a redefined fuel use model that reflects changes in fractional aerobic scope with body size. We find exercise defined as % aerobic scope is a better predictor of fuel use across a wide range of quadruped species from mice to dogs and to running humans.