Purpose To investigate the gross efficiency (GE) and delta efficiency (DE) during cycling and running in elite triathletes. Methods Five male and five female elite triathletes completed two incremental treadmill tests with an inclination of 2.5° to determine their GE and DE during cycling and running. The speed increments between the 5-min stages were 2.4 and 0.6 km h −1 during the cycling and running tests, respectively. For each test, GE was calculated as the ratio between the mechanical work rate (MWR) and the metabolic rate (MR) at an intensity corresponding to a net increase in blood-lactate concentration of 1 mmol l −1. DE was calculated by dividing the delta increase in MWR by the delta increase in MR for each test. Pearson correlations and paired-sample t tests were used to investigate the relationships and differences, respectively. Results There was a correlation between GE cycle and GE run (r = 0.66; P = 0.038; R 2 = 0.44), but the correlation between DE cycle and DE run was not statistically significant (r = − 0.045; P = 0.90; R 2 = 0.0020). There were differences between GE cycle and GE run (t = 80.8; P < 0.001) as well as between DE cycle and DE run (t = 27.8; P < 0.001). Conclusions Elite triathletes with high GE during running also have high GE during cycling, when exercising at a treadmill inclination of 2.5°. For a moderate uphill incline, elite triathletes are more energy efficient during cycling than during running, independent of work rate. Keywords Triathlon • Cycling economy • Running economy • Incline • Metabolic rate • Mechanical work rate Abbreviations α Treadmill inclination DE Delta efficiency DE cycle Delta efficiency during uphill cycling DE run Delta efficiency during uphill running ΔMR Change in metabolic rate ΔMWR Change in mechanical work rate g Gravitational acceleration GE Gross efficiency GE cycle Gross efficiency during uphill cycling at the lactate threshold GE run Gross efficiency during uphill running at the lactate threshold LT Lactate threshold, i.e. the mechanical work rate at which the blood-lactate concentration increased 1 mmol l −1 above the lowest measured value m tot Total mass of participant and equipment MR Metabolic rate MWR Mechanical work rate RER mean Mean respiratory exchange ratio μ Rolling-resistance coefficient of the bicyclė VO 2max Maximal oxygen uptakė VO 2mean Mean oxygen uptake * Tomas Carlsson