The purpose of the present study was to investigate whether mechanical efficiency (ME) relates to wheelchair propulsion capacity and wheelchair performance tasks during and after rehabilitation of people with a spinal cord injury (SCI). Eighty participants with a SCI were tested during rehabilitation (3 x) and 1 year after discharge. Two 3-minute submaximal exercise blocks, a maximal wheelchair exercise test, and four wheelchair performance tasks were performed. ME, peak power output (PO (peak)), the sum of the performance times of a 15-m sprint and figure-of-eight, and the heart rate reserve (%HRR) during 10 s of wheelchair propulsion on a 3 % and 6 % slope were calculated. The relationship between ME and PO (peak), %HRR and performance time was tested with a multilevel regression analysis. ME showed a significant relationship with PO (peak) (p </= 0.002). A 1 % higher ME related to a 1.6 - 2.2 W higher PO (peak). ME of exercise block 2 was related to the sum of the performance times of a 15-m sprint and figure-of-eight; the tests were performed faster by participants with a higher ME. No relationship was found between ME and %HRR during wheelchair propulsion on a slope. ME showed a significant effect on wheelchair propulsion capacity and wheelchair performance time during and 1 yr after SCI rehabilitation although the isolated effect of ME is small.