Experimental investigations of thermal and fluid flow characteristics of a spiral grooved tube (SGT) fitted with twin overlapped twisted tapes in counter large/small combinations (TOTT-CL/S) have been conducted. Turbulent flow with Reynolds number (Re) ranging from 8000 to 22,000 by using air as working fluid was considered, and overlapped twisted ratio being 1.06, 1.56, 2.44 and 3.22 was analyzed. The experimental heat transfer and flow characteristics of plain tube (PT) and SGT alone were obtained for comparisons. The result shows that the heat transfer rate of the SGT fitted with TOTT-CL/S is higher than that of the PT and the SGT but the former presents larger pressure drop than the latter two tubes. Both Nusselt number (Nu) and friction factor (f) increases with decrease in the overlapped twisted ratio. The SGT yields higher Nu than the PT up to around 86% while the Nu of the SGT fitted with TOTT-CL/S with overlapped twisted ratio being 1.06, 1.56, 2.44 and 3.22 is respectively 131-177%, 125-168%, 120-153%, and 113-140% higher than that of the PT. It is found that the f for the SGT fitted with TOTT-CL/S is about 13.01-19.02 times of the PT depending on overlapped twisted ratio and Re. In addition, the observed results show that performance evaluation criterion (PEC) of the SGT equipped with TOTT-CL/S is lower than the SGT while it is higher than unity under low Re when compared with the PT. The PEC is approximately 1.05-1.14 for SGT and it is about 0.87-1.01, 0.88-1.04, 0.88-1.07 and 0.87-1.06, respectively, for SGT with TOTT-CL/S with overlapped twisted ratio being 3.22, 2.44, 1.56 and 1.06. Finally, the empirical correlations are developed for the SGT and the SGT with TOTT-CL/S with deviations from experimental values within +/- 5% for both Nu and f. (C) 2016 Elsevier Ltd. All rights reserved.