A study of heat transfer in simultaneously developing flow through rectangular tubes is presented in this article. Heat transfer coefficients were measured for three different tube sizes and shapes (D-h = 2.21 mm, alpha = 0.050; D-h = 3.02 mm, alpha = 0.108; and D-h = 1.74 mm, alpha = 0.029), which correspond to typical dimensions used in automotive heat exchangers. For each of these tubes, several different tube lengths were tested to measure the effect of developing flow on the Nusselt number. The results demonstrate that developing flow enhances Nusselt numbers, especially for short tubes. For the geometry range studied, the effect of aspect ratio was not very significant. Heat transfer correlations that accounted for the effects of Reynolds number (118 < Re < 10,671) Prandtl number (6.48 < Pr < 16.20), and bulk-to-wall property variations (0.243 < (b)/mu (w) < 0.630), and geometric features such as tube length, hydraulic diameter, and aspect ratio, were developed from the data.