In this work, new improved models for calculating heat transfer in inclined and vertical tubes are presented, developing a new analysis procedure to consider the influence of the viscous-gravitational effect on non-isothermal laminar flow, in order to obtain a better fit index from the available experimental data and to increase the validity range. Three models were developed, two for vertical tubes with ascending and descending flow and one for inclined tubes. Available data from 14 different fluids, including water and organic liquids, were used to validate the models. The first model is valid when forced and natural convection are in the same direction, obtaining a correlation fit with an average error of 11, 6 % for 77,8 % of the data. The second model is valid when forced and natural convection are in opposite directions and provides a good correlation with an average error of 12,8 % for 74,4 % of the available experimental data. The third model is valid for inclined tubes with an inclination angle with respect to the horizontal line between 1⁰ ≤ θ ≤ 88⁰ and provides a good correlation fit with an average error of 15,9 % for 75,0 % of the available experimental data.