Interference-fit finned tubes depend for their heat-transfer capability upon a contact pressure between fin and tube. The bond resistance under such conditions is relatively small compared to the other resistances in the heat-flow path. At elevated temperatures, however, differential thermal expansion between fins and tubes completely relaxes the contact pressure and introduces an additional gap resistance; this may become a significant part of the total resistance to heat transfer. A theoretical method for predicting the gap resistance is derived in terms of the fin and tube dimensions, their physical properties, the fluid temperatures and heat-transfer coefficients, and the initial contact pressure. Test data on five finned-tube units representing embedded, tension-wound, and muff-type fins are given in graphical and tabular form. Very good agreement is found between theory and test in two of the interference-fit units. The other two show only fair agreement.