Arns85, Steel TUBING is measured with O.D. sizes and steel PIPE is measured with I.D. sizes. That said, the Fuel line on the 6 cylinder Fieero is a 3/8" (nominal) tubing with an outside diameter which is .375". The inside diameter of the rubber connecting HOSE will fit snuggly over the outside of that, So the hose I.D. should accomodate the O.D. of the steel tubing. Depending on which gauge steel tubing is used (22 ga. has a .319 I.D.....16 ga. has a .245 I.D.) the higher number gauges have thinner walls with the same nominal outside diameters. So 3/8" O.D.is your nominal steel tubing size.