Hello, IsabelM. Let's assume you have two attributes on your source feature class that represents rural roads: Meters and RoadClass.
1. Create a network cost attribute.
2. Set the evaluators for the to-from and from-to directions of the roads source to be field evaluators and set their pre-logic section to the following:
result = 0
Select Case [RoadClass]
Case 1, 2, 3
result = [Meters] * 60/5500
Case 4, 5, 6
result = [Meters] * 60/3500
End Select
3. Set the "Value =" text box to result.
4. Build the network.
5. Optionally, verify the values for roads are what you expect by using the Network Identify tool on the Network Analyst toolbar.