Write down information on mileage signs when you come up to them. For instance if you come to a sign that says "Sacramento 57, Fresno 243, Los Angeles 516," write that down accordingly.
Use the information from the sign to figure out the distance between those three place through subtraction. For instance, from the information given in Step 1, to figure out the distance between Los Angeles and Sacramento, you subtract the distance from Sacramento from the distance from Los Angeles, which would give you a distance of 459 miles.
Repeat Steps 1 and 2 for every mileage sign that you encounter. Keep in mind that the town names are going to change on the signs, especially when you drive past the towns which used to be featured on mileage signs.
Repeat steps 1 through 3 until you have enough towns that you want to map along the interstate.
Take the mileages you have obtained from the towns to map out a chart featuring the distances between each of the towns.
