@Bongo Most interstates are 65 or 70. Densely populated areas, as you near cities, they are less, wide open spaces with no one around for miles the limits can be higher. Interstates generally have a speed sign as you enter the interstate, so you should not have to guess. Many Eastern states the people say highway meaning the interstate, the word is used interchangeably, in fact out east they almost never say interstate. Towards the center and southern parts of the country they use interstate, because “rural highways” are much more common. It seems very confusing, but honestly, I don’t think you will have any problems. Interstates are also called freeways depending on the state you are in. We have Turnpikes and Toll roads also. No matter where you are if you say I90 pronouncing the letter, eye ninety, people will understand that to be Interstate 90, or whatever number you are trying to get to.