215-70-15 to 215-75-15 what is the difference?

This 215 is the height of the tire in comparison with the sidewall size of the tire (see below). 70 and 75 is the distance between the edge of the tire where it meets the rim and the edge of the tire where it touches the road 15 is the inch diameter of the rim that the tire fits on. It was so much easier when the tires were determined via pure American standards before Carter adopted the metric mix up.