On my MGA, I was trying to get as close as reasonable to the original tire size. The rules I run under say the car is supposed to be set up as it was raced "in the day", so 1959.
At 165 the width is a little wider - about 5.5 inches vs. 5 inches. The 70 is the aspect ratio height vs. width. So 70 is a taller tire than say a 60. Again, I went with the 70 to get closer to original. I think originally, the tires had an aspect ratio of 80.
The 15 is the wheel size.
On the MGA with these tires, my mph should be just about what the speedometer says. A different aspect ratio or a different wheel size will throw that off.