205 is the crossection width of the tire (not the tread) in milimeters. To convert to inches, divide by 25.4:
205 / 25.4 = 8.07 inches.
Now the height of the sidewide is a percentage of the width, ie..
8.07 x .70 = 5.649 for a 70 series tire
8.07 x .60 = 4.842 for a 60 series tire
Being you have two sidewalls in the total height of the tire, you have to double that number and then add the wheel height/diameter for the total tire height:
5.649 x 2 = 11.298 + 14 = 25.298 for the 205/70 - 14
4.842 x 2 = 9.684 + 15 = 24.684 for the 205/60 - 15
The difference in overall tire height/diameter is 0.614"
Now as far as the width of the wheel as you have highlighted, it only very, very slightly affects the crossection width of the tire because it controls how far the beads of the tire are "spread apart". Ideally you would want to match the wheel width to the tread width, that way the tread lays flat on the ground when looking at the tire from the front of the car.
Hope this helps. Tire Rack has some more good info on this subject. Maybe easier to understand too!
15x7 wheel on your '68, I would say probably some where around
Tirerack's recomended rim width for this size is 6.5-8.5" on their chart for Firestones's Firehawk SS20 (these are what I have on mine)
The installed dimensions are as follows:
wheel width = 7"
section width = 9.5"
tread width = 7.7"
over all diameter = 26.1"
Gary, I have a 67 coupe, and on it I have 15 x7" rims all the way around. On the front I have 235 60 15 and on the rear, 255 60 15. The back tires are tight, but they fit and do not rub. I have a pic if you would like to see it.........Kalin