Tire size
Tire sizing is dependant on what rim width you will be using. Most tire manufacturers will give a recommended rim size for the tire size. They will say 6-7" or 7-8" or, in some extreme cases 6-8". If you try to put a tire that is too wide for the rim, the center of the tread will want to bulge out. To get the tire to sit or wear properly, you'll have to run a lower inflation pressure. Not a good thing. Tire wear will get all funny and handling gets weird.
The good thing about the current tire sizing is that you can easily match tire width. Actually, that's what that first number is, the 'section' width, or bulge width. This is a measure of the widest part of the tire on a standard recomended rim, measured in milimeters. Tread width is just that - the width of the normal contact patch.
The next number in the size is the aspect ratio. A 75 series tire is 75% as high as it is wide, a 60 series is only 60% as high as it is wide. Then we get to the really extreme ratios like 35s. These will look like rubber 'rim protectors' they're so short. Generally, the wider the aspect ratio, the better or more precise the handling of the car will be due to less sidewall flex. Ahhh, yes. The less flex in the sidewall also makes for a harsher, stiffer, less comfortable ride. Everything has tradeoffs. Sidewall stiffness can also be dependant on construction, but that is throwing other variables in here which may make things a bit confusing.
You will want to keep the overall tire diameter of your new tires somewhere near what was 'factory' on your car so the speedometer calibration stays close. Change tire size too much and you'll have to change the speedo gears, either in the tranny or at the end of the speedo cable. Yes, there are two gears. One is called the 'drive' gear that is on the transmission output shaft. The other is the 'driven' gear that is on the end of the speedo cable that goes into the transmission. By changing the number of teeth on either gear you will be changing the speedometer calibration. Since the average driven gear has about 18 teeth, changing this gear by one tooth will have about a 6% change in speedometer reading. Hmmm, but what if you're off by 3%? well, there are several ways to 'fix' the problem, but I won't go into that here.
OK, back to the tire size issue... If the recommended tire size for your car is an E-78-14, like most of our Cougars were, that translates to about a 205/75/14. Pretty wimpy looking, huh? To maintain the same diameter, you can go with a 215/70/14 or maybe even a 225/60/14. The F-75-14 works out to be 215/75/14 or 225/70/14. Most of our Classic Cougars came with 14x6" wheels. Anything wider than the 225 will probably need a 7" wheel width. Again, check with the tire manufacturer's rim width recommendations to be sure. The problem is, once you start deviating from the original, some things just don't look right. That's why they have the 'plus-1 and plus-2' idea. If you have a 225/70/14, you can step up to a 245/60/15 and maintain the same overall diameter. Anyway, your tire retailer can help you make the right choice here.
As for tire/body interference, that depends on several factors as well. Width, diameter and wheel offset. Those topics we'll discuss at a later time. Its late and I have to get to bed!