Having worked at a tire store for several years, mathmatically speaking, the sidewall hieghts for the tires you listed are almost exactly the same. The second number is the aspect ratio of the width to the sidewall hieght. Normally speaking, what that means when the first number (the width) goes up, to keep the same overall hieght, you need to go down in the second number.
Here is an easy example. We did this conversion all the time for people with small cars. A customer comes in & wants a wider tire. Say they have 155/80R13s on it (far from pro touring, but bear with me.). The first numbers go by 10s and the second number by 5s. The next size wider (which they don't make) would be a 165/75R13. The size after that would be a 175/70R13 (much more common). This gives you a wider tire with the same tire hieght.
If the manufacturer was the same, you would be choosing 2 different widths. The only problem is that different manufacturers measure the width at different places. Theoretically speaking, it should be the widest part of the tire. Some manufacturers make the widest part at the shoulder of the tread and others make the sidewall buldge out to get the width.
Sorry for going on, but I hope that helps.
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