While I don't know for sure what you are asking about the different tire sizes affecting anything while driving I can tell you how tire sizing works. With a tire size, 235/75R15 the 235 is the width of the tread face in millimeters. The 75 is the is the ratio of the sidewall height to the tread face, in this case it is 75%, the R is radial and the 15 is the rim size. So, for that tire, the sidewall is 176.25 mm tall. If you were to go to a 215, for the same sidewall size you would have to have a 215/82R15. But tires are only manufactured in increments of 5 such as XXX/65, XXX/70, XXX/75, etc. From geometry we know that when you increase the sidewall height for the same rim size you increase the overall diameter , radius and circumference.
So when you change tire sizes you will change the circumference and thus the number to times it rotates per mile and thus the speedometer calibration. For example, if you go from a 245/75R15 to a 215/75R15 you will be off by about 6%. Not major, but some. This is not as important as changing rear gear ratios.
Or as another example, my daily driver Colorado Centennial Edition came with 265/60/18 tires. For next winter I plan to get the 16" steel rims to take off the polished aluminum rims. The factory tire size for the steel rims are 265/70R16. As unbelievable as it seems, the two tire circumferences are less than 1% different even though the rim sizes are 2" apart.
I hope that helps out a little bit.