The original 7.75 - 14 tires on the 67 GTX should have had a 26.3 inch diameter. This is equivalent to a 75 sidewall on a 14 inch wheel. When you go to 15 inch wheels and a 75 sidewall, your tire diameter increases to over 27 inches, your speedometer reads low and your car goes faster at the same RPM. If you want to keep the speedometer accurate, you need to drop to a 70 sidewall if you go to 15 inch wheels. This maintains the 26.3 tire diameter of the original factory tire.
"Filling up the wheel well" can be taken two different ways. The 15 inch wheels do visually help since there is more actual "wheel" in the wheel well and a 14 inch wheel in the days of 15-20 inches being normal do look rather small. A 70 series tire will not fill up the gap between the top of the wheel and body in the front, but will maintain the original diameter and make the rears somewhat easier to get on and off. If you ever used the old bumper jack, you have to jack the car ridiculously high, but the rear axle drops plenty enough for wheel removal. The old bumper jack also has a tendency to bend the bumper. This is not the case if you use a floor jack on the frame in front of the rear wheel and why the wheels are so hard to remove, particularly in a 67 with tight wheel wells and if the rear tires oversized in width and diameter.
I think I will go with 205-70 15's all around next time, with Magnum 500 wheels and redlines including the spare tire. This is closer to original tire width and height, I can rotate the tires like the old days without staggered tires, and the car will look good and just the same with the spare on in the unlikely event of a flat.
What will happen if I actually have to use my bumper jack since it is now all powder coated is a worry for when that time comes. Some will undoubtedly crack and fall off. I am not going to carry a floor jack around in the trunk, even though it is bigger than the typical "tiny house".
