Originally Posted by
RiddleOfSteel
I have Swift 27x1 3/8" tan walls--lovely looking tires. Ride is nice--haven't ridden them enough to see how they handle at the limit, but no squirreliness thus far. They do run small, certainly on 27x1 rims (Champions). Price is great, too. I know Continental used to make a Grand Sport in 27" of some sort. Their very recent website redesign is complete and utter garbage for navigating and seeking information, so I don't know what's available there.
Those are the old Wolber/Super Champion Modelle 58? I have a set of those, 23 mm outside width and 16.7 mm inside width. For the similar Ukai rims there are similar dimensions. If you have a rim which actually has a 25.4 mm inner width (I wonder this because your description of the rim is actually very vague.
If you really mean you have a common-variety 27 x 1 ¼ rim without hooked beads, ok. Then the inner width will be around 17 mm. This will be the case because rims marked as such (27 x 1 ¼") are spares replacements for old Raleigh, Peug, Motobecane, and many other old, common models.
Why is this significant? If you are trying to say something significant about compatibility of the 27 x 1 ⅜" tire with a rim which measures 25.4 mm inner width, the difference is 25.4-16.7 = 8.7 mm. That can be looked (just an estimation here) as an 8.7 mm increase in installed circumference and hence a corresponding increase in air volume. That would suggest there is a significant additional improvement in ride quality.