110 x 18 is the closest metric equivalent to the original 4.00 x 18, and it's a very common size. There's lots of them around. My '78 came to me with a 120 out back. Once I used it up I switched to the more correct 110 and haven't used anything else since. I like it better than the 120. The bike rides just fine with it but steers a bit quicker and handles nicer (easier) compared to the 120.