Actually, the modern convention of a single space is not due to saving space.† A proportional computer font is kerned in such a way that the space after a period is wider then between words. For example, a standard space is 1em‡, but the space after the period is kerned to give a width of 1.5em‡‡. A typewriter on the other hand only has a single 1em space, so either use 1 space or 2. Also, I read once that they used 2 spaces on a typewriter to reinforce the ending of the sentence in case the period did not strike properly.
† Though newspapers are famous for tricks like this such as the missing oxford comma according the AP Stylebook, and I wouldn't put it past the newspapers to have done single spaces years ago for this reason. There are several differences between AP Stylebook and Chicago Manual of Style to account for saving space.
‡ An em is the width of the lowercase m in the font.
‡‡ 1.5em isn't a hard and fast rule. The rule is wider than an em and less than 2 em's. It could be 1 1/3 is common as well. In reality, a designer spends agonizing amounts of adjusting the kerning on a font til it looks how they want.
An em is the width of an upper-case M. If you look at printed text you'll see that spaces are much narrower, in fact narrower than ens - compare a space with an en dash.
> Actually, the modern convention of a single space is not due to saving space.† A proportional computer font is kerned in such a way that the space after a period is wider then between words.
That's not "the modern convention of a single space"; that's "a return to the original convention of wider than a normal character width spacing". Note your reference to the width of the space being 1.5em, for instance.
† Though newspapers are famous for tricks like this such as the missing oxford comma according the AP Stylebook, and I wouldn't put it past the newspapers to have done single spaces years ago for this reason. There are several differences between AP Stylebook and Chicago Manual of Style to account for saving space.
‡ An em is the width of the lowercase m in the font.
‡‡ 1.5em isn't a hard and fast rule. The rule is wider than an em and less than 2 em's. It could be 1 1/3 is common as well. In reality, a designer spends agonizing amounts of adjusting the kerning on a font til it looks how they want.