Back when I wanted to do a trike conversion to my Tempo in the worst way I came across the math and calced it out. Projected r.r. reduction was so small as to be almost pointless.
My project would have lost maybe up to 80 lbs from the deleted wheel/suspension- which doesn't do much- and the r.r. like Bob says has more to do with load. I'd expect some gain from elimination of scrub on one end but absent any testing of that theory, my sources say that factor is very negligible too.
I wasn't going to mess with trying to taper the body. For one thing it would be difficult to achieve a nice curve inwards in plan view. For another I have my doubts about the effectiveness of extreme boattailing in plan view in the real world due to... get ready... the dreaded
CROSSWINDS.
