There will be a slight difference in drag, maybe a watt or 2, depending on how many jockey wheels the SS bike uses and assuming you don't mean fixed, you mean freewheel but only one gear. It also makes a huge difference what your local terrain is and therefore how many g.i. you use on the SS. SS and fixed are usually geared as a compromise between flat and hills. Normally the geared bike will be much faster on the flat. I rode SS with a geared bike group many years ago. I could sit in at 130 cadence, but could only pull at 105, though I could have taken short pulls at the 130 cadence speed with my geared bike at a 90 cadence. Cadence eats HR unless you're LA. OTOH, if you only ride on the flat and thus would use the same g.i. SS or geared, then very small difference, like <1%.
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