I think it's 4.5:1 but I recall Mario saying they run at about 10,000 RPM at 12v so they're a bit faster than Graupner S900s or something along those lines, I'm running them at 4S. I think they're Johnson 900 size motors as opposed to the Graupner ones (I just call them S900s for the ease of it) but I could be mistaken there... I can't really remember!