There used to be a link for gear rat on here but I've not seen it for awhile you might ask Anton.For single speed setup's even with the engines own gear reduction the math is quite simple,when you do it enough it gets easier! all I do is work from the rpm so if we have 6000 rpm with 4.1 internal reduction and a driven reduction of 4.4 (stock) on a 26" wheel then it's 6k divided by 4.1 divided by 4.4 times circumference divided by 12 then the mile in feet to get the distance covered and that converts to mph being based off a mile per minute ,now that I've tried to explain it maybe it's not so simple but don't we all use a calculator whether on our phone or online LOL to aid in this type of endeavor.