I got this info from the FAQ over at SOHC4.net:
CB750 Speedo 2240:60 Tach 4:1
CB650 Speedo 2240:60 Tach 4:1
CB550 Speedo 2240:60 Tach 20:3
CB500 Speedo 2240:60 Tach 20:3
CB400 Speedo 2240:60 Tach 20:3
CB350 Speedo 2240:60 Tach 20:3
I'm guessing the 1:7 (or 7:1) is about the same as the 20:3? I've read somewhere that it's actually 6.67:1 and most people round it off to 7:1.
That kind of sucks, because I plan on swapping a complete 650 engine into my '71 CB500/4 and I was hoping the twins had the correct ratio for the 650 cam/tach drive. I'd really like to keep my small, early guages.
Guess I'll have to look for a pair of matching 750 gauges or something aftermarket.