Using a CB750 Speedo on a CB650

[Strife]

So my speedo is busted on my 1979 CB650 that I am working on, but I'd rather not by a $300 NOS part, so I'm looking on eBay. My question is, as long as it's cable operated, wouldn't one from a CB750 work on a CB650? I mean, it seem to me that the gear that determines how to move the needle would be the important part, which is part of the speedo.

Anyways, one I am looking at is from a 1977 CB750A Hondamatic. Would that work with my bike? What determines whether or not one particular speed/tach will work, aside from it being a cable operated one, and that it uses the same cable?

 

[Strife]

A side question, does anyone know where to get aftermarket speedo/tachs that would work with a 650/750? I wanted to get some new ones, but all I could find were ones that weren't cable operated, they all used electronics.

 

[rockcitycafe]

I'm pretty sure all the honda speedos of that "era" had similar specs, the difference will be that the front wheel circumference has an effect on the speedo reading, so if that size is different, the speed will read different as well... you could probably use one and have a friend in a car lead you at 55mph, so you could put a mark on the speedo for reference... most old speedos were off 5-10% anyway

 

[Strife]

Yea that's what I have read as well, that the CB750's and CB650's use the same ratios. The one thing I'm curious about is if I get a 750 speedo that goes up to 160 mph, and the original 650 one went up to 120mph, is there going to be a problem there? Does that indicate a different ratio? Because I have seen in the forums people posting the ratio's of CB750' and CB650's, which say they are all the same, yet some 650's (like my '79) have a speedo that goes to 120 mph, while most 650's only go to 80 mph, and most 750's I see go to 140 or 160 mph on the gauge.