Topic: tach and speedo ratios for cb550

[jon44]

Does anyone know what the ratios for a tach and a speedometer on a 76 cb550 are?

thanks

[Clyde]

The tacho is 20:3 or 6.67. The 350/400/500/550 are all the same. The CB750 is 4:1

In the case of the 550 - 10,000 rpm at the crankshaft means the cable turns at 1500rpm and the tacho reads 10,000.
Most honda tachos are driven from the camshaft which turns at 1/2 crankshaft speed. Thus there is inherently a 2:1 ratio anyway and the difference is through the tacho drive gears

The speedo is 2240rpm (at the cable) =60mph (or 1400rpm = 60kph)
Most speedos of the time were the same (Honda, Suzuki Yamaha,etc) and also a lot of newer speedos.
Providing you use the original speedo drive on the wheel it will not matter whether you use a 16 or 19 inch wheel-it should still read the correct speed.
Where problems occur is when the rim is changed for a different size to the original for that wheel/speedo drive

[jon44]

Thats great news.  Thanks.  I can now purchase these mini guages i've been looking at.  drag specialties makes them.  it seemed by everything i read that 2:1 was a harley ratio but thanks to you now i know that i can get it.  and the 2:1 mechanical tach is cheaper than the other ones for some reason so yeah me.  Thanks again
jon

[Clyde]

Jon,
Sorry if I misled you. The tacho ratio is 6.67:1 for your bike. not 2:1.
The tacho drive gears that I was referring to are in the head and combine with the camshaft rotating at half engine crank speed to give a 6.67:1 ratio
2:1 will not work

[jon44]

do you know of any mini gauge tachs that are 6.67:1?  Looking and not really coming up with anything.  The only thing I can find is on this japanese chopper website that says that most hondas up to 550cc's are 7:1.  Is this close enough or totally not ok?  And of course, thanks

[Clyde]

Most people round it of at 7:1 and yes it is close enough.
6.7 to 7 is only about 5% different.  Most tachos are only as accurate as 10% anyway, from what my speedo man tells me.

[jon44]

Thanks for all your help.

[Sporkfly]

Sorry if this is a stupid question, but what are some sources for a tachometer? The one on my 550 is a 750 tach, thanks to TT for pointing it out, but now my problem is replacing it with a more accurate tachometer.

[jon44]

Well this is the one that i'm getting.  It is here. http://crc2onlinecatalog.com/chopper.htm

[jcorrill]

So the speedo needs to be 2240 = 60mph  &  the speedo needs to be 7:1  or are the both 7:1?