140 mph speedometer from xs750

[stevo]

geezer said all jap bike speedo ratios are the same ..what is that ratio i'm wanting to put some aftermarket gauges on my 79 std and don't know ratio of speedo..also. correct me if i'm wrong but doesn't ignition pulse tach twice per revolution thanks stevo

[randy]

On the XS11's, the tach pulse is taken from the alternator, not the ignition, so it's a true engine rpm, not iggy/dividedby/something. Don't know about the other models configurations.

[TopCatGr58]

Here's a quote from an earlier thread I posted about aftermarket tachs in the Tech Discussion Forum:

As for the TACH, he CAN use an aftermarket type, as long as it's electrical, and runs off the coils. They are a bit pricey($100.00+) in the models that go up to 10Krpm, needed for the 8.5krpm+ range, vs. the cheaper auto 6k ranges. Also, they usually have a selector so you can pick from 2cylinder,3, 4, 5 ,6, 8, but because it'll be reading 1 coil for 2 cylinders, vs. 1 coil for 4 or more on many cars.

Okay, I had figured this out before, but forgot which setting he would need to pick!? Let's, see, our coils fire 1 each revolution, and a 2 cylinder 4 stroke also fires once per revolution, yeah, that's it!

The above is correct, because for triggering an aftermarket tach, you only need to take a reading off of 1 coil, which includes the waste spark process!!
T.C.

[TopCatGr58]

Speedo ratios

Hey there Stevo,

Most speedos for our Japanese imports are all very similar, so a gauge for almost any model of street bike in the 70's-80's with 19" front wheel should be okay, preferably Yamaha. According to models in the PartsnMore catalog, the speedos are rated at 60mph = 2240 cable rpms!

My calculations follow:

tach cable rpms=2240=60 mph

110/90-19 Dunlop tire = diam 26.5 inches
83.25 inches circumference
6.9377 feet per rpm

60mph=1mpminute=5280 feet/minute
divided by 6.9377=
760 wheel/tire rpms

2240/760= ~3, so....
~1:3 ratio of wheel to cable rpms

T.C.