Cable driven tachometers

The other day a friend wanted to buy a cable driven tacho for a racing Norton SS650. He ended up buying one of the $87 replica Smiths Chronometric which are $87 on Ebay - he paid $140 for one from a dealer. In the process I found another Suzuki RG250 tacho for $75 similar to the two I already use on my Seeley Norton and Egli Kawasaki. I believe they are both 4 to I ratio units. Another friend is considering buying this RG250 unit, however he raised the question of the ratio. I seem to remember that the 315 Suzuki two stroke had an odd ratio unit on it. I also believe I've seen units on Brit bikes driven straight off the end of the cam with no right angle drive. So in that case the head would have 2 to 1 ratios instead of 4 to 1.
My question is - are there many cable tachos around which don't have 4 to 1 heads ?
I've been using this unit and have assumed it is 4 to 1 ratio. When it shows 7000RPM, the motor sounds as though it is doing those revs. But I could be wrong - how embarassing ?
The direction of rotation of the RG250 unit is no problem - you simply undo two screws and reverse the gearbox on the back of the unit.

acotrel said:

I've been using this unit and have assumed it is 4 to 1 ratio. When it shows 7000RPM, the motor sounds as though it is doing those revs. But I could be wrong - how embarassing ?

Click to expand...

If you give it the berries and it (tries to) show 14,000 rpms, you know you have a 2:1 unit.

If you are only going half as fast as you think, postie bikes would be overtaking you on the straights. !!

Unless you have an electronic rev limiter, it will give you the best part of your 7000 rpms available,
regardless of what the tacho shows.

It is not the 2 to 1 or 4 to 1 tachos which worry me. It is the very odd ratio ones which have been used on some Japanese bikes. It is possible to unknowingly buy one and find it is reading high or low by several hundred revs if you ever get it calibrated. I don't think I have that situation with the RG250 tacho, however I will probably never know unless I take it to a speedo specialist.

Electric drills often run to a very precise published rpms.
Try it against that and see what it indicates.

Thanks for that, I would never have thought of it. I think it's possible to get a fairly cheap hand held engineers tacho which can be held against the end of a shaft. That way I could check the drill.