Howdy all! This may seem like a really stupid question, but what's the difference between a Mr. Smiff's 1.25:1 ratio and a 15:12 ratio? My '72 T120 used to have a 15:12 but it bit the dust and I've replaced it with one with a 1.25:1 ratio which I figured would be the same. But now the bike reads too fast - possibly installed wrong or just wrong box altogether??? Thanks for any insight!
There have been several speedo drive ratios over the years that must be matched to the speedo itslelf. Personally, I am most familiar with the 2:1 and 1.25:1 (which is the same as 15:12) ratios of the speedo drives. Usually the Smith's speedo will have a number on the face at the bottom in small print. For the 2:1 ratio the number is 1600, I belive, and for the 1.25:1 the number is 1000. The way I remember this is if you divide the Smith's tach number by the speedo ratio you get 800 (i.e., 1600/2=800, 1000/1.25=800). Of course you have to match the proper speedo drive to the proper speedometer.
This is just my 2 cents, I am sure we will hear from many others more knowledgable than I on this subject. Good Luck.
Thanks for the info Mike - but I guess my question got a bit muddled. Theoretically, a 1.25:1 and a 15:12 should read the same, correct? But is there a physical or mechanical difference between the two or is it just a different way of saying the same thing?
From a ratio point of view, the 15:12 should be the same as the 1.25:1 I would think. However you never know with the British motorcycle industry. I have a '72 TR6R and have replaced the speedo drive. It was a 1.25:1 ratio originally and I replaced it with an identical part (part number 60-3213). I did not have any problems with the speedo readings with the replacement.
hi , i could be wrong here but i thought that you used to be,( but not now) able to buy repair kits for these,so is it possible that your 15:12 ratio was actually another ratio,or maybe your new one labelled 1.25:1 is different, is it still possible to count or is it totally destroyed, i've been trying to think about it because as you say the ratio's pan out to be the same, what percentage increase are you getting on your speedo?