Can the Tach and Speedo readings change over the years?

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Mar 19, 2005
Relating somewhat to the thread, "How fast is a Norton"....

Amazing to see some of the power and speed readings from some of you guys! I did drag mine once out the other side of Prag but I only got something over 12 in the quarter...interesting experience though.

Back when I got this bike and it was almost used to scare the living daylights out of was ungodly fast and the adreneline rush after midnight, fueled by a nice bowl ....of bringing it up through the gears between towns was worth getting out of bed for at two so the cops would stand less of a chance of catching me. It would do 105 in third gear...forth gear I rarely even bothered to shift into....shut it back down....I'd had my rush. Once was enough. Worked like magic every time, too. One run...and I could go back to sleep like a baby.

I remember back then, the speedo read 50 at 3000, 60 at 3,500, 70 at 4,000...etc. Every 500 rpms, it picked up 10 mph. You didn't really need a speedo to tell your ground speed.

My question.

Can the speedo or tach, change readings over the years? Mine has gradually begun to read 60 at over 4,000 and doing 70 will get it up towards 5000. The bike used to do 90 at 5000 rpms. The gearing hasn't been changed...still a 21 sprocket, still the same speedo gearbox. What's the story? Am I risking getting pulled over for speeding because the speedo reads too little...or...much to my disbelief, can a tacho start to read HIGHER over the years. Neither instrument has been opened, or played with. I know I can check ground speed by sweet-talking someone into driving thier car at a set speed and seeing what the bike is doing at the same speed and the tach can be checked by an electronic tach...but what interests me is if these changes are possible...if these instruments can really change readings over time. Or are my memories just a bit foggy? :lol: :lol: :lol:

Open to any ideas...thanks!
instrument readings can change over the years because the hairspring will eventually change tension from workhardening or fatigue...thats why your bathroom scale has an adjustment on it and is 'not legal for trade' would have to get it recalibrated at an instrument shop but it is hard to say if it was accurate in the first place...does anybody really know how fast their norton is/was?unless you had it dynoed originally and were able to do it now under the same conditions using the same gas and tires on the same ashphalt and you weighing the same and on and on and on just be happy that the bike has a lot more potential than most of us will ever have the ability to use...i think the the important thing about nortons is that they posess the unmeasurable quality of niceness and all of us have got that s...eating grin on our faces about that first of wanna have the hotest norton on the block? spend another grand on it and you will until somebody spends 2 grand on theirs and then where are you....nice is where its at and the fact that they really are a hot machine is only a little icing on the cake isnt it? now go out and shift it at redline a few times and hit some twisties and enjoy the thing :wink: barry
FYI, My tach needs to be rebuilt. the failure mode is that it is all over dial. It does not ever seem to fail to report rpm's, just randomly more than it is. Imagine sitting with a nice low idle tick-over, and it says 5 grand! I agree with Barry, somethings up between the cup, magnet and spring... is nice to know there is somebody else out there that understands what i am saying and not take offense to my way of thinking.....nbmbruce...ya might try giving it a shot of lube...3in1,lps2 or if you are really desperate wd-40[which is not a very good lubricant but is convenient to spray in as an intermediate will clean things out so you can put some proper lube in after the wd runs out]....sometimes the bushings just get a bit dry after 30 years and the needle will bounce around
Thanks for the ideas guys....
So if I read it right...the general consensus is that my tach might be reading more than the motor is really turning. I know most of us rely on our ears to tell us when to shift...a habit that just happens over time....but my ears aren't what they were and I have started to look more at the tach the last year or so. Guess I'd better go to the trouble of checking both instruments out before I either get a ticket or blow the lady up. It's still such a rush to bring her up through the gears once in a while, that I would maybe feel better if I knew what the truth was before something irreversible happens. If I know what the story is...then I can adjust my habits to compensate. It is though interesting to know that I most likely don't remember so incorrectly after all. It used to be quite nice to just look at the tach and be able to mentally calculate the speed. I have to do that anyway here....road signs are in KMs and the speedo is in miles. But you get used to that. My only concern was that the readings were starting to get so unpredictable that I just couldn't be sure of them. By the way....if the tach needle is jumping around in a rhythmic manner...oil the drive shaft or whatever it is called inside the black cable running to the gearbox on the motor....called a "Welle" in German...but in English...can't remember. Been here too long, I guess...when I can't remember something in my mother language...but can in a language I only halfway can speak in.
I'd be also a bit cautious about just spraying something into the tacho or speedo....with a watch or clock, lubrication is only placed exactly where it is needed because the other parts will get gummed up and collect dust and dirt. The daughter is a watchmaker. The instruments are also a form of clock, if you get what I am saying. Might do more damage than good. Perhaps you can view the parts through the hole the light fits into and use a thin knitting needle dipped in light oil, to get to just the bearings...don't yet know...spect I will find out soon, as mine will need attention too....just make darned sure you clean the knitting needle off before the wife finds out :lol:

thanks for the help! Ride safe!
Agree with all of you here, but thought I'd cover my speedo woes recently that are now fixed - the speedo drive on the back wheel was the culprit in my case. Been through two in 10,000 miles (the original plus a secondhand one of unknown vintage) as the build quality is not great, they are non-rebuildable and people tend to forget (that is, previous owners or course!) to squirt some grease into the nipple on the bottom once in a while. Failure of the speedo gearbox exhibited itself as reading low and then within a couple of hundred miles refusing to register anything over 60mph even though I knew I was going faster! Once a new pattern gearbox was installed, I temporarily stole my sons' $20 bicycle computer to check all was well - got a "10% high" reading from the Norton speedo against the bicycle computer which satisfied me all was now well......
Cheers for now
Thanks for the info...will check mine...but they are indeed able to be taken apart and cleaned and greased...mine has just had this habit of somehow deforming a bit and grinding a groove into the nice aluminum plate covering the hub of the rear wheel. The entire housing of the gearbox has a couple of times gotten, somehow into a bowl form and this makes it simply thicker and it rubs on the hub...but they can be cleaned and greased, despite this idiosyncrasy. I've had to carefully put mine into the workshop vise and delicately squeeze it back into form a number of times over the years.
From your description...your old one must have blown the drive gear or the worm gear and the teeth wern't making enough contact to drive the speedo.....what with a good 120,000 on mine.....even if it was greased regularly by me.....could indeed be that it is plain shot, and has too much play between the gears....will check it out...Thanks!
Speedo/Tach errors

I had to rebuild my speedo again recently, first time 7 yrs ago the needle shaft came loose on the brass bushing and would not read over 45mph. This time the cup was so loose on the bushing that it was rocking and touching the magnet, giving me high readings when I pulled away. That might explain the higher than normal revs. Strangely my tach has never been apart and is rock steady and when the local speedo guy reglassed and calibrated them both he commented it was unusual to see old Veglias that worked so well.....just luck I guess!
Under speedo gearbox there should be a spacing washer, this will stop it rubbing on alloy plate. Also it is not unknown for pattern speedo cables to have too small square drive formed on end so drive slips when corners round off a bit. I have heard of magnetism in tachos & speedos going weak & causing inaccuracy, but this would cause low readings not high ones. I would avoid red lineing the old dear, how would you like it if someone rode you that hard at that age?
Geoff said:
how would you like it if someone rode you that hard at that age?

I could...but won't...make a naughty comment to that...think I'll leave it alone out of respect for those readers that actually might agree with you.:lol:

The spacer washer has always been there...were it should be. What has happened several times is that actually, the body itself gets deformed and then the body makes contact with the nice alu hub cover. Eats it's way right into the hub too and has messed that up....but it hasn't done this now for a few years...somehow holding it's shape......but never trust the lady though...she will surely do it again soon.....just to let me know she wants a little TLC.

Speedo cables will indeed get rounded off on the tach cable is that way. I folded THIN sheet metal around it and stuck it back together so it can't turn in the little end piece it fits for about a year or so...then the sheet metal wears through and it starts to slip again.

Wishing all a great weekend! Might get a bit aof time on the machine on Sunday...if I can get out of bed soon enough...before the traffic starts. It will be a late night on Saturday...Band is doing a gig.... :D
Can the Tach and Speedo readings change over the years?
At least your instruments are reading fairly close. On the prototypes, we had a speedo drive gearbox that was putting out 1600 revs/mile, which was Smiths current standard, but we had a speedometer (from the factory surplus parts bin) that was calibrated for 1000 revs/mile. The speedo was therefore reading 1.6x actual speed. The odometers were off by the same ratio.

Trying to do road tests where you had to rely on the tach for anything above 65 mph or so was a real PITA. We got impressive gas mileage, though, despite having main jets that were 20 percent too big.!
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