Hitchcocks Forum

I've been comparing my speedo with those roadside warning signs, and with my mobile phone satnav (as best I can), and I reckon the speedo is a good five or maybe six mph fast at 30mph. (This is done with in-head averaging, which isn't always foolproof, of course).



This suggests it's way more than the legal limit of 10% fast. Has anyone else noticed this? Has anyone else tried and/or succeeded in fixing it?

I have never really believed the 10% rule, as that can be far too small a margin at low speeds... so I just checked. EU law says speedos cannot show a speed below the actual, and have to indicate no more than 110% + 6.25 mph. Plus... your tires might be flat

Btw the fix would be bigger diameter wheels/tires or an alternative speedo drive methinks. Do you have the grey plastic one or the steel upgrade? If plastic, lubricate the speedo cable threads!!! You will never get it off undamaged otherwise.

Thack - a speedo that far out on a new bike is surely a warranty claim for a new, and accurate, speedo head.

Are you talking the LED signs that light up with a number as you approach? We've got them around here and they seem wildly inaccurate. I've set up both my speedos on the lathe having measured the actual distance covered by a rotation of the driving wheel and related that to the turns per mile (on the Bullet) or kilometre (on the Ducati). Both have been calibrated to be most accurate at 60mph or 100 kmph but the Bullet has a real Smiths Chronometric which is pretty bloody good right throughout the range. The Vegila on the Duc goes a bit pear shaped below 20 Kmph and above about 140kmph but in the middle range is pretty good. I think therefore that the signs are a bit dependent on who set them up or perhaps some other atmospheric variable. If though you are talking about the measured mile signs where you hold a speed between marked posts and see how long it takes then as long as they've been put in at a genuine spacing there can be little argument.

Presto: You make a good point. I'll take some more time with a GPS so I can get a better picture of the true accuracy. I'm only hesitant to go back to the dealer because I suspect a replacement may be little better.



Simon: You've done exactly what I was thinking: setting it up on my lathe. How did you perform the actual calibration on the non-chronometric speedo?



The manufacturers and reconditioners have a machine with large electromagnets in them, which they use to either permanently strengthen or weaken the rotating magnet inside the aluminium cup. Another approach for those without the proper kit is to adjust the length of the spiral hairspring. Did you do that, Simon, or something else?

Michael: thanks for that tip about the threads - I will check tomorrow.

Enfield speedos have never been accurate can't imagine why all of a sudden they would get better

Nooo way simpler than that Thack, I calibrated the speed it should indicate at a specific rev range (as near as I could get to the 100kmph figure) and set the needle there. You need to make a little needle puller to get it off without breaking or bending the spindle but it is a relatively simple tool. I then checked it at other rev ranges and worked out what the bias was. I think its running about 5kmph slow at 160 and a similar amount fast at 20. The regularity is vastly improved with a wash in an appropriate spirit (one that doesn't dissolve the paint or plastic and then a lubrication with some suitable grease or oil. I used Isohexane and Garrad Turntable spindle grease but fortunately for the rest of the world these aren't the only options!

A worn front tyre, or changing the front tyre will change the accuracy of the speedo, due to a change in rolling circumference. looser road surfaces will also contribute due to micro skidding of the front tyre.Needle damper springs also start to drag as the speed increases , causing the needle to read increasingly under as it goes up the scale.I once passed a roadside warning sign on a Ural combo, the sign said "Give way" Then the speedo needle fell off .......