Speed vs RPM

Smiths or Vaguelia clocks, the clue is in the nickname for Veglia. My MK2a with Vaugelia clocks over reads speed by 10 mph at 30 mph, never hooked the engine up to a calibrated or electronic tacho to compare but I doubt its any better.

Speedometers usually over read but its normally about a 10% error through the range.

However, if this was the case with John's Commando, it would be showing approx 78 mph at 4000 rpm. not 60 mph as he claims it does.

If the bike and speedo drive gearing is correct then I would suspect the speedo, maybe some grease has got into the head. I once had a Honda CBX250 RSE which read about 5 mph slow at 35 mph. A "new" secondhand speedo soon corrected the problem although a Commando speedo will not cost £15 like my Honda's did a few years ago.

Further to my earlier post, it looks like the rev counter could be dodgy. It is showing 66.7 rpm for each mph at 60 so it should show 4667 rpm for 70 mph. John says it showing just under 5000 rpm so something is wrong which may be due to it being an old instrument.

Look on eBay. There are some electronic rev counter/hour meters that are quite cheap and easy to connect up - you just wrap the cable round one of the plug's HT lead and then adjust the instrument to one pulse for every two revs. The unit has its own self contained battery. Prices range from under a tenner (including post from China) to a good one from the USA (Works Connection) but sold in Britain at £50.

It does sound like the speedo is underreading. At 4000 the engine will/ should sound like its starting to work but at 5000 you know theres a lot going on in the engine. If it had been a Veglia the brass shaft (with needle attached) running thru the aluminum cup comes loose and lags behind the rectangular spinning magnet attached to the drive end. However, you say the speedo is new so its not one of those. What make is it? Maybe an incorrectly calibrated aftermarket part? Have you got the correct rear drive box - 15/12?

There is a guy in this months RH who claims 4000 at 90 mph. That would take some gearing up in the primary and final drive to achieve 22.5/1000, with a 3.33 overall compared to 4.38 std. Something like a 28T gearbox cog if the primary was std. Why not hook up a low cost bike speedo and get a baseline against the tach.

Have you got the correct rear drive box - 15/12?

Does this also apply to Dominators? My 650SS, with aftermarket instruments, under-reads the speed, as well. I know the speedo is slow, and assumed it is the head, but maybe it is the drive, which is a 2:1?

Ian, 2-1 is correct for a Dommie, inc the 650.

If you are using a 150 mph magnetic speedo it is likely to be 15/12.

120 mph chrono is where you want to be but it might cost you!

It may have been mentioned on the Norton site recently, or one of the other bike related forums I visit, but there is a free app available for 'smartphones' called Go Pro Analogue Speedo. Gives you a needle on dial as well as a digital sped readout. Works really well. Tape your phone to the handlebars and go for a ride. This way you will soon see if your speedo is ok or not.

George.

Try using a sat nav to check your speed (on a long straight road), as has been mentioned, the speedometer should underread by about 10%.

Previously neil_wyatt wrote:

Ian, 2-1 is correct for a Dommie, inc the 650.

If you are using a 150 mph magnetic speedo it is likely to be 15/12.

120 mph chrono is where you want to be but it might cost you!

Neil, you have solved my problem, at least! Having just trawled the 'net, I have identified these as being my instruments.... This speedo requires a 15/12 drive ratio, as you rightly said.

My rev-counter, which matches, apparently requires a 4/1 drive. Is the 650SS drive 4/1?

Many thanks. Ian

Another thought; as my speedo, with a 2/1 drive ratio, reads 60mph when other bikes riding with me a clocking 70-75mph, won't a 15/12 drive make much more difference than that?

Hi Ian, if the 150 mph speedo uses the same gearing as the genuine smith instrument then it should be 15/12 @ the gearbox, like on my Mk3 Commando with a Veglia and the same set up I put on my Model 50. (Black faced 150 mph like shown but genuine Smiths)

I know these replacement rev counters use 4-1 drive, though I have never used one.Are the Speedo's 4-1 too? 15/12 = 1-1.25

I bought a smiths electronic rev counter for my Poor Man's Manx. These things are set up for twins but need a lot of fiddling through 3,4,5,6,7 & 8 cylinders to get back to 1.

Anyway, the Smiths rule is 120 mph = 2-1 and 150 mph speedo = 15/12. (1- 1.25) I dare say there will be exceptions to the rule and 650's changed from chrono to magnetic during production. Changing the wheel size by an inch won't make so much difference if that is taken up by the profile of the tyre. I learnt a lot from the Gag Brothers about Speedo's but you can't get hold of them these days.

Got to say the new electronic Smiths (Puca) instruments look the part.

Previously John Shorter wrote:

Try using a sat nav to check your speed (on a long straight road), as has been mentioned, the speedometer should underread by about 10%.

Over read I hope, so if the Sat nav says 60 the speedo should be showing 60 to 66 or you are in line for a ticket.

Neil. The makers of the aftermarket 150mph speedo's say they are for a 15/12 drive, and if that adds 25% to my speed reading, that should be ok.

Many thanks.Ian

When I was playing with gearing on my Inter I made the attached spread sheet to convert RPM to road speed. (Sad I know but I was a student at the time so it seemed more interesting than urban drainage systems!)

There will be some small rounding errors but I think itâs pretty close, it wonât tell you which instrument is wrong but it might be interesting to have a play with it and see what sort of error youâve got.

The table is filled in for my Inter at the moment so you can see how it works if you want to try it.

Incidently

Previously john_holmes wrote:

Previously John Shorter wrote:

Try using a sat nav to check your speed (on a long straight road), as has been mentioned, the speedometer should underread by about 10%.

Over read I hope, so if the Sat nav says 60 the speedo should be showing 60 to 66 or you are in line for a ticket.

Oop's, I did not put my brain in gear, before releasing the clutch! On a French motorway, (speed limit 130 km/hr) my sat nav indicates 130, when the speedometer reads 140.