Mileage checks

[bikerbob]

Has anyone ever checked the accuracy of the mileage readings on speedos. The reason I ask is I have a 1962 A65 and a 1956 A7, couple of days ago took the A65 for a run on local country roads did 29 miles the next day did exactly the same run on the A7 but it indicated that I had done 33 miles. I have had both bikes for quite a few years and have never before checked this out, am not really bothered as such but was surprised to find such a difference both speedos are the original items.

[Ian C]

At BSA camps when filling in the mileage sheets, I always jest 'Smiths' or actual!!

[DAVE BRADY]

Hi,

There does seem to be some variation.  I always think that chronometric are the more accurate than magnetic but there are a few things to consider.  Different tyre profiles and tyre pressures will make a difference over a long distance.  Less reliable seems to be the speed shown and this get less accurate the faster you go.  I have found that at 30mph all seem to be about right but at 70mph the reading is probably 5mph fast.  A satnav/gps speedo is a good way to see how much they are out.

Dave.   

[chaz]

maybe a previous owner changed the gearing or speedo drive to a different one with a different ratio.

[Calum]

Different sized sprockets will also affect the reading, if the drive is taken from the gearbox (obviously different primary drive gearing wouldn't affect the speedo)
As said different tyres and pressures will affect it also, but I have to say 3 miles over such a short distance is a big discrepancy - near enough 10%

[BEEZABRYAN]

Flash chronometric is good enough, the Enfield 10% not so but there is a difference with Indian Sub Continent sub quality control. Possibly tyres have a minor effect possibly not 10% enough.
But, am I bovvered?

[Pete A10]

I don't think any speedos are really accurate. My Triumph Explorer when showing 100mph is running at 95mph on the satnav. Similarly my BMW 320 consistently shows slower on a separate satnav than on the car speedo. I think the normal allowance for speed checking is the speed limit plus 10 percent plus 2mph - so even Plod doesn't seem to think speedos are accurate.

[bikerbob]

Thanks for those replies, just to answer some of the queries both bikes are geared as per BSA original specifications no alteration to speedo gearing or sprockets and both speedos are the originals form new. As to tyre pressures I always check the tyre pressures before going out if the bike has stood for a week or more both bikes have 28psi in the rear and 25psi in the front. Tyres are avon 3.25  ribbed on the front and Mitas3.50 on the rear they are similar to Dunlop k70. Sometime over next few weeks I will do the exact same run in the car and see what the difference is.

[Group Leader]

The Odemeters/Tripmeters are operated by pawls with a sharp point engaging in the  fine teeth of the "gear" (not really a gear as it doesn't mesh with another one) on the end of the odemeter drum.    The pawl levers are driven by an eccentric bush that rotates driven ultimately by the input cable.    When the point wears on the pawl it will sometimes fail to advance the drum causing it to record a lower mileage than actually covered.   When the pawl is worn completely it fails to drive the drum at all and the Odemeter and/or Trip stop working completely.    The picture shows a replacement lever I made and fitted to a friends Chronometric to replace the worn original.  The video shows the drive mechanism in operation.

https://www.youtube.com/watch?v=_U2Hb9GrtdU

HTH.

 

[bikerbob]

Thanks for that info I will look into that when I get the time maybe a job I can do over the winter as to do it now I would have to take the Avon handlebar fairing off and it is a bit of a fiddle. That sonds to me as though the A7 is the problem asit has done 73000 miles from new and the A65 has done 15000 from new the A65 still has the original pistons fitted and stamped BSA.