Speedo inaccuracy

[Kitch]

The speedo on my kettle for some reason reads around 20% too fast. So I’ve done some checking using my battery drill at 1500 rpm the speedo shows exactly 40mph which is bang on as the speedo runs at 2240 revs per mile. I’ve also run it at a constant 40 mph on the drill and it covers 1 mile in 90 seconds also correct so why is it so far out. I can only assume the gearing in the front wheel is inaccurate.does anybody know if there are any different drive units or am I stuck with this

[PaulD738]

Probably the tyre size? If you’ve got modern rubber on it the rolling diameter might be different than the oe. My Z1300 reads a good 8mph faster than it shows on the sat nav because it’s got an oversized tyre fitted

[Kitch]

Front tyre is 90/90x19 I’ve worked out the wheel revs per mile x the speedo drive and it works out at 7% fast so don’t know we’re the other 13% is coming from.

[malky]

I think they all read a bit low - certainly my T500 and Kettle read 5mph slow at 30 mph and around 8mph slow at 70mph both running 325 X 19 front tyres

[teazer]

I seem to recall that speedos were allowed to read up to 10% fast and zero % slow. The idea was supposedly to protect riders and drivers from going faster than they "should" but I suspect that OEMs were happy to have owners boast about how fast their bike was compared to say road tests of the time.

You can check the gearing in the speedo drive gearbox and measure the rolling diameter of the tire which will be less than the static diameter. I am told that it's possible to recalibrate the speedo if it's dismantled. Fortunately modern electronic speedos can be programmed to show the correct speed or use a GPS speedo.

[750ynot]

I seem to recall that speedos were allowed to read up to 10% fast and zero % slow. The idea was supposedly to protect riders and drivers from going faster than they "should" but I suspect that OEMs were happy to have owners boast about how fast their bike was compared to say road tests of the time.

You can check the gearing in the speedo drive gearbox and measure the rolling diameter of the tire which will be less than the static diameter. I am told that it's possible to recalibrate the speedo if it's dismantled. Fortunately modern electronic speedos can be programmed to show the correct speed or use a GPS speedo.

I remember this 10% thing too from back int day, lots of people saying they had speeding tickets but they thought they were going 10% faster than ticket speed.

Lets not forget the famous X7 that was supposed to do a ton, but didn't, only on the speedo.

I have checked many speedo's against sat nav and found 10% out on most older vehicles, can's say about newer ones as I don't have any.

[Kitch]

I know speedo’s read fast but I would like it to be a bit more accurate I’ve noticed the drive unit on the front wheel looks the same on the 750,550 and the 380 but as there all 19in wheels there probably all geared the same.the 250gt as an 18in front wheel but I don’t know the gearing or if the hole for the wheel spindle is the same