Actual Speed

bikesnbones · Post by **bikesnbones** » Fri Aug 23, 2013 11:28 am

Has anyone checked to see how accurate their speedometers are.
I suspect mine is over reading.
My other BMW is.
I often use a sat nav on that one which shows an indicated 70mph, to be an actual 63mph (assuming the speed reading on the navigation device is accurate)
I have heard that this is common with BMW's
Until I figure out a way of rigging up the sat nav to the 12S I don't know for sure, so was wondering if anyone else has tried it.

eyore · Post by **eyore** » Fri Aug 23, 2013 11:41 am

Yes, the R 12S reads between 5% and 7% below actual mph according to my gps.
I don't think its just a Beemer thing, the Ducati does and my Triumph Sprint was the same.

ned1 · Post by **ned1** » Fri Aug 23, 2013 11:56 am

I have always been under the impression that speedos are always over the top for safety (the speed limit) reasons.
Also with a sat nav you have to be on the level and not going up or down a incline with constant speed for accurate measurement.

Ned

John Coles · Post by **John Coles** » Fri Aug 23, 2013 12:01 pm

In the good old days of 'weights & measures' department - don't know what it's called these days - speedo's had to be calibrated to 'within 10%' as this was as accurate as they could mass produce mechanical ones without having to hand calibrate each one. Knowing that got me off several tickets when talking to the cops back in the 1970's. As far as I am aware this has not changed and it gave the 'zero tolerance in 30 mph zones' no chance. However I did not argue the fact with the Swiss police who politely sent me a ticket for 55 in a 50kph zone ( very obscured 50 sign good old hidden grey camera ) as the pretty ticket stated they caught me at 55 the alowance was 3kph over so they fine me for the 2kph

also no points here but the fines increase massively the more over the limit you are.

McBoxer · Post by **McBoxer** » Fri Aug 23, 2013 12:17 pm

It's true. They can't make a mechanical speedo that's 100% accurate all of the time due to differences in tyre diameter (different brands, tyre wear, temperature etc) and other tolerance variations.
In my experience (gained in the car world, mainly) most manufacturers err on the side of caution and overstate the roadspeed slightly. This way you can't blame them if you're caught speeding!
Your speedo will be most accurate on new tyres but the gap will increase as the tyres wear.
Coincidentally, my current car (Saab) is the most accurate speedo I have experienced, to the point of overreading sometimes (compared to my sat nav). It also triggers the 30mph warning that I drive past most days, when indicating 30. I have to slow to around 26 (indicated) to get the sign to go off. This might help to explain the ticket I got recently, which I would forward to Saab - if they still existed.......

Boxered · Post by **Boxered** » Fri Aug 23, 2013 12:29 pm

Yep, my R12S speedo is around 10% out at most speeds, I rely on my sat nav for a truer idea of my speed.

Steve

tanneman · Post by **tanneman** » Fri Aug 23, 2013 12:43 pm

Don't rely on the safety signs to indicate your true speed as these devises has to set up and maintained. The Ford Explorer of the 1990's under read on the speedo.

Speedometers has to be accurate to within 10%. The accuracy of your sat nav will depend on a few factors, like the height above horizon, angle of the antenna, terrain where you are, number of satellites engaged and the timing accuracy of the sat nav. Even a 0.001 second of timing difference will produce a big distance error on earth surface. Riding up or down an incline and around bends will also influence accuracy as the refresh rate of the sat nav determines the straight line displacement that is used to calculate speed. If you are riding in the hills then accuracy will be affected as you move around the contours and the sat nav has to handshake with different satellites as they rise above the horizon meaning that you'll be in dead ground to some satellites while visible to others.

Most vehicles nowadays has very accurate speedometers with a difference of only 1 to 2 mph as manufacturers have moved away from mechanical means of speed measurement. What you observe while riding/driving is a digital signal converted to an analogue output.

bikesnbones · Post by **bikesnbones** » Fri Aug 23, 2013 12:47 pm

ned1 wrote:I have always been under the impression that speedos are always over the top for safety (the speed limit) reasons:

tanneman wrote:Most vehicles nowadays has very accurate speedometers with a difference of only 1 to 2 mph as manufacturers have moved away from mechanical means of speed measurement.

A 7mph discrepency as is the case on my other Beemer, is quite a lot.
My car is over reading by 1mph at 30, and 3mph at 70.

Hayden · Post by **Hayden** » Fri Aug 23, 2013 4:27 pm

The speedo on the Sprint is suprisingly accurate, there`s one of those safety signs down our road, the ones that go from green numbers to red at 30mph, if I go through there at 145 indicated it lights up to show 142, not bad

...........what......

tanneman · Post by **tanneman** » Fri Aug 23, 2013 5:35 pm

bikesnbones wrote:A 7mph discrepancy as is the case on my other Beemer, is quite a lot.
My car is over reading by 1mph at 30, and 3mph at 70.

How do you measure it or what instrument do you use to confirm your suspicion? Maybe the statement should be a 7 mph difference between the speedometer reading and whatever gadget you are using. The reading on speedometer falls within the 10% at 70 mph and is ok for a mechanical device. If you want accurate GPS then get yourself the kit a surveyor use. The GPS on that thing is accurate to within 5 mm in height and distance. The commercially available sat nav can on a good day pin your location to within a 3 m radius. A 3 meter calculation error at 70 mph or 140 mph will give you various readings. For true accuracy you'll need an instrument that is synced to an atomic clock with the ability to do complex calculations every 0.1 second. The point is that a sat nav cannot be used to accurately determine speed. You can get a ball park figure and the accuracy will improve when you travel at a constant speed in a straight line. However for us folk, that travels on a road wide enough, the sat nav only has to determine that you are in proximity of that road so the protocols within the sat nav take care of placing the icon on the road. The effect is that your speed can be calculated so the protocol can announce warnings, direction changes etc in advance.

Hayden, the safety signs have only 2 displays.

Boxadog 2000 · Post by **Boxadog 2000** » Fri Aug 23, 2013 5:59 pm

At an indicated 70mph on the speedometer my twat nav shows shows about 67

Hayden · Post by **Hayden** » Fri Aug 23, 2013 7:37 pm

tanneman wrote:
Hayden, the safety signs have only 2 displays.

really, then its either engine vibes or my eyes are deteriorating....

KenG · Post by **KenG** » Fri Aug 23, 2013 7:52 pm

hitting the limiter in 6th on the blackbird,, who cares lol

claimed 186 "forgive or overtake" the gixer thou while getting your arm ripped off pulling a birdie

tucked in, and both cheeks clenched.

best 10 seconds ever.

McBoxer · Post by **McBoxer** » Fri Aug 23, 2013 8:42 pm

Hayden wrote:The speedo on the Sprint is suprisingly accurate, there`s one of those safety signs down our road, the ones that go from green numbers to red at 30mph, if I go through there at 145 indicated it lights up to show 142, not bad

...........what......

Hayden, do you live round the corner from me? That sounds very familiar....

bikesnbones · Post by **bikesnbones** » Sat Aug 24, 2013 3:02 pm

tanneman wrote:
bikesnbones wrote:A 7mph discrepancy as is the case on my other Beemer, is quite a lot.
My car is over reading by 1mph at 30, and 3mph at 70.
How do you measure it or what instrument do you use to confirm your suspicion? Maybe the statement should be a 7 mph difference between the speedometer reading and whatever gadget you are using. The reading on speedometer falls within the 10% at 70 mph and is ok for a mechanical device. If you want accurate GPS then get yourself the kit a surveyor use. The GPS on that thing is accurate to within 5 mm in height and distance. The commercially available sat nav can on a good day pin your location to within a 3 m radius. A 3 meter calculation error at 70 mph or 140 mph will give you various readings. For true accuracy you'll need an instrument that is synced to an atomic clock with the ability to do complex calculations every 0.1 second. The point is that a sat nav cannot be used to accurately determine speed. You can get a ball park figure and the accuracy will improve when you travel at a constant speed in a straight line. However for us folk, that travels on a road wide enough, the sat nav only has to determine that you are in proximity of that road so the protocols within the sat nav take care of placing the icon on the road. The effect is that your speed can be calculated so the protocol can announce warnings, direction changes etc in advance.