Speedometer accuracy

[bmwski]

I have a 2004 R1150R that I have had for almost a year. In my opinion and experience I feel like the speedometer reads faster than I am actually going, around 5 mph. Any info on this. I have heard that some read this way.

[wncbmw]

Yep - sounds average. Mine is off the same.

Fortunately, the only time I usually check the speedometer is on long interstate runs.

[MIXR]

The Roadster runs (as I recall) a 2.6 ratio speedo head (the bit at the front wheel, not the instrument panel). There is no lower ratio that will fix the error (unless you go to a specialist shop).

The GS that replaced my R has a 3.0 ratio speedo head which gives a 6-7 mph error at highway speeds.

There is also a 2.9 (actually 2.875) ratio head from some R1100 and RT bikes. This helps minimise the error on the GS, but not eliminate it. The 2.6 ratio head actually makes the GS speedo under-read by a few mph, which is not a good idea. Both the 2.9 (nominal) and 3.0 heads would give the opposite to what you want and increase the error.

Swapping speedo heads is easy and cheap, but you need a LOWER ratio to reduce the error. The 2.6 is as low as they come (so I'm told), so no dice with the R. You need to run a GPS alongside the speedo and use it if you want accuracy as the R has the lowest ratio BMW head already.

You can also get better speedo accuracy with different brand front tyres. I'm not talking changes to the specification, but just the brand and type. For example, the Avon Azaro gave me my worst error. The Avon Distanzia gave me an exactly correct match to my GPS. Other brands also gives variations due to their profile (and therefore height above the rim) which changes rotational speed.

Live with it I suppose.

[garr2]

I recently had a policeman check the accuracy of the speedometer on my 1150RT.
The speedo was showing a steady 130 k.p.h. and he only booked me for 117.
Assuming his radar was accurate and he wasn't doing me any favours, that's an error of +11%

[bmwski]

Thanks for the feedback. I thought that I was right it in being off. Any more input would be great.

[NoRRmad]

I think it's required that speedos be optimistic. Reduces liability for people who want to sue about speeding tickets. Counting seconds per mile marker, I figure I'm just about ten percent high.

[CRazyCam]

Cheapest and simplest way of getting an accurate speedo is fitting a pushbike trip computer such as those made by Sigma.

They allow you to enter the exact distance the bike will travel for one revolution of the front wheel.

They have a small magnet which mounts someplace on the front wheel and a wee sensor which you have to mount on the front fork someplace.

Older Sigmas which I have used have been good for upto 280 kph, by repute and 220 kph by experience.

Some of the newer units only read to 199kph, but presumably, also 199 mph (it's the digits that are the limitation.)

regards,CrazyCam

[garr2]

Riding a m/c especially when touring is a great time to think, to make connections between thoughts, and to come to conclusions.

Thought 1

I recently had a policeman check the accuracy of the speedometer on my 1150RT.
The speedo was showing a steady 130 k.p.h. and he only booked me for 117.
Assuming his radar was accurate and he wasn't doing me any favours, that's an error of +11%

Thought 2 The tread depth on my newly fitted Michelin PR4 tyre is 6 mm. The tread depth on the old tyre it replaced was 2 mm. So the diameter of the wheel fitted with the old tyre is 8 mm less than the diameter of the wheel fitted with the new tyre.

Connection How much of the over-stating of the speedo (on a bike whose speedo is driven off the front axle) can be attributed to the circumference (and therefore distance travelled per rotation) on a worn tyre being a bit less than the distance travelled in one rotation of a new tyre?

Conclusion Using a tape measure, some middle school maths (C=pi X d), and a calculator I concluded that only 1.25% percentage points of my earlier 11% speedo over-statement could possibly be attributable to a worn tyre.

On my next ride I am thinking about thinking about cold fusion.

Ian

[captaincable]

I have heard that a good GPS is accurate for speed but have not confirmed .
Mike

[AZBMWRIDER]

We've got informational radar sites on a few of the streets that I commonly drive / ride on .
So I assuming they are close to being accurate .
All the airheads and the oilhead that I have all show +3 mph, from 30 mph, to 60 mph .
It's a bult in 'error ' from the manufacturer .

[peels]

just noticed this same phenomon, and wondered, so I tested it.

against my phone, and a regular gps, my speedometer shows a 4 mph optimistic view at 80 on the speedo. My tires are slightly worn...maybe contributed...

anyways, i wish i had never wondered. lol now I know about it, so ill exploit it...

[towerworker]

My '04 reads a consistent +4 (compared to my GPS) at 70 mph.