Faulty Beemer Speedos - or?

[dc lindberg]

Over the years I have been concerned about my bikes poor mileage, as I see at least two entries here on Horizons Unlimited present to.

The last two-three weeks I have had to go back to my beemers stock final drive 37/11 since my 32/10 suddenly developped a need for a Bevel-Dentist... Mileage looks a lot better with the 37/11 speedo -
What I like to use is the 32/10 from R75; easier to fill and better ratio.

Friend of mine just bought a R100RT 1983:
www.buffalobill.se/uppland
He noticed that his speed is showing 17km/h higher speed than the GPS...
If we assume the R100RT to be expected to run at 5.5L/100km at 90km/h even speed this would mean that the true mileage is 6.4L/100km... and that is what I have!

I have also checked tires sizes and that do vary too!
It seems as if the Beemer speedo is made for 100mm high tires, and not for 90mm high...
Those of you who have been really worried about high mileage, correct my memory if it is way off, but did you not have Continental TKC 80 i.e. 80mm high tires ?

I used to have a bicycle speedo on my beemer - but the pic-up stoped working. With that and correctly set data it showed the exact same speed as the 37/11 speedo with 32/10 bevelbox between 50-105km/h. At this time I used Continental 120/90x18" and Michelin 120/90x18" H tires. Using Sheng Shin C-186 and Pirelli MT43 fully studded ment +5-10% speed - so I used a differently configured bicycle speedo when studded tires where on the bike.


So my question to you all now is:
Have you checked your beemer speedo ?
Do you know the accuracy of your speedo ?
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Second part of this is of cause - how much can we trust the BMW vehicle data on mileage ???
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A third part of this question is directed to all of you who have fantasic mileage - the dream mileage for some of us: what are the specific vehile configurations in your case ? How come that you can get 5L/100km whereas some of us get 6.5L/100km ?

[Carl P]

It seems as though BMW have a long tradition of optimistic speedos, airhead or oilhead they always seem to read a little high, maybe 3-5%.

I guess its to be on the safe side in speed limit situations, could be bad if they read under.

I use a Sigma bicycle speedos on a R80 & an 1150GS, both set accurately, funnily enough both bikes do 16km to the litre, 100km/6.25Lts, 45 MPG(UK gallon). Both bikes are run without fairings.

A friend has an RT, same year, same engine as my GS, obviously he's fully faired, at about 140 kmph he is running nearly 1000revs less than me!!
I've got the enduro(low) top gear, anyway on a recent long run he used about 15% less petrol than me.

[AliBaba]

The R100 is higher geared then the R80, it also have 18” wheels compared to the R80GS 17” which makes the gearing even higher.
The higher gearing and the faring reduces petrol consumption, especially in speeds above 80km/h.

In my experience 20-30 years old tachometers isn’t very accurate.

When it comes to the bikes’ mileage the odometer is far more accurate then the speedometer. When I used a BMW-speedometer the odometer was spot on when using Pirelli MT21 (130/90-17), with Michelin T63 (130/80-17) there was a very small error, maybe 3%.

It’s not a problem to get a mileage around 5l/100km on an airhead if the bike is properly tuned (even with T63/MT21). It requires maintenance and that you drive carefully….
Normally my mileage is 5,8l/100km 6.8l/100km, it depends on my mood

[oldbmw]

The following is based on my observations of my own bike only, a 1985 R80RT.
When I first got it I got 280 Km to teh tank before needing reserve.
At my first service I changed the needles and needle jets as per motobins suggestion. After that it does 320 km without needing reserve.
Generally I get about 16 Km to the litre.
mpg is greatly affected by throttle openings and revs. The best I ever got was 57mpg (imperial) following a friend on his diesel Enfield at about 55-60mph. (he was getting about 150mpg) also in the pyrennean mountains where I went up most of them using the gears (down to first gear on many of the turns) but with light throttle openings and came down mostly on the overrun in high gear.
Note high revs, high throttle openings not speed seem to consume more fuel. Naturally at high speed you are using both the above, but in my view riders habits are more important than the actual gearing.

[farqhuar]

As AliBaba has pointed out, do not confuse odometer accuracy with speedometer accuracy. Typically bike speedos (and brand bike) read between 5 and 15% optimistic, with the average being 10%, whilst odometers are accurate to within 1-2%.

Even cars are out too, my GPS shows my car being 2% optimistic.

I always run a Sigma on all my bikes but my recent GPS acquisition has shown this is no longer necessary as the GPS is consistent with the Sigma for all speeds and distances.

Garry from Oz.

[dc lindberg]

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Nice debate. Most interesting and quite helpful! I still think with my limited experience that 5L/100km is utopia, though I am very much aware that there are many factors contributing to the mileage.

I have tested the Sigmas - and love them!
Mine have however stoped working; the induction pick-up does not respond to my magnet... and the alu-plate I used to bolt the induction pick-up on have been all chewed-up by road salt...

[Frank Warner]

Originally Posted by dc lindberg

the induction pick-up does not respond to my magnet...

Errr it is a magnetic reed switch that should respond to the magnet. It is not inductive.

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On my R80G/S at 80 km/h and a light throttle I can get 4.5l/100km ..

Slow down - you'll get better economy. Of course there is a lower limit where you'll need to go down a gear .. don't do that .. stay in top gear but get a speed where the engine is just happy .. I think 80km/h is about the optimim speed .. for economy on most of my bikes?

On a bicycle the optimum speed is around 21km/h that is where the rolling resistance and the air resistance equal one another .. both those depend on many factors - rider shape, weight, tyres used, tyre pressure etc etc..

[lecap]

Not sure about speedo but my R1100GS odo & trip counter read about 10% more than DR650 / KLR650 / Triumph Tiger 900. Always causes k@k with the roadbooks but helps BMW to make 10% more on service and increases the milage a BMW does until the final turn of the crank by 10% :evil6: