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 ?