Speed / Revs

[ME 109]

Radicalised speedometers.

[Duane Ausherman]

Gidday,

I just ran a test along the Hwy with the GPS on the tank,

Speedo is slightly lagging behind the actually speed of the GPS by about 5kms in the lower 40, 50 60 kph range

At 110kph on the speedo, the GPS shows 112kph @4500rpms - is this normal or not??

Any thoughts and the bike is a 6th Month 1978 R100RS

Normal? No, but common for the error to not be linear. Many times I have seen a speedo be accurate as some speed and above that it may be slow and below that it may be fast. The /2 was quite accurate. The /5 was really bad, but the /6 was much better.

[Kurt in S.A.]

I understand the 85-mph speedos were also fairly accurate.

Kurt in S.A.

[Ken in Oklahoma]

I always thought the civilian speedometers were on the optimistic side.
That is, they would show a reading higher than actual speed.
I think this is true of most manufacturer vehicles.

I understand that in the U.S. and surely other places as well, there is a legal requirement for speedometers to be at least a certain amount optimistic. So many of us live in nanny countries.

Ken

[Kurt in S.A.]

BMW's service bulletin #2756 states: a certain amount of "speedometer advance" is necessary to compensate for the negative tolerances in tires sizes between tire manufacturers, electric speedometer indication, and other factors." That way the risk of it displaying a speed less than the actual speed is reduced. The maximum amount of advance in speed reading is 10% of the actual speed plus 2.4 mph. If the actual speed is 55 mph, than the speedo should display somewhere between 55 and 62.9 mph.

Kurt in S.A.

[Duane Ausherman]

I understand the 85-mph speedos were also fairly accurate.

Kurt in S.A.

Could be, as that was after my time.

[bbelk]

Thread degradation alert: My truck, and just about all new cars have a GPS in them and for a fee each year, they connect to the mother ship and can tell you all kinds of things. I don't pay the fee, so the only one that hears anything from my truck is GM. My bicycle GPS has a sensor on the back wheel as well as its primary GPS. If it loses signal for any reason, it takes its values from the wheel sensor. The first thing it does when you use it is to calibrate the wheel sensor against the GPS. I want my truck to do that. I also want to have a way to "Enter" a number of gallons of gas on a fill up, so that my MPG calculator could calibrate itself.

[tsa]

Speedo is slightly lagging behind the actually speed of the GPS by about 5kms in the lower 40, 50 60 kph range

At 110kph on the speedo, the GPS shows 112kph @4500rpms - is this normal or not??

Grant, your final drive ratio is the corrrect one for the R100RS/RT, but a previous owner seems to have replaced the original speedo (a .691 ratio one) with one from an R100/7 (.711; equipped with a 34/11 final drive). Thats the reason your current speedo shows a lower speed than what the correct .691 ratio one would have done (difference: 33/34 = 0.97).

You could either try and find a new (correct ratio) speedo, or just put some tape stickers around the periphery of the one you have at the important speeds (limits?). If you're concerned with precise speed indication, use the GPS, or fit a cycle computer. The latter will probably be left in peace on the bike longer than a permanently attached GPS would.

[grant81rs]

Gidday,

Many thanks for the replies and info, I'll start looking about for the correct one as I like things original or close to..

Ok then but still a little confused...

So I now know the bike is doing 110km on the speedo and 112km on the GPS but the revs are showing 4500rpms on the taco when SteveD said his is doing close to 4000rpms, maybe a replace gearbox has been fitted to my bike at some stage and upped the revs.

I will install another speedo one day just to compare, this is now sit on my mind and piss me off no end...

[Bob Bennett]

Morning Grant,

Your problem may be the tacho over reading mate,

I do have similar problems with my bike with speedo over reading so used a GPS with similar
results to you (speedo was/is at least 20kph over at 100 k and I can live with that). My bike came with
the original owners hand book and it has some graphs re gear/speed/revs and looking at
the graph 100kph should be about 3750rpm. With the GPS at 100kph my tach is reading
about 4500.
With these results on my high mileage bike I think that both instrument return springs have
succumbed to the ravages of time and vibration.

To confirm we should use a known tach to compare, either mechanical (on the end of altenator)
or electronic (signal from plug lead)

Your bike is looking great and I'm looking forward to seeing it and you at our next meet up

Regards Bob.