1. I will throw in another value for this algebra solving equation.

Last week i got new rubber installed. Same Michelin GT4's that were on there before. The used ones were 85% used according to the dealer. (i bought the K1600GT used in December).

So, on the used tires there was a 9 km/h difference. When going 120 on the speedo, the GPS read 111 km/h.
On the new tires i now get a 5 km/h difference. When going 120 on the speedo, the GPS reads 115 km/h.

How does that even compute ? There is no way that the difference in tire diameter from new to 85% used could do so much difference.
Is there ?

(i also used a GPS speedo phone app to compare with the NAV iv and both were exactly the same. So Nav not faulty).

2. Originally Posted by forlornmtl
I will throw in another value for this algebra solving equation.

Last week i got new rubber installed. Same Michelin GT4's that were on there before. The used ones were 85% used according to the dealer. (i bought the K1600GT used in December).

So, on the used tires there was a 9 km/h difference. When going 120 on the speedo, the GPS read 111 km/h.
On the new tires i now get a 5 km/h difference. When going 120 on the speedo, the GPS reads 115 km/h.

How does that even compute ? There is no way that the difference in tire diameter from new to 85% used could do so much difference.
Is there ?

(i also used a GPS speedo phone app to compare with the NAV iv and both were exactly the same. So Nav not faulty).
Measure the circumference of used inflated tire on the rim and then measure the new inflated tire on the rim. Calculate difference as a percentage. You might be surprised.

3. [QUOTE=26667;1048444]
Originally Posted by greenwald
Oh yeah! I brought that up to my BMW dealer when the '36,000 mile' warranty expired on my R1200RT before my 3 years were up, which really only had 33,500 miles on it, due to a 7% margin of error, as verified by GPS.

He just shrugged and said "Oh well - can't fight City Hall (in Berlin)."

So...some of those mileage awards are ...??? undeserved?

and what about IBA people? Do your miles match up between ODO and the IBA route?

where's the emoticon for "can of worms?"
Sorry for the delay. Have been in Colorado for a week.

As for "...mileage awards deserved...", the effort to accomplish so many miles justifies the recognition, in my mind. To each their own, as to those amateur achievements.

As for any sanctioned Iron Butt award, your route is verified by Map Quest or some other distance template that the IBA people in Chicago trust. That is why, for the certificates that already hang on my den wall, I always finished my rides about 50+ miles over the minimum distance, to account for odometer error.

Motorcycle odometers as well as speedometers are, by design, inherently inaccurate - we deal with it.

4. ## Speedometer Error

Of course, what seems to elude BMW and other OEMs is that they could use motorcycle GPS to report speed and the issue of tire size, inflation pressure and so on goes away. Speedometers are "driven" by a cable that usually comes off of the transmission so to the extent the radius of the wheel varies, error is introduced into the speedometer reading. I am sure the day will come where our speedometers will be "drive by wire" as well -- and the issue will go away.

Kent

5. Been awhile since a BMW speedo was cable driven. Using GPS is an intriguing idea but I suspect there are some issues that make it somewhat impractical. Cost is one and lack of reception in some places is another. One would lose reception in tunnels and possibly in deep gorges and mountainous areas.