Fluctuating odometer

Question
When I go riding, I follow the same path every time. Based on my first ride
this spring, the distance according to the odometer was 26.65 km. Over the
course of the summer, as the pedals and gears became gummed up with grit
and dirt, the odometer slowly showed the total distance of the route to be
increasing, eventually reaching 27.1km. After I degreased the chain and
generally cleaned the grit off, the distance dropped back to around 26.65 km.
On subsequent rides, the distance measured began to slowly increase again. I
recently had a complete and proper tune-up, however, and now the odometer
shows the route's total distance to be 26.1km. Given that the odometer is run
off the front wheel, which is unconnected to the pedals or rear wheel (i.e., the
propulsion mechanism), why would changes to the propulsion's efficiency
affect the front wheel's odometer?

Answer
Well as you know the Earth is expanding and what you noticed was the expansion of the Earth's crust...

I can only offer a guess here: The fact that you said your drive train "gummed up" leads me to think that there is interference between the magnet and the sensor. When the magnet on the wheel comes around it closes a switch which sends a electric pulse to the computer - so many pulses so much distance. The only thing I can think of is that the grit is causing a double read - two pulses when there should be one. But this isn't happening every time thus it is not doubling the mileage.

One other thing, check the distance between the wheel magnet and the sensor it should be 1 -2mm of gap in between the two. And make sure that the surfaces of the two are parallel to each other.

The last thing is to check the contact on the handlebar mount and the computer. A light cleaning with a pencil erasure often helps.

Why did it go back to proper readings after the degreasing of the drive train? I really don't know, my only thought is that you may have cleaned your entire bike and the computer started to work again. It s a simple device and if it doesn't work properly there aren't that many things that can go wrong - that is things that can be fixed. If it is the electronics inside the computer... time for a new one. Check the things I listed above and if that doesn't help I guess it is time to start shopping for a new bike! (Well, it is a good excuse to get a new one, isn't it???)

Good luck and let me know how it works out.

Tad