What's up with my computer?

[Maizie]

I think this is probably a stupid question, but as we've been encouraged to start a new thread today, I'm going to ask it...

When I first got my cycle computer (July 2008 ish), my commute was 20.6 miles (round trip). After the first couple of days, it became 20.7 miles.
That's OK, I thought it must be on the borderline. And it's been 20.7 miles for over a year now.

Then last week, my commutes were 20.6 miles. This week they've been 20.4

Sensor thingies are still all close together. My obvious thought (which is what makes this the stupid question) is that perhaps the battery is on its way out - it is still on the first battery from July 2008.
But then I thought I'd start a thread to see if there are any other ideas!

PS I am pretty sure there has been no significant tectonic activity resulting in my office moving closer to my home.

 

[Theseus]

Maybe you are cutting the corners a bit more and shaving off distance a bit at a time.

 

[dmoan]

Are you keeping your tyres fully inflated? Have you changed tyres recently?

Perhaps different tyre pressures / sizes are leading to different wheel circumferences, which might account for the differences?

 

[Chamfus Flange]

Do you pick your bike up during the commute. If so the cycle computer can still record a spinning wheel as distance travelled (or no distance if wheel not rotating) while you carry it.

 

[Maizie]

Nope, bike remains on the ground throughout. No new tyres, they are generally kept up at the right pressure - do them each weekend.
And I can't believe I could have suddenly lost 3/10ths of a mile in a fortnight just from tighter cornering!

The only difference is that for a couple of the manky steep bits, I do now stand up, but the wheel surely goes round the same number of times...