I bike commute an identical route most of the time with few variations. Yet my daily mileage often varies by 0.1-0.2 mile over the 22 mile round trip. Why would this be? I am using a wired computer, so it's not due to signal interference that can sometimes affect wireless computers. I pump up my tires to the same pressure every morning before I leave. I travel the same roads because I don't have many other options. One-tenth of a mile might not seem like much, but if you roll it out on your bike it is pretty noticeable -- 528 feet. I can't believe that the actually distance I am riding varies by that much each day.

Any theories as to what could account for the variations?

That does seem like a lot. On my GPS my 9.17 mile (each way) commute doesn't vary by more than 2 or 3 hundredths on any given day.

A 0.5-1% difference seems pretty small to me. You aren't travelling the same exact path. That would account for some of the variation (who knows how much).

Without actually seeing the data, the 0.1-0.2 mile difference might be representative of the typical variation (maybe, these are just the largest differences) and we have no idea what "often" means.

The GPS might actually smooth the measured path out some (and, thus, reduce the variation). To illustrate, if you happen to ride in a weaving way, the actual path traveled by the bicycle might be larger than what the GPS reports.

My theory is that the differences are due to changes in the tire diameter from varying temperatures and air pressures. The distances on my computer seem to be getting shorter as the days get warmer.

That makes sense. One of these days I'll have to compare my GPS with my computer.

Put 10lb less in one day and 10lb more in the next. See what you get.

I wonder if it could just be the variations in the path each day, some days you zig zag a bit more to go around parked cars, take wider corners, etc.

To check this, I propose that you get a paint brush and some yellow lane marking paint and paint a solid yellow line from door to door and ride on that everyday. Then check for deviation. ;-)

I've noticed variations too. i always think is funny but never really worried about it. I chalk it up to taking the inside line on some curves, even subtle curves like up hills etc. sometimes I go back and forth to get up them. you should try marking the ground with chalk and riding he EXACT same line each time each way and see what the computer says ... hahaha. oh I just the read the post before mine .... hahahaha duh ... what he said

My daily morning commute is 8.48mi +.01/-.00. If my front (readout) tire gets 10psi below my normal 110psi I get a consistent 8.49mi commute. If I pump up to 115 I consistently get a 8.47mi commute.

Of course if I make memorable changes to my normal line I see differences as well.

Non-memorable changes from slightly different turn radius or start point of merges or lane position don't show up.

what if one direction has more left turns and you are on the longer outside of the corners, and the other direction has more right turns and you are on the shorter inside of the corners?

One thing to note is if your computer uses a three or four digit calibration - also if the mileage read out is to tenths or hundredths.

I once got a replacement computer with three digit calibration and/or (I don't recall exactly) tenths resolution and I returned it as it would not give the consistent read out my previous one had probably due to rounding errors. Of course one doesn't need to know hundredths, but once you are used to getting within .02mi every commute seeing 8.5 one day then 8.4 the next and 8.6 some other day ain't right.

Tire pressure is huge. When I first got my road bike I didn't pump the tires up. My route to the sea was 23.22 miles, a route I had done a million times.

The next time I pumped my pressure up to 120 psi in both the front and rear. Same exact route and the mileage was 23.04. That's a pretty big difference based soley on tire pressure!

That's less than 1%. Since the relationship between diameter and circumference is, of course, linear, a 1% difference in effective diameter would produce that much difference. Basically, we're talking about your tire compressing about 3-1/2 millimeters extra (3-1/2 on each side = 7). About 1/8".

Makes sense.

most bike computers have a minimum speed at which they begin (and stop) registering. So if on some days you spend more time going very slowly up to or from a stop light or stop sign you're getting closer to your destination, but the computer isn't tracking it.

throw in minor path alterations (such as taking a curve wider than normal) and differences in tire pressure as mentioned above and I think you have your explanation

I don't know man - its crazy -could be the "twilight zone effect" or may a ripple in the space-time continuum - glad it doesn't happen to me - I'd freak - like totally. God bless - good luck getting over it.

As regards Accordian's two rides, the one A made the tires pumped to a higher pressure should result in shorter mileage on the computer, should it not?

Slightly larger diameter, slightly longer rollout ... fewer turns to complete the trip, fewer turns = shorter mileage, as far as the computer knows.

According to Accordion it did.

I'd suspect that this "minimum speed" is very low.

Keep in mind that computers aren't registering speed. They are registering wheel rotations. And one or two (a small handful) of rotations should be enough to cause the computer to start counting.

OT: Hey njkayaker, is that you on the Cyclery USA BikeSpeak forums?

Cateye Strada registers speed the 2nd time the magnet passes the sensor. I can't see how it could with just one pass.

Two wheel vehicles lean when turning so there is some change in the diameter of the wheel. For motorcycles this makes the speedometer inaccurate and may be the same reason your bicycle computer reads differently. If you lean in more sometimes and less other times that will affect the reading.

I know that some computers will let you set the pause speed. Garmin does and my old VDO did. When I first got my Garmin, I went riding with a friend with a Garmin also. At the end of the day his average speed was about 2MPH faster than mine. He had set the pause speed on his Garmin to a value higher than my default from the factory setting.

For crying out loud - if you really want to "see" if the bike ODO misbehaves - then go to a track and walk the bike along a lane stripe directly after reset (.1 of a mile) -then turn the bike around and walk it back - staying on the stripe - this should rule out any "rips" in the space-time continuum -but not interference from Romulans....using cloaking technology to spy on you.

A little info on the "minimum speed" issue: a Cat-Eye Strada will show slow speeds as zero, but still measures distance (by counting wheel revolutions) while doing so.

Experiment: lifted front wheel off the ground and spun it slowly enough that speed showed as zero. After a number of spins, the distance incremented by a hundredth of a mile.

(No, not me!)