Bicycle Mechanics - avg. speed error on the Cats Eye digital odometer?

My spouse and I both have the Cats Eye digital odometer. Although distance travelled and current speed seem to be accurate by outside measurements, the average speed often decreases when I am going faster than the average speed. For example, if I'm going along North Avenue at a steady 19 miles per hour, it's annoying to look down and see my average speed DROPPING from say, 15.3 mph to 15.1 mph and then staying there or dropping even more.

Is this just a side-effect of using a cheap bike computer, or is there something that I can do to correct this problem?:crash:

Pedal Faster!!!:p

I've got a couple of Cateye's (mighty 2 or 3 I don't recall). I've noticed the average speed seems to do odd things when I ride. I've never checked to see if my mileage divided by my riding time equals the average MPH at any point during the ride. (note that would only work when you are in auto mode where the timer runs when the bike moves).

If you have a choice between auto mode and non-auto mode, you need to choose auto mode. Otherwise it will do just what you are describing, especially in city traffic. It will count time spent at stop lights, stop signs, convenience stores, fast-food joints, etc. as time against your average speed. I doubt that there's anything wrong with the computer itself....

My CatEye Mity 2 (several years old) shows "AUTO" in a little box when that mode is selected--which of course is what I prefer.

My CatEye Velo2 doesn't have any choice. It's always auto mode.

JonR - On my Cats Eye, when I stopped at a stop light, the avg speed does not change. The average speed only changes when I am in motion. So if my avg speed display reads 15.3, stopping at a stoplight won't cause my avg speed to go down, but travelling for several blocks non-stop at 19 mph will cause my average speed to go lower, perhaps to 15.1 or 15.0.

Perhaps there is a delayed reaction, wherein the lack of speed at the stoplight doesn't show up in one's average for several more minutes.

Well, the display never gets updated until motion resumes, and I suspect there's no calculation done till you're underway again, either--but even so I don't see how there could be that much difference. Anybody else got any ideas? I wonder if Sheldon Brown would know the answer--I think he invites questions on the Harris Cyclery website as I recall...

Oh, good question for Sheldon. I run my cateyes in auto mode and still see strange stuff with the average speed.

I just don't worry about it as I figure my distance and time with the tires rolling is accurate and I never mess with average speed. Next time, I'll calculate the average speed and see what I get.

Average speed is calculated thusly
http://sportsfigures.espn.com/sportsfigures/lp_average.jsp?iAm=c

I hope you're using the same techniques to calculate it.

Sorry for sounding like a teacher, but average speed is not total mileage divided by riding time.
Average speed has to do with time spent in intervals and distance covered during each one of them, as the above URL explains.

I completely agree with the website

Directly from the URL:
To find the average speed for a multipart journey, we must take the TOTAL distance traveled divided by the TOTAL elapsed time. Emphasis by the writers, not me.

If a ride 30 miles in 1.33 hours(wheels moving) my average speed is 22.6 mph.

If I include the extra 10 minutes to fix a flat:
30miles/1.5 hours equals 20 mph.

I always set my cyclometer to auto mode so I don't mess up riding time by including stops.

the Cat Eye averages according to distance and speed to average the time, when you slow for a stop or when you accelerate from one your speed falls.
Thus the Cat Eye re-computes your avg. Time accordingly, I've used them for years and found them to be far more accurate than your cars trip meter that avg.s time and speed.

Thanks for the input everyone. I have checked, and AT (auto) is the mode that is set to. The explanations of how average speed should be calculated were helpful, although I may have to investigate further to see exactly how my Cats Eye is using it.

My question was based on the fact that there's an Oak Park cycling group whose rides average 18-20 mph, and I was just trying to figure out my average speed before I showed up for a ride. It will probably be easiest just to show up for a ride and see how I fit into the flow.

I think I may stop looking at the Cats Eye average speed altogether. It still seems counterintuitive why my avg. speed goes down in Auto mode during the moments that I am going faster than the average, but I don't care anymore. I just want to enjoy the ride.....

Right on !!!! :)