I've hooked up a bike computer to my commuter. It reports my ride to work as 6.24 miles. My previous measurement was 6.1 miles using gmap-pedometer and a car odometer. The car odometer only has one decimal place, so conceivably it could be something like 6.15. My questions:

How much of the difference in the distances can be attributed to all those little extra turns etc. the bike makes compared to a car?

Is this close enough? If not, how can I get greater accuracy?

Assuming that the reading is off by .1 after six miles, how would this affect the speed reading? For example if the ride's top speed says 25mph, how much slower was I actually going assuming the car odometer is correct and the bike is off a bit?

Thanks in advance!

Your top speed would have been 24.5 mph. The computer setup instructions should include a section on setting the wheel circumference. You need to measure the distance covered, with your weight on the bike, in 2 wheel revolutions and dividing by 2. Then enter this number (mm or cm). Your setting is about 5 cm too high.

Mike

I am an engineer, and worry about such things to an extent bordering on OCD.
Your easy questions first -
If you are off by .1miles after 6 miles, then the error is 0.1/6 = 1.7%, your speed will be off by 1.7% too. Since your car spedometer probably doesn't read finer resolution than 1mph , this would mean that you would have to be going 60mph to have a 1mph error in the speed read.

Things that I have done to convince myself that my computer is set up right:
I will map out one of my routes using DeLorme Street Atlas - there are plenty of others too - map out a good long ride of 25 miles or more, then ride it and compare the number that you get on your computer to what the mapping software says that you should have done.

I have also ridden with a GPS unit attached to the handlebars and compared what the GPS says to what the computer says.

You can adjust the computer by changing the number that you load for tire size - i.e. if you loaded a number of 2125 for your 700c35 tires, and you found that the computer was reading 10%too fast, then you would decrease the number by 10%.

Or you can take the advice of my wife, and just not worry about it.

Generally, if you plugged in the correct tire size, your bike odometer should be more accurate than any car. It is pretty easy to get new tires for a car that differ from the originals by 10% in circumference. Even tread wear can make a 3% difference. That would equal 6.1 miles +/- 0.18 miles for the car just on tread wear. Regarding the geometry of your ride. Swerving around catch basins etc. adds negligible (less than 1%) difference to the ride. Even crossing streets etc. does not add to much. Most of the difference will be in the circumference of the tires. That said, why the interest in 3 decimal points of accuracy. Just tell folks you commute a little over 6 miles. By the way, based on your numbers your top speed may have been only 24-3/4 mph....or, it could have been as high as 25-1/4. You can check your bike odometer setting by marking the wheel and rolling the bike forward. Check it against several revolutions on a smooth surface.

The bike computer probably came with a table of estimated tire circumference values. Don't use these. Get an accurate measurement by directly measuring the circumference with a tape measure, or roll-out test. You can also measure the diameter and multiply by pi, but this isn't quite as accurate. With the proper circuference in place, your bike computer should be much more accurate.

Thanks for the quick & helpful replies everyone!

why the interest in 3 decimal points of accuracy?

No good reason. A little compulsive I suppse. Often I just enjoy the ride but sometimes I like to figure out which routes are faster/better/shorter etc. Also, it is interesting to see how fast I'm going on different parts of the route. For example, I was not too surprised to find myself going about 25mph down a short downhill, but was surprised that I was maintaining that speed on an earlier (flatter) portion of my route. I've only been commuting since June, so quite honestly I am surprised to be hitting 25 mph much at all (:

Your setting is about 5 cm too high.

This is more or less what I expected because I had to mount the sensor/reader a bit farther away from the edge of the wheel than I thought in order to get them to pass closely enough to register. The manual I had was not very clear about how to set things, so I'll have to give it another read.

It is pretty easy to get new tires for a car that differ from the originals by 10% in circumference. Even tread wear can make a 3% difference.

Good point here that I had not considered. I also have a GPS, so I'll take that for a spin as well.

Thanks again!

My commute this morning was 17.60 and yesterday it was 17.61

The poster was referring to the tire circumference dialed in, not the position of the sensor. For example 220 cm vs. 215 in your computer. It doesn't matter where the sensor is since that only counts revolutions.