Electronics, Lighting, & Gadgets - computer distance accuracy

I just got a Specialized Roubaix with a Giant Continuum Wireless computer. I noticed my distance on the computer is much shorter than the distance I get from Google maps : 6.16 vs 8.54 miles. I checked the circumference setting and it is set correctly for that tire size [I did not measure the circumference directly] Does anybody have a clue which is the distance that I actually traveled?

Seems like a large difference on such a short ride. If it were me---I would (1) check circumference of my tire and the computer setting and (2) since it is such a short distance---I would drive it and check my car's odometer. Surely you would get two out of the three mileages to be close. On most of my group rides of 40+ miles---most all of the group's computers are within 1/2 mile or so of each other.

I was just playing with Google Earth and measured a common 20 mile ride that I do a couple of times a week and the measurment was right on what I get on my cycling computer. The math doesn't work out quite right----but I have to ask---Did you make sure you are set for MPH and not kilometers/hour?

I did check the circumference setting and it was correct - at least, itwas what the manual stated it should be for my tire size. BTW, the bike shop set it up for me. I'm pretty sure it was mph too. I did notice that it was mounted a little low. According to the instructions, it needs to be within 1.8 feet. And, a quick measurement was 1ft 9in or 1.75 feet. Could this be an issue? Anyway if it was km/h wouldn't the number be higher?

As I mentioned---the math did not work out right in the KPH/MPH issue---but I thought I would ask. A 2 mile difference in 8 miles is too much. It seems that either the circumference is wrong or the sensor isn't getting input on every revolution of the wheel or you have some wireless interference problems. I have a Cateye Double Wireless and every once in a while my screen will go to zeros very briefly when I go under a powerrline or something else that gives wireless interference. But I am still real close to a measured distance.

I rode with a friend yesterday about 80 miles.

We were close to 5 miles different when we returned.

Its been my experience that with these cheap electronics made in china we are lucky they work at all.

They were made by women and children who are paid by the piece and then spent 4 months on a boat and then 1 month getting to the LBS.

It's easy to set a computer to be accurate to within 0.5%, and with a little work, I've even managed to get a consistent <0.3% maximum error on one of my bikes.

0.5% over 100 miles is of course half a mile off. That's good enough for most people, I would think.