FYI, the Garmin speed sensor is not an accelerometer. It is a magnetometer which senses its own position relative the ambient magnetic field, typically the earth's magnetic field.
I can be affected by strong magnetic fields such as magnets for other sensors. Some have reported that it does not read correctly when riding over bridges built with large iron beams. Dynamo hubs can also cause interference.
http://support.garmin.com/support/searchSupport/simpleCase.htm?caseId={f8f275e0-1284-11e4-eaba-000000000000**&kbName=garmin
The URL above has to be copied and pasted. For some reason it doesn't work when linked.
-Tim-