A couple of months ago, I recalibrated both my bikes' speedo-puters by measuring the wheel circumference along a driveway seam, rather than the tire size guidelines. Previously the GPS readings were three-tenths of a mile shorter than what each bike odometer said over 9 miles. And both odometers matched each other. I had had some random GPS oddities so I figured the GPS was innacurate due to signal strength, reflection, etc. Well, after the calibration, they all match now perfectly. However my speeds now read slightly slower, about 1.5 to 2mph under what I was used to. Such a small inaccuracy, but such a big difference.