For any of a number of technical reasons, the ONLY way to accurately and consistently measure speed and distance ridden is with a wheel mounted sensor.
GPS simply can't duplicate that because it it calculates what might be called "connect the dots" distance vs. Actual distance ridden. It's actually pretty good for most purposes, but will never satisfy data obsessed people who look too closely.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.