Not having a GPS I check odometer accuracy whenever I get to travel continuous highways with reliable km signs (old roads patched with bypasses won't cut it). I also keep an eye on my normal commute trips' measured length. This was that showed me that the new to me car had wrong sized (too small diameter) tires, so I corrected every distance by -3% until I got new (winter) ones. Then the commute length became pretty much the same as our 'unit bike' (Ciliegia, Hyosung GV250 - has the most accurate odometer among our vehicles).
My bike (Teresa, BMW F650CS) is quite a bit different: she seems to be too modest about distance (the speedo is still over, of course), last time I checked (on a Czech motorway) the error was ~2.5%. I usually correct by +2%, to be on the safe side (I don't want to exaggerate my mileage
) - and add the distance I coast with the key off...
When I bought new (Michelin Pilot Road 2) tires I hoped they'll be more accurate than the previous Heidenau K73s (I used +1% correction back then), but the difference just became larger. At least I got a tiny bit taller gears which I couldn't otherwise
This difference should get smaller as the tires wear, but now, ~1mm down from new I still can't measure the effect.