Folks on the Connie forum seem to have ODO readings that are anywhere from spot on to 10% high (none have said theirs reads low).
I checked mine early last year, but have since changed tire model on the front, which I understand will increase the inaccuracy. But I haven’t measured it.
I understand the interest in high MPG, but I personally worry about it too much as it relates to my riding and bike choice. I’ve been able to increase my mileage about 8-10% by riding at a very mild pace and keeping the revs low. But a more natural pace puts me at 38 average, which is fine with me.
At the end of the day, the bike gets nearly double the MPG of my car, gets me to work 35% faster and is provides 175% more smiles. — (OK, I admit to having no stats to back up that last measure)