I used a Garmin GPS with the C5 and stock 18" wheels, and front tire at 18 psi.
RE speedometer GPS
30 mph 25-26 mph
55 mph 50 mph
65 mph 59-60 mph
It is definitely off by 10-11% and appears to be a bit more optimistic than the numbers seen by others on their G5s with 19" wheels. The C5 should definitely get a bit more accurate with a 19" front wheel conversion.
On that note, what is the largest diameter tire that anyone has safely used on the stock 18" front wheel?