No, i have not used a gps unit, what i have done is matched the distance traveled to the nearest mall with my grandpas 07' chevy, his odo had 48, mine had 36 and google maps had 47.4
48/36=1.33
from: Noel, MO to: joplin mo - Google Maps
so i multiply by 1.33, i know my speedos off but know one around here really cares to give me a ticket if i am speeding..
I think im going to go with 4.88s and zing everywhere!