Very impressed with gas mileage

Verify the odometer againast a GPS. Suprisingly my odoometer is quite accurate. The speedometer is all over the place. My stock '68 225 with 145K has been a consistant 18.5 MPG in mixed driving after a real good tune up (valve lash, float level set,weights in distributor not stuck, vac advance works, heat riser not stuck, etc.).