/6 Mileage ?
Drove a 64 Valiant 225 with a three on the tree for 84,000 miles. Kept track of all maintenance and all gas consumed. In 84,000 miles it had averaged 20.1 MPG for every mile it went and gallon it consumed. The best I ever saw was 27 mpg the worst, 13. Those below zero mornings where it idles for who knows how long because a slant six never warms up tend to get it close to the 13 mark. 22 to 25 was my normal highway expectation.
The 225 Valiant convertible I have now with a 225 and 904 has averaged 17 but it's almost all around town and I haven't driven it very far. It's probably been run on the driveway at least once for every time I've actually driven it on the street.