I have a 65 custon 500 (cheap galaxie) that has a 70 302 (500 CFM holley 2 bbl) with a 3 speed manual trans with 3.00 gears,I took it on its first long trip which I put 147 miles on it and used 8.5 gallons of gas which comes out to a little more the 17 which I thought was not too bad but would like to get better. I have a 5 speed from a late 80s mustang and I am planning on installing a 600 CFM 4bbl when I replace the couple noisey lifters so I am thinking the low 20s could be possible with the 5 speed and 3.00 gears. I thought about putting a slightly taller gear in it but the little 302 would have a hard time getting it going so the 3.00 will stay. Even with the 3.00 gears at 70 the 302 sounds like its turning a bit and I do not have stock mufflers so its time for a tach to see how fast the motor is actually turning but overdrive will help a bunch and hopefully not slow the motor too much. Jeff