What mileage does your slant six powered ride get?

Some "points of reference" from back then

Friend of mine had a stock 69? Charger with a 318 routinely got OVER 20 mpg on a trip from San Diego to LA (We were both in the Navy, then)

The ill-fated 440 in my 70 RR (was a 6bbl, had a Holley 800/ Edelbrock/ headers) would get 13.8 and that was with "my driving" which was not "careful." Later I swapped a 340 into the car and it would get 17 1/2 plus, routinely. Again, not being very careful. "Getting on a ramp" was not an idle out onto the freeway deal

OTHERS HAVE MENTIONED ^^ timing!!! The 0 BTDC was fro SMOG and no other reason. Search up on what sort of performance advance curve you really want for a slant. One start would be to look up specs for the distributor on a 65/66 slant which is BEFORE emissions. Remember that distributor degrees is HALF crank degrees. Get your distributor set up with a shorter curve

Also make sure the distributor is actually in good shape........Worn bushings and advance mechanism can CHANGE the evident advance curve, along with a busted vacuum advance.

Check the ACCURACY of your timing marks..........piston stop

Exhaust...........Even a low HP vehicle can benefit by a good exhaust, working properly.....no frozen heat riser, etc etc

Fuel........A good properly jetted, properly adjusted carb......including the choke will make a tremendous differenc in fuel mileage. "I ain't the expert" on specifics of slant carburation

The last thing to consider THIS IS JUST an example. Years ago, l a friend had a big 'ol heavy 68 Chev 'Burban, had a 292 six. Got XX mileage, he thought it was terrible. I think it was 12, 14, ?? He spent ALL kinds of time and money converting it to a 283, and guess what?? IT STILL GOT terrible mileage and didn't have ANY more power than when he started.

The point? Sometimes you can shoot yourself in the foot!!!