My '63 Valiant convertible with pushbutton automatic, 225 aluminum block (recently rebuilt and installed), and 3.23 rear gave me 20 mpg (19.947). I drove 2 - 80 mile trips, one on Monday and one on Tuesday and the test included stop and go driving and highway driving (30%/70%). One trip was solo, the other was with 1 passenger, both days with the top down. I'm happy with that. Does that seem reasonable?

Was the top up or down?

Cruising in a '63 Vert with the top down, and you're thinking about fuel economy? Think smiles-per-gallon!
Sometimes, it's about more than money.
Not too bad, though. Aerodynamics and the 3.23 gears probably hold you back a bit on the open road. Mileage might improve a bit as the motor breaks in.

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

The smiles-per-gallon are off the scale right now . I'm thinking that mpg is just another measure of overall operation and efficiency. On a single road trip maintaining even speed, I'm sure I could do better but just want to compare my mpg to what others are getting out of their slant 6's. BTW I'm running the stock 13" X 4.5" wheels with 185/80 13 radials and, so far, have never driven the car with the top up thanks to the weather channel's "local on the 8's" and the fact that the car has only been roadworthy since May.

It sounds like you have a fun car!
My '64 Valiant V200 4-door sedan, with a 225 and pushbutton 904 has hit 26 MPG, loaded, on a long run at 55-60 MPH. Most of the time it's in the low 20's on the highway at 65-70, high teens in town.

_________________
"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

That's really pretty amazing considering the technology of the day.

Yeah and gas was 28 cents a gallon and 'tree huggers' were still in their diapers, sort of speak

Never say "Never". 30% chance of rain yesterday turned into a downpour while in route to my favorite watering hole. Put what's left of the top up. I stayed dry but since the back plastic window is out and the top is ripped on both back sides - Drats! Good thing my new carpet came in. I saw 2 hoses routed in the trunk from the well for the top. I pushed them back in the rubber grommets that directs the water to the ground last week when I was in the trunk trying to figure out why the power top doesn't work.