I'm just looking at a Studebaker Engineering factory chart discussing the mileage >at steady speed< for 1959 Larks with 6 and V8, with conventional, overdrive, and automatic transmissions. For the 170 CID 90hp flathead six (starting at 2577 lbs for a 2dr, about 200 more for a 4 dr, up to 3400 for a wagon) I compared a 53 Aero Ace 4dr (2588 lbs, with the 161 CID 90hp engine in the 685A F head seems a good comparison. Both are about the same weight, about the same aerodynamic profile, and both have essentially the same sorts of transmission/overdrive. The Aero had a 4.10 final drive, and the Studebaker 6 in 1959 'normally' came with a 3.54 for automatic, 4.10 for conventional, and 4.27 for overdrive, but there was also a 3.73, and it seems that any of these R/E ratios could be ordered for either conventional or overdrive, and the Studebaker engineers tested all of them. Now, the F head should have been a bit more efficient than an L head, but the Aero only had 7.6 compression ratio to the "improved 59" Studebaker flathead 8.3 compression ratio. The Aero made torque of 135, to the Studebaker 145 (in 1954, the Studebaker 170 had compression of 7.5, HP 85, and torque of 138 so almost identical, but I don't have the mileage figures) Anyway, I would think that they are essentially a wash comparing the two.
CONVENTIONAL
MPH 3.54 3.73 4.10 4.27
20 23.3 23.1 22.2 21.6
30 25.6 25.5 24.6 23.9
40 23.8 23.5 22.5 21.7
50 20.7 20.2 19.5 18.4
60 17.7 17.3 16.0 15.1
OVERDRIVE
30 29.8 29.7 28.7 27.7
40 29.0 28.9 27.8 26.8
50 25.5 25.2 23.9 22.8
60 21.8 21.6 20.8 20.0
AUTOMATIC
30 23.7
40 21.9
50 19.2
60 16.5
Interesting! The key to getting good mileage with an L or F head is to having overdrive, a numerically low R/E ratio if possible (to lower the revs) even further, and to keep the engine speed down at its maximum torque range, which in early 50s terms for a little six, is in the 40 mile an hour range. Nowadays, you could probably increase the mileage a fair little percentage by running on higher pressure radial tires, which have less rolling resistance anyway. As a side note, I drove my automatic 1957 Studebaker Champion, 3.54 R/E, last summer over two days on a 1000+ mile trip at 55-60 mph all the time on 4 lane highways, and it averaged about 19.5 MPG (US size gallon, not Imperial) running on modern radials and Ford rims.
The Aero as I said has a more efficient engine design, and being a bit lighter and bit more aerodynamic, would likely improve on the Studebaker figures a small but noticeable amount, so 30+ MPG might be possible if the engine and tires are fresh, the overdrive works, and maybe a tad more on radials---if you keep your speed down to 50 MPH.