I don't think I've ever checked my 38 for an exact number. Others will have to chime in but I believe most of the posts I've read about mileage indicate an all around mileage of 16-18. It's not good by today's standards but the cost of gas and pollution weren't a big issue back then.

On my 32 with the original carb my mileage was about 10-12 MPG. I think it was a 212s carb. I now have a 569S #421 and best Ican tell average is from 16-20 MPG depending on the type of driving-flatland or hilly. The 569S #421 is a replacement carb that came out in 1946 for the earlier downdraft carbs. It has a straight throttle arm whereas the later carbs have a bent throttle arm.

My experience is overall mileage on early six cylinders and late 20s is 15-18 for moderate speeds (up to 40-50 mph on the road). The early 1932 is closer to 10-15 depending on speed. The later properly set up gets the 15-18 as above. That is based on 30+ years driving several thousand miles on VCCA tours. As an example I calculate 18 mpg for my '28 Coupe on tours and that has proven to be accurate enough to have 1-2 gallons in the gas tank when the tour ends.