Miles per gallon?
These cars never got 18-22 mpg, even when new. 15 mpg on the high end and 12 is normal average in mixed driving. 10 mpg in the city is common. None of the more than dozen 225 slants I've owned ever got better than 15mpg, and that was only after looong trips at steady speeds.
I think you are trying to check your mileage too frequently. Average it out over about 500 hundred miles to see what you are really getting. Too much error in a 130 mile trip.
You don't mind if I find that a bit tough to believe? Maybe in the 73/ later smoggers, I would think the earlier cars would do better
Let's have some perspective.
My old 70 RR originally a 390hp, was swapped to a 71 junkard 340 in about 73. So we are talking here about a B body weighing close to 3900 with gas, tools, me, etc. ready to drive. This had a Dana 3.54, ran 3000 RPM at 70, a 4 speed. That 340 on the highway would get an honest 17.5 MPG
My friend when I got out of the Navy in 74 and a nearly new Dodge 1/2T 4x4 with a 360. that thing empty would get real close to 14. We are talking here, a bout a nearly 5K lb pickup with whatever the "big" Armstrong tires were back then, 33" tall or so, and 3.55 gear.
Of course in this case, there is a LOT of "headspace."
What condition is the engine, compression, etc?
Is the carb clean and works right?
Is the distributor advancing correctly, including the vacuum advance?
Just what conditions is the OP getting this mileage? You need a sustained, clear highway cruise to average this, a "long day trip." Around town? I agree, much lower. I would thing you could do 13-14 around town, depending on how you drive, and THAT is a very loaded issue.
Tire size and gearing, and speedometer accuracy is a no brainer. That is, they MUST be accurate.
