Actually, my aero mods are worth over three percent efficiency improvement - lifetime of the car. $5 bucks is a one time expense. A meal at McDonalds, not nothing, but no change of in efficiency. You will have many fewer fill ups than me so efficiency gains are somewhat less meaningful on your model. Just pointing out you don't have to sweat small gas price differences.
As an example in my own life, my hybrid gets a max of 58 mpg in the summer, my motorcycle 78 mpg in the summer. Car holds 12.7 gallons, and I do time fill ups to save a couple bucks out of pocket (just as you are doing). Motorcycle holds 3.6 gallons, most I can save is about 50 cents. So I don't sweat getting the least expensive gas for the bike.
As most of your miles are electric, your price savings on gas (other than trips) may be the most minor expense you have on your car. I understand the OCD nature of this and where you are coming from, but the whole reason I bought the hybrid is that gas price increases are future proofed - this car gets double the mpg of my last car, so gas is now effectively half priced. Your car reduces the effective price of gas by over 90%! You are spending more on electric costs than gas.
If it was me, I'd consider $5 well spent if you get to another OCD goal like 2,000 miles per tank.
2018 Kia LX HEV Metal Stream with Advanced Tech