With a MPG meter in both of our newer cars I have it without overly close study that we get 1 mpg more at about the 33 mpg level with no alcohol gas.
Are you talking about the cars MPG computer?? If so…stop using it. They are notoriously inaccurate. The BEST way to determine your gas mileage is to divide the number of miles driven since last fill-up by the number of gallons you just added.