No, it wasn't a mistake.....it will help. It's just that they'll never be 30 plus MPG beasts, no matter what some people claim. Simply not gonna happen.
We've fought this battle here before, but I documented my feather getting 31.4 over several tank fulls on a road trip after putting a new clutch in. It had 288,000 miles on it at the time and was so tired you could turn the motor over easily by hand by grabbing the flywheel ring gear. Admittedly, this was with 36 PSI in the tires and it was still running the 195/78/14s.
For what it's worth, the car still has it's unmolested, unrebuilt, unmodified original powertrain so far, is still going to work everyday, and the odometer rolled over for the third time at three thirty in the afternoon two Saturdays ago.