Change from 31" to 33" mpg

[Tiny Yokum]

75 sii, stock 1968 345 tq, stans headers, flowmaster 40s, 727, 3:73.
The only change I made was going from 31"x10.5x15 to 33"x12.5x15 on alum wheels.
Driving to and from the trail, highway mileage went from 15 down to 9 mpg.
I was turning 2800-3000, now I keep it at or under 2500.
Would changing to 33"x10.5x15 help or just go back to 31" and let the engine suffer ?

 

[Robert Kenney]

You May have caused the engine to require a higher manifold pressure to run the same speed and the carb to run in power mode afr's. Which are 20% richer.

Also multiply tour odometer readings by 1.065 to comp for the larger tires circumference 97.5 - 103.7". You are getting closer to 10 mpg.

 

[Tiny Yokum]

Thank you Robert.