With winter gone, I'm thinking of trying these old 15" Civic wheels that I have in my junk pile. Civic tires are 185/65/15, where the Mirage stock size is 165/65/14.
I plugged the tire numbers into this tool:
https://www.blocklayer.com/tire-size-calculator.aspx
And it tells me that change would reduce engine RPM (in all gears) by ~8%.
At 80 km/h (~50 mph), the engine is turning around 2250 RPM on the stock tires. Those Civic wheels & tires would drop it to ~2070 rippems!
The
final drive swap I did to my Firefly dropped cruising RPM by 20% and increased steady state cruising fuel economy by about 5%.
So all else being equal (it's not), this might net around 2% better MPG's.
Assuming I remember to adjust speed because the speedometer will be off (reading lower).
Pontential downsides that might cancel the gearing change:
A: They're heavier.
B: Multiple aero penalties: extra width, plus they'll raise the ride height a bit.