185/75 - MPG improvement?

[Chubby]

New member here, just picked up a new/used Mirage, currently at about 30K miles and looking into first set of new tires.

I do a lot of highway driving at about 70-75 mph and would like to see lower rpms and better mpg (I could go slower, I know). Was wondering if larger tires (and their effective re-gearing) would help? For sake of discussion lets not worry about the spedo or mileage indicator being off.

For example a 185/75 tire would do about 78mph when the stocks would be doing 70 mph and reduce rpm by about 400.

Question is
1- Would it even fit?
2- Is there any mpg advantage under steady state highway driving in switching to a larger diameter or does the increased width negate this?

I have 185X65 X14 tire and kia rims on my 2015 mirage. Make a big difference. lots of room for bigger tire