honestly I don't have enough data to say for sure. I just noticed that I was getting around 370wh/mi when going 75mph for about 10 miles with these extreme summer tires.
another thing to consider is actual tread width.
235/45/18 MXM4 tread width: 7.8"
255/35/18 MXM4 tread width: 9.6"
265/40/18 ADVAN SPORT A/S tread width: 8.7"
As you can see it all depends on the tire for the actual tread width that you get. And this tread width will also play a factor when it comes to rolling resistance. If you get a very efficient 265/40/18, my guess your range hit will be around 8~10% compare to stock 18"
I think a good compromise for look and range is 9 inch wheel +35 offset with 245 wide tires. Overall diameter is only 0.77% larger than OEM 20" and you get 11.4mm more poke so the stance is more flush with fender.
Will They Fit - Online Wheel and Tyre Fitment Calculator