Well I have run a set of 235/85R16's Michelin LTX M/S They are an E load rating..10 ply tire. I recently bought some new tires and went back to the factory P265/70R16's.
I recall when I put the 235's on that I didn't notice a change in ride but now that i went back to the factory size passenger rated tire ..wow..much smoother.
The overall height is about an inch difference and I think this definitely affects your mileage. For what it's worth a Michelin tire is also typically a smoother riding tire than most brands so an E rated tire in say a GoodYear is likely going to ride much rougher.
The stock tire is a 30.7 inch tire. The 235's are almost a 32 inch tire. If you want a LT245/75R16 is a 30.7 inch tire and the width is in between the other 2 options. I think a 245 can be bought in a D or E load rating as well. If you go to a 265/75R16 they are basically a 32 inch tire the same width as the factory 265/70R16's.
If you are concerned about mileage do not buy a taller tire and if you go to a narrower, smoother tread design you should reduce the rolling resistance of the tire, thus best chance for better mileage.
Michelin LTX M/S are excellent tires all year round, wear well and forever almost, are quiet and balance up with usually minimal weights. Check out the tire reviews on Tire Rack for any tire you may consider.
Here is a link to Michelin tire size chart.
MICHELIN | LTX® M/S | Tire Sizes & Specifications
There are several smaller size options in a 16 inch as well. Going too small can bring up you rpm's to get the same road speed though so there is a limit.
that's my 4 cents