Ideas For Running Two Sets of Tires/Rims—Street & Offroad?

Living in Arizona, we have stark differences in terrain. I have it in mind to purchase two sets of wheels & tires later this year and was hoping to gather all y'all's thoughts on my line of thinking. This question is moot once I have the money in a few years to just drive two separate vehicles. Anyway, I budget several months in advance which is why I'm asking now. Is this a prudent decision or am I wasting time/money because it makes negligible difference?

1. Lightweight Street Tires on Lightweight Aluminum Rims (Keep stock TRD rims?)
   1. Daily driving (Estimated 80% of vehicle life... Gosh here is Pareto creeping up again!)
   2. Maximize mpg
   3. Cheapest reputable tires
   4. Load Rating: B or C (I may be revealing my ignorance here of load ratings)
   5. Appearance is totally irrelevant
   6. Towing is so rare that this isn't a consideration
2. Offroad Tires on Steel Rims
   1. Offroad driving (Estimate 20% of vehicle life)
      1. Capable of serious wet and dry trails and heavy weight, while considering AZ is mostly dry
   2. Load Rating: E
      
   3. MPG is nice to have, but not important
   4. Will probably be installing 6112/5160 or similar according to this thread
   5. Price of tires is irrelevant if based on functional performance alone; I don't care how anything looks.

 

Personally I'd just get a good set of all terrain tires and not worry about swapping wheels out each trip you want to take. This just adds to wear on the wheel studs and adds possibility of breakage or forgetting to tighten the lugs. It happens. If you don't get an E rated all terrain tire you can still get a P rated tire that will provide everything you're looking for in a tire for those 20% trips. Another thing that crossed my mind is losing the ability for those impromptu offroad trips. It would suck to have to go home and change wheels. If you're talking pareto charts then your the kind of guy to put some numbers down and compare the theoretical mpg to dollar savings for a set of light weight street tires vs a set of all terrains for daily use. Don't forget to factor in your time and possible frustration in swapping these wheels on the weekends. I know your going to be 80% street driven but what it will come down to is is the savings (from street tire set) large enough to offset the cost of purchasing another set of wheels and tires? And will you be satisfied changing out wheels that often?
I'm running 285/70/17 on mine with the lift and roof top tent and still pull 17.5 -18.5mpg around town. My buddies stone stock offroad gets 21mpg. Just a super small sample size for some mpg numbers to use. I'm curious to see what your final decision will be! Either way take some pics!

 

I average about 80%/20% on/off road usage. The advice @OmegaMan73 laid out is really about as clear as it can get.

I am running 265/70/17 KO2 C load and only lost about 1 MPG

 

Steel wheels are heavy and not stronger than alloy.

For pavement Michelin Defenders.

For off road, the 255.85.16 seems to be a popular selection, and I think most brands in that size are E.

If you make sure the wheels are the proper dimensions they will fit with no lifting and no rubbing/cutting.