Hello sports fans- I'm new to 4runners, just bought a 98 SR5 4Runner. Lots of miles (201K) but well maintained, one owner. Had my mechanic THOROUGHLY check it out front to back- and it seems all the issues/ recalls have been taken care of- trans cooler/ radiator, head gask, water pump, timing belt, brake rotors, rear springs all replaced. Compression within spec., no major rust, etc.....The only thing I dislike are cupholders and GAS MILEAGE! So heres the question- will I get better mileage (and how much) if I switch from the stock toyota wheels (16 in. split 3 spoke) with 265/70/16 tires to the stock 15 in. 3 spoke wheels (toyota) with 31/10.5/15 inch tires? I do very little off roading (some dirt, forest service gravel), and mostly city and hiway driving....BTW, love this website- very informative!!!! Thanks for any and all advice, for a newbie...

I doubt you will see any gains in MPG. A 265/70/16 is essentially the same size as the tire you are planning to switch over to. If anything the 10.5 is wider, and may actually hurt you fuel economy.

First, make sure that the 15" wheel will fit over the front brake calipers. I think most, if not all, 98s came with the smaller brakes. It may have been an option for larger brakes that might not fit in a 15" wheel.

Larger wheels will actually slow the performance of a vehicle and will decrease effectiveness of the brakes.

If you take the same car and drive a race track. Run it with the smallest wheel and time laps. Also run it with a large wheel and time laps. (both running good quality tires and both tires are the same diameter) With no other changes, (brakes or engine) the laps with the smaller wheels will have better lap times. This is due to the rotation mass (mainly the outer rim of the wheels, which is the heaviest part of a wheel/tire package) being closer to the hub. Now if you put bigger brakes on (the larger wheels will allow), this setup will have better lap times. Acceleration will be slower but with better brakes, you can drive deeper and faster into corners before braking.

With that said, you should have slightly better performance with the 15" wheels. This should translate into better gas mileage. A 265/70-16 and a 31/10.5-15 are about the same size.

LT tires will weigh more than a P series tire. Higher load range tires will weigh more. More weight will reduce performance/gas mileage.

What kind of gas mileage are you getting?
If it is really low, I would look into another issue. Sometimes replacing the upstream oxygen sensor (the one between the engine, upstream of the catalytic converter) can make huge improvements.

The proper sparkplugs can make a big difference too. These engines require duel electrode plugs (the NGK or Denso).

Yeah, I knew the 31/10.5/15 was basically the same diameter/ size (albeit slightly wider) tire as the 265/70/16. I can pick up 4 stock Toyota 15 inch split 3 spokes for $200 from CL and they are in like new condition. I also have 4 BFG All terrains, (the 31/10.5/15's) w/ 90% tread, that I took off my last truck, so its a cost effective switch. I didnt think about the brake caliper clearance, thanks for that info.
I wonder if the BFG's will rub on the wheelwells, and if some trimming may be needed. Anyone have experience with that size tire rubbing?
BTW, my '98 4R appears to be bone stock- but it looks slightly taller than the '96 and "97 s- was there ever a "factory lift" for 98?
Getting about 15 city, 19 hiway- with new plugs & wires, air filter, cleaned MAS- didnt check or replace the O2....

I don't think you will see a difference.

The 4Runner gets 15-19 MPG real world.

If you cannot get 15 in the city you have a problem.

Different year 4Runners came with different sized coils. Some years "stock" appears higher than others.

About the only thing you're getting is a higher sidewall.

Quite possible your speedo will be slightly off as well.

That MPG is about right depending on how you drive and what you haul around all the time.

Not that you can't go back later but a lot of people have done the "Tundra brake upgrade". These require a 16" wheel. Don't get me wrong, tons have not done the upgrade.