I have no idea how some of you are clicking 20 mpg and 400 miles per tank. I do the math by miles ran and gallons filled every time. Bottom line, my 8000 lb dodge 4x4 diesel on 35's gets better milage then the old 3rd gen 4runner.
I'd probably get better mileage too if most of my driving wasn't stuck in traffic from Orange County to Los Angeles everyday. And when I do get a break from traffic I tend to drive it pretty hard and fast.
Are the fuel tanks identical on all 3rd gens? Toyota didn't modify them at all throughout the run? Those of you who don't get more than 15 gallons.. how empty is your tank? Do you always stop after the nozzle clicks? I can get as much as 4 gallons in after the click.. All of that can't be in the filler neck right? I just pump slow after it cuts off.. It will typically click 4 or 5 more times even pumping slow but I can tell when it's from slosh.. Wait 5 seconds and keep going. Been doing this for years.. Ive never put less than 18 gallons in if the needle is on the bottom hash mark.
... I was thinking of running higher octane during the winter months to reduce the amount of times I visited the gas station.
Is this a good/bad idea?
Unless the higher octane is ethanol free (or cleaner in general), you won't notice a difference in anything except your wallet. The only thing higher octane does is prevent pre-detonation on higher compression-ratio engines... of which ours is not.
I have no idea how you're getting anywhere near 20 gallons. I have run mine dry more than once (i.e., ran out of gas) and the subsequent fill - to the pump click - has never been above 18.5. I don't think the filler neck is 1.5 gallons... no idea how you're doing that.