So, let me throw a friendly wrench in the works..
I generally agree with the points Havik made. Yes...mileage sux. Period.
Yes...running 80 mph will make the truck louder than it normally is; not to mention the additional wind noise!
The mathematical difference between 4.56 and 4.88's is only 6.56%.
at 3500rpm the drop is only about 230ish RPM. Your mileage difference, between the two ratios, will be so small as to not hardly notice it at all.
Especially when you consider all of the other variables that can affect mileage on a 4Runner.
FWIW, I run 4.88's and use 33inch tires. I do so because when I hook up the camper I am pulling 45-4800lbs and the camper is 9 feet high. I want ALL of the torque I can get.
Locked in 4th gear, the 4.56's are producing about 1277 Ft Lbs of torque.
By comparison, the 4.88 are producing about 1366 Ft Lbs.
(assuming you are near the engine torque peak in RPM)
I'm a "better to have and not need, than need and not have" guy.
Running 4.88's and 33" tires allows me to run 65ish on the highway at 3500ish RPM and get 10-10.5 mpg pulling the trailer.
IMHO, 4.56 are great for 33' tires, 4.88 work nicely for 34" and 35" should use 5.29's. especially if the truck is 6000lbs or over .
Good luck which ever way you go!
AMLOR
