[E.Roy]

Having oversized tires can kill mpg when going down in gear ratio, especially on gas engines in my truck experience. Having higher numerical ratio often means the engine has to put out less work, as long as the rpm isn't too high that engine has to labor to turn itself.

I just recently went from 4.10s to 3.55s in my 7.3 diesel E-350 van with 31.7" tires vs stock 30.5" tires, thinking I'd get a decent 2-3mpg boost in economy, especially steady state highway, best I got was 1mpg improvement in combined driving, probably does worse below 55mph. Maybe 70-75mph it would do 2mpg better.