uner/over drive
 

who make the best one and how bad r they to put in??
also what can i expect from a mpg point i know over will gain mpg and under get ugly but i have a 3:73 rear end and i dont use the bed or hitch all that often but when i do it is usualy a hard work out pulling a car on a trailer or a bunch of tools and scrap form one job or another so i want to help it pull better and we all want better mpg what else can i do does anyone know for sure ??

thank you

Gear vendors is supposed to be really strong.