2nd Gen Non-Engine/Transmission tires and gears?

[vaicase]

i have a 94 with a 5 speed and3. 55gears,with255/85r16 tires they are about3inches higher than the suppose to be tires245/75r16. the speedo is about10percent off. i was thinking of going with the proper tires. i know a guy who will trade me. the tread is better than mine. also they are10 ply mine are8ply. what advantages will i get?i plan on towing a lot. i now get 17mpg without figuring the 10percent. i believe that is reading slow. so i am actually going 10 percent farther than it shows right?and i do beat this truck good. i believe in off or to the max. thanksOo.