I drive 20 miles highway and 4 city everyday i got 4.10 gears with 265/70/17, would i see a mpg increase or decrese by going to 275/70/17? or 285/70/17? i know i woudl definatly see some loss on the city but how bout highway? I have a lightfoot on the city so it dosn't matter much.

 

That's about a half inch difference in outside diameter (31.6" vs. 32.2"), which would translate into next to nothing in mileage, either in City or Highway driving.

 

If i put a bigger heavier tire will my rpm go down or up on the highway? how bout a smaller tire? (not so smart when it comes to tire and those kind of calculations)

 

Your rpm will not change relative to your speedo reading unless you get your tune updated for the change in tire size. You will be going faster than indicated by your speedometer if you go with larger tires. If you get your PCM programmed for the larger tires your rpm at a given speed will drop and visa-versa for smaller diameter tires.