I have a 2000 f250 cc 3.73 6 speed at 70 im running 2100 rpm and 80 about 2400. I have 285 75 16 that are 31.25 tall ( checked with tape).
I was looking to go to a 315 75 16( should be 34.5 tall). How would that help or hurt my power and mpg. I get about 16 to 18 now. I tow about once a month 10000# about 100 miles. Any input? No tuner yet after tires will be getting one soon.

 

what engine?

 

i'd bet your MPG drops to around 10 combined (assuming ur truck is 4X4)..

those tires are a BIG jump from stock and those 3:73's are AWFUL gears in a SD.

i know, cuz i have almost the exact same config. with a V10 mind you. but a 5.4 will be dam near the same MPG and even less power. u basicly will HATE ur truck with those tires.

unless you dont mind regearing...if so then i say jump to 4;88's

 

I have the 7.3 4x4

 

The only thing you'll likely help is the aesthetic properties of your truck. Never have seen someone get better fuel economy, and power would definitely be down without regearing your diffs.