Right now my truck has 6" with 325/65/18's but I'm thinkin bout throwin on a body lift and some 37's. How bad of an idea would it be to run this setup with the stock 3.55's? How much would it kill my gas mileage? Right now I get about 13-14 and I have to drive 35 miles one way to work so I can kill it too bad.

 

well my 97 fl50 i got 25 -22 hiway on 255/70/16 then i went too 285/75/16 and now im at 15 if i have a rear wind too push me along lol anytime u had biger tires it realy guna mess with ur milege and goin with a biger lift means ur less airo dynamic so even worse i would guess u would be at around 10mpg. i drive 30 miles one way too school so i went out and bought a cheap little gas saver so now i can left my truck as high as i want and not worry too much bout the milege. i paid 200 for my gas miser and have another 400 in it and it a great car that will last along time so its not that hard or expisive too do.

 

Lift and going to bigger tires and gas mileage should never be said in the same breath. Gears will help preserve some of what you're looking for. The larger concern here is the strain on the transmission. With 3.55's it puts a lot of force on it and will lead to failure. For 37" tires look at 4.56 gears.

 

Exactly. X2.

 

Ok that's what I thought but I've never had a truck with anything bigger than 35's so I've never re-geared. But there is no way I can afford $1800 in gears so ill prolly just stick with the lift and tire size I have now.