i have currently bought the 3" suspension lift with the 3" body lift and the 35x12.5 r15 bf goodrich tires. i know as well as anyone that my gas mileage is going suck now. but it's worth it to me. my question is does anyone have a suggestion to what kind of rear end ratio i should switch to now. i do a lot of towing in the summer. rumor has it that if i switch my rear end ratio it will make up for the lost gas mileage, have less trouble turning the huge tires, and make it more reasonable for towing my 4,500 lbs boat. any ideas? and perhaps how much it will cost me? thanks for the info.

 

Changing your gears will definetly bring back your performance, and help a bunch with towing too, but you will likely not make any improvments in mpg. Yes, your engine will not have to work as hard with with new gears, but the additional weight and rotational drag of the larger tires is usually too much to see anything but mpg decreases (probably 2-3 mpg at most would be my guess).

You never said what year/model your pickup is, but if it has the stock 3.55 gears (which most 2nd gen Rams do), you will probably want close to 4.56 gears to run the tires you stated (those are about 34.7 diameter). 4.11's are idea for 33" tires, and 4.56 are good for 35's. Here is a nice chart that shows where you are at with various gear/tire combinations.

http://www.4lo.com/calc/geartable.htm

If it is a 4x4, you will need to change gears in both differentials, so expect to pay $1000-$1500 if you have a shop do it.

Good luck, and post some pics when you get her done.