I have a 1999 SD 4x4 Supercab with the 5.4L. I am planning on buying a electric cooling fan, throttle body spacer and Hypertech programer. I don't know if I want to lift my truck or just run 315/75R16 tires under factory. If I decide to lift my truck, what is the best method without taking a huge hit on my wallet? Also, I have 3.73 gears in my truck now, if I was to put 37x12.5 tires under my truck what size ratio would I need to improve my gas millage?

 

Well, about the only way to lift a truck the size of a superduty is with a suspension lift. Lucky for you, a suspension lift for a superduty is a lot cheaper than a lift for an f150 due to the superduty's solid front axle and leafspring combination. You can get shorter suspension lifts or leveling kits for superdutys for very reasonable prices, but expect to pay more foe taller lifts, which you would need if you went with 37's. If you did go with 37's, I would recommend you getting either 4.56 or 4.88 gears. Don't expect factory gas mileage (could achieve it, but I doubt it), but your performance should be pretty close to factory if not better.

 

Where to start......315/75/16s will be a very tight fit on a stock SD, especially if the springs have sagged any like most do. They will most likely rub the body, but will rub the leaf springs as well. This will also leave no room for any flex. If you're not sure you want to lift the truck I'd look more at some 305s instead of 315s. I've put my 315s up against my 99 and with U code springs there was no way they'd even fit without a leveling kit. I'd also look at getting a set of 8" wheels for the 305s or 315s. You will get better performance out of the tires offroad, longer road life, and a better ride than if they were mounted on the stock wheels. As far as lifts there are leveling kits out there for $200-300 which gives 2-3" out there. There is also the Edge 4.5" hanger kit which is a little more expensive, but not much. The hanger kit and the Edge mini spring pack leveling kit can be used together for more lift. 37s will take the 4.5" hanger kit at minimum to fit them. If you go with 37s (even 315s) forget putting "good" and "mileage" in the same sentance again, even after gears. The only advantage gears will really give you (might get a rise in mileage after the huge hit the tires will take) would be a return in power. For 37s I'd look into 4.88 from an offroad stand point. For 315s I'd look at 4.56 gears.