I have a 2002 Screw with a 4.6L and it has 35" tires with a 7.5" lift and I currently have 4:56's installed. Would 4:11's or 4:30's be a better option? I drive 60% highway and 40% city, rarely tow anything and I don't live in the mountains. I just don't think I need to be as low as 4:56.

 

The higher the better mpg. You will have to get the speedometer re-calibrated with any gear ratio change.

 

My speedo is already calibrated and can be done again very easily. I already have the 4.56's and I don't like the upshift/downshift so frequently, especially at freeways speeds. I drive around 80 mph on the freeway before its starts vibrating in the rear so thats another reason I want a different gear ratio. I guess I should have just asked 4.30's or 4.11's?

 

The truck will upshift/downshift MORE frequently with the higher (lower numerically) gear ratios because it will need a lower transmission gear to accelerate at any point.

There is also no reason the gear ratio should make the truck vibrate; check your tire balance and roundness. If that's not it, check driveshaft balance.

George

 

Before I lifted my truck and re-geared it, I could drive over 100 mph without any vibration. Since lifting it, putting on larger tires and re-gearing it, I can get to about 80-85 before it starts to vibrate and I'm afraid I'm going to damage something. I want to run at higher freeway speeds without vibration.

 

Yeah somethings not right, I have 35's with 4.56's and my trans doesn't search at all. I do however get the vibration at about 90 but I also need a new driveshaft

 

Dumb question.... have you checked your U-joints? And when you redid your gears, the backlash was set properly I'm assuming? Sorry, just had these two things happen to my truck and I learned the hard way

 

I would stick with 4.56s in my opinion. Im sure you have done the math already but if you havent, consider this. I have 4.56s w 35in tires and a 4" lift w a 5.4L....it really equals out to an "Effective Ratio" of about a 4.10-4.20 gear ratio, doing the math. Esp with you having a 4.6L and 7" of lift....that creates alot more wind drag than most people think. The best I have got on the highway is about 17mpg running 70-75mph. I average 13.5-14mpg in the city. Good luck!

 

I had the gears done by a reputable shop in town so I'm assuming the job was done right. In fact, they checked their installation about a year later because of the vibration and everything checked out okay. The U-joint on the other hand, hasn't been checked so that's my next step with that. Thanks for the heads up.

 

It's not so much about MPG for me but that certainly would be a benefit. After my lift and tires, I drove it around with stock gears for months before the 4.56's and although it did improve in "get up", it wasn't that impressive over the 3.55 stock gears. The best improvement I saw was with the Hypertech tuner. This truck is a pig too, it won't spin the tires on dry pavement, even with the 4.56's so I don't know if going to a 4.11 is going to make things much worse. Would anyone agree?

 

Your truck will never have "impressive" acceleration. It will never spin 35" tires with the 4.6L engine, no matter what gears you put in the axles. The vibration is almost certainly the tires. Those tires probably wiegh double what came on the truck. In addition, with tall sidewalls the wheel balancing wieghts are a long way from the source of the imbalance. You might try the wheel balancing "shot" that is placed in the tires. It balances them as you drive, works good on big tires.

I am trying to figure out what you are trying to cure by changing the gears again. The gears are not causing the vibration. The tuner may be the source of your trans shifting issues, can you alter shift points with it? Otherwise, it is just the seriously overwhelmed 4.6 engine. Time for a supercharger.

 

The vibration is had with 3 sets of tires so far, 2 new sets and one used set. I was also thinking it could be an out of balance drive shaft . The vibration seems to come from the driveline and not the tires. You can hear the humm coming from the center of the rear.

I think I may be trying to cure some mileage issues more than I originally thought I was with the 4.11 idea. And the engine has too many miles for a supercharger...165k.

 

I had the hum too and it seems to be something that's just part of the locker axles. Have the pinion and side bearing been replaced? Axle bearings? What kind of shape are the CLutch packs in?

 

When you lifted the truck, you changed the angle the u-joints had been running at for quite soem time. This can definately induce vibes because they are now at a different angle. A set of new joints and a balance is probably a good place to start. It is also less expensive than you might think.

You killed the milage with the lift and the tires. I doubt moving backwards with gearing is likely to pay for itself. Not to preach but, 80 mph in a lifted F150 on 35's with what I assume are stock brakes is not the safest way to travel. Dropping your speed to 70 would be much safer and more efficient. If you can keep cruise rpm at or below 2000, you will likely do better. Driving a lifted truck 80 mph is not going to give good milage regardless of gearing.

 

I agree. A 7.5" lift will change a lot of suspension geometry and driveshaft angles and I would not expect it to be smooth and steady at high speeds.

If you want a truck for fast cruising, you need to move in the direction of a NASCAR truck, not Bigfoot. At 85 mph with your ultra-high center of gravity, you are an accident waiting to happen. The stock brakes are going to have a harder time with the larger tire weight and diameter, and if you hit a sharp corner or have to do a quick evasive maneuver, your truck will be a hazard. Got a roll bar? You may need it for when you put the truck on its head...

For fast cruising, lower the truck to stock height or lower, go with performance road tires close to the stock diameter, and even consider air dams, etc. Think NASCAR truck.

You say you drive your truck 60% highway and 40% city. I see no mention of off-roading. If you drive on the road, you should optimize your truck for the road, not for mudding... Sounds like you have lifted your truck in order to "wear" it, not to use it.

George