What is the difference at highway speeds like 55 to 90 mph going to the 456 gears. I was told i really need them since I am going up to 35's and i have a 2wd w/ a 4.6.

 

You are aware that the difference in circumference of the tires cancels most of the gear change?
That is why you are putting in the gears after all.

Assume you have 3.55 gears in the truck now, going from 30 inch tires to 35 inch would mean that your new 4.56 gears would equal 3.90 gears with stock 30 inch tires.
Ford sells the truck with 4.11 gears on 30 inch tires stock.

35 inch tires and 4.56 gears would have about the same acceleration and freeway speed as a stock truck with stock 30 inch tires and stock 3.73 gears.

It won't stop as well, of course, because of the huge rotating mass of the big wheels and tires.

So you will hear more gear noise, hear more tire noise, accelerate fractionally better, stop a bit worse.
Chris

 

thanks for that info,, that is what my bud had told me,, what i was really trying to find out was the difference in mpg, rpm, etc at high range speeds, i understand i will be able to get up to those speeds the same but is there going to be any difference in the trucks performance at those "top end" speeds?

 

You will lose MPG mostly because of the much heavier tires/rims. You will lose braking ability, both at normal speeds and higher speeds. At higher speeds it will be more pronounced.

This is just a fact of adding a huge mass of weight to each wheel and it is compounded by the lower gears. Keeping it simple, the lower gears will convert more of your power to heat than the higher gears.

This does not mean lowering the gears is a bad thing. It's more a matter of raising the truck and putting monster tires on it will hurt the handling, power, gas mileage, braking. You are upgrading the gears to get back to where you started in the power department. You are going to lose 1-4 mpg on the freeway and around town. People who say jumping to 35 inch tires and 4.56 gears will not hurt your mileage are either mistaken or kidding.
It's a decision you need to make.

Now putting slightly lower gears in these trucks (say jumping from 3.55 to 3.73, or jumping from 3.73 to 4.10) without changing the tire size can increase your power, cut maybe 1 MPG of freeway and either break even or just cost you a fraction on around town gas mileage.

It's the big tires and lift that are going to reduce your mileage, power etc. Not the gearing. The gearing is just to bring you back to where you started.

Contra wise, if you put the big gears under the truck and did not change the tire size, but did reprogram your PCM so that the truck shifted correctly, you would get a HUGE power increase, probably wheel hop if you floored it. You would get a two or three MPG drop in around town, and about a five MPG drop on freeway mileage.

No matter what you end up doing be sure your PCM is correctly adjusted to report the new tire/gear size. Unless you end up exactly where you are now (that would be for example putting on 38 inch tires if you jumped from 3.55 gears to 4.56 gears) you must get that adjusted. Not just because your speedometer and odometer would be way off, but more importantly, you would be shifting at all the wrong points, leading to early transmission death.

Do read up on it before you undertake tire and gear swaps. It is not a trivial thing to do.
Chris

 

now see that is a damn good answer, thank you very much, I am having the install done at a proffesional shop. being that i have the 4.6 w/ 2wd xlt what initial gearing am i currently at???? Im sure that they know but i would like to do some searching and research on my own also.

 

I suspect you have the common 3.55 gears. It's on your door sticker. Get the code there and the key has been posted.

The 3.73 is common on 4x4 trucks, the 4.10 gears only come with the seven lug big axle.
Chris