I have a 2008 with a 5.5 and its 4x4 with 3.73 gears. I recently put on 305/55R20 Nitto Terra Grapplers and as you can imagine (and I knew it would happen and yes I know I have a truck) my mpg and performance declined. If I were to re-gear to 4.54 would that do anything for my engine performance and my mpg? Will it bring it somewhat back to stock?

 

It will definitely bring the performance back, but it won't likely improve the gas mileage.

 

it brought my mileage back up especially on the highway.

 

Gears are the best solution but 4.54 may be a little much since those are about a 33" tire. I would get a programmer if you do not already have one, and I think you will be pleased with the results.

 

will 4.54 be ok? or 4.11? I will do a programmer when my warranty is up so my warranty doesnt get voided.

 

If you take the time to return the vehicle back to stock programming before going to the dealer (~3-5min), they will never know. Therefor you can do it now. It will also adjust your speedometer, etc.

 

I wouldn't do more than a 4.11 on that truck.

 

x2 forgot to post earlier, would stick with 4.11 unless you plan on running at a minimum 35". I think 4.11 would be a good option if you plan on keeping somewhere between 33-35" tires.

 

Thing is, there's not a lot of difference between 3.73's and 4.10's. Too bad you can't get 4.30's for the front.

 

Get a programmer with custom tunes.

You only increased your overall tire diameter a little over an inch over stock height.

If you plan to go 35"+ then go with 4.55 gears.

 

Not to mention if you plan on having four wheel drive work right you have to regear the front too.

 

One reason your mileage has gone down the tubes is because you're not getting an accurate reading. The truck only knows that it is burning way more gas per tire rotation. What the truck doesn't know that it is also travelling further. But the longer distance is not going to completely offset the poorer mileage.