550 requires lower gearing to better handle it's maximum payload should you decide to run it loaded to the max. I'd be content with the 4.3 gears, it's a good blend of around town pulling performance, and low speed highway cruising. Anything over 65mph, mileage will be below or around 10mpg depending on weight, aero drag of trailer, etc. Most trucks with the 4.88gears top out at around 75-80mph, at which it's not mpg, it's gpm.
If you want highway mileage, you can always look into getting a larger diameter tire when the time comes to replace the stock set. A 3" taller tire can give you less performance around town, but lower rpm on the highway. Mileage won't be much more noticeable unless you're easy on the throttle and keep speeds down. The extra height can work against you some, as it will create more drag when air passes under the truck and smacks into the trailer.
If you want highway mileage, you can always look into getting a larger diameter tire when the time comes to replace the stock set. A 3" taller tire can give you less performance around town, but lower rpm on the highway. Mileage won't be much more noticeable unless you're easy on the throttle and keep speeds down. The extra height can work against you some, as it will create more drag when air passes under the truck and smacks into the trailer.
