Here is my dilemma… I have the 4.7 with 3.92 gears and 35” tires. If I stick to the roads it usually takes to get to work, I get around 13.5 mpg. 50/50 city/hwy. Not long ago I went on a trip up to Ga. I filled up and then reset the trip meter. My whole trip was on the highway. Here comes the issue, I got worse mileage on the hwy! I was only getting 10.9 mpg going 70-75mph!! It seems that if I stick to roads that are 55mph I get the best mileage. So the faster I go the worse it gets and as for the city well we wont go there. I wanted to go to 4.56 gears to help out with the 35”tires but now I’m afraid if I do that, the next highway trip will get even worse! What’s going on here? What do I do? By the way I hand calculated my mpg. I know that many of you are going to say “who cares about MPG dude, you have a truck” but that’s not the issue. I am trying to figure out this whole gear ratio thing and if it would be beneficial to me to go with 4.56's.
The other issue I saw on my trip was that sometimes it seems at though the truck didn’t want to shift into that last gear. What I mean is that usually around 55-60mph I’m usually turning 1600-1700 rpms, but occasionally the truck would be turning 2000 rpm and I would have to back off completely and coast it down to about 50mph and then go back to 55mph and it would finally shift into the last gear putting the rpms back to 1600 or so rpm. It’s not like I’m dealing with hills or anything. I’m on flat Florida highway roads. Do you guys think that this issue could be contributed to the bigger tires and the gear ratio??

 

AirRam should be able to help you out with any info and parts related. I seen in a post a few weeks back that you want to be about 2,000rpms with your truck for the best mileage below 70mph.

 

4.7 with 35's cruising at 75? your truck is struggling to keep up the speed, that's why it won't shift. first step should be a programmer. it might help quite a bit.

I am also going to venture and say that the gears will help you because they will let you sit in 5th instead of 4th. the programmer might be enough to do that though.

but don't get gears expecting to make the monye back on mpg, you'll never make it back, but you'll have a lot of fun trying. your truck will feel like a new machine, like the 4.56 gears should have been there all along.

 

Thats good advice! I forgot about getting a tuner so I can catch the computer up on what size tires im running. thanks!! I'll get the tuner then update on what I find.