Ok, with the 6.0L and 37s, 4.88s are definitely a little low. However, you could get 4.30s with stock tires in the "Tow Boss" packages, and in the F450/550 you can get 4.88s with stock tires. Turning 3k on the highway will definitely hurt your mileage, but so do a 6" lift and 37s, so your numbers really aren't that horrible. One thing though, based on the comparison between our gears and tires, and what RPM I'm turning at 65, you would be turning 3k at 90 mph, and you certainly can't expect to get decent mileage at anything over 70.

 

4.88 is a very low gear!

I paid attention on the highway today and at 70mph I am turning 1750 rpms as shown on my Insight. This is with 37s and 3.73 gears. I'm convinced that 4.30 gears are what I need, but I'm still wondering if they are going to help me on highway MPG. I'm sure the around town stop and go MPG will improve because the engine won't be working so hard to get the truck rolling, but I'm wondering if raising RPMs on the highway is going to cost me Mpg or if the increase will get me into a"sweet spot" where the 6.0's fuel efficiency will go up.

 

yeah i was a little out of it when i posted that. when my spedo is at 55mph, im just shy of 2k rpms, but with the tire difference its about 62mph. If i could go back and do it again i would go with 4.30s and i think my overall average would be better due to a higher highway mpg.

 

My experience:

I bought a 05 F-250 with 3.73 gears and 275/65/20 tires (roughly 34"). I drove it that way for 60k miles then lifted it 8" with 37/13.50/22's. I was waiting on a TruTrac and had to run the 3.73's for a couple of months before I could swap in the 4.30's I had sitting on a shelf. After the gear swap, I lost maybe 0.5 mpg. But my EGT's went down 150* at 70 mph, my trans temp went down 15*, and the truck was a lot more fun to drive. Much better acceleration, both from a stop and from 50-75 mph. Compared to the OEM set up, I'm turning maybe an extra 100 rpm's at 70 mph and more. That's really the only time I notice it (FL highway's are 70 mph speed limit and I typically run 75-80 mph).

Everyone who's said 4.10's for 35's and 4.30's for 37's is spot on, in my opinion. I've driven the truck with the 4.30's for 90k miles and you couldn't pay me to go back to 3.73's, or even to 4.10's.

 

Awesome info there- same size tires as me and a truck about the age of mine. Are you saying your MPG went down on the highway or around town? I would have figured the gears would help in town, but I'm not sure about the highway. Still haven't heard whether lower RPMs on the highway always mean better MPG or if the diesels do better with a little more revs on the highway.

Also, did the gears improve the transmission shifting pattern? I notice mine likes to hunt in and out of OD at certain speeds.

 

In general, fuel economy goes down as RPM's increase because there are more fuel injections. However, as the RPMs get too low, more fuel is needed to produce the same amount of power. There is a sweet spot in RPMs for every level of power output and in general, for the 6.0L cruising on the highway, this tends to be around 1900 RPM. There's a reason why Ford used the 3.73 gears with stock tires for most applications, to get the best mileage around 60-65 mph.

 

Glad I could help a little, and I had a good friend push me re-gear when I lifted my truck. He has the exact same truck as me and I value his experience and guidance. My driving is mixed, and if I keep the truck at 70 mph or lower, my fuel economy is better than when it was on 3.73's. As Cartmanea mentioned, there's a sweet spot for every engine. With 37's and 3.73's, you're out of that sweet spot at almost every speed. My motor was lugging at 55 mph in overdrive, and the truck had to downshift to accelerate with any type of authority. The best way to describe it, and there's no quantitative data I can offer, is that the truck never felt settled on 37's and 3.73's. Once I swapped gears, the truck was back to how I remembered it.

The gears definitely improved the shifting pattern. Without a doubt.