'08 F-450 w/ 4:88's. Black Maxx, four inch SS exhaust, delete from turbo back. 77 mph, a little head wind 11.89 mpg
71 mph, no wind, a few hills 11.82 mpg
May as well run it hard. Fueled up today in Colorado for $3.92 per gallon. Knew I had to take credit card inside so that pump would not shut me down at $75. But, it still shut me down at $300, so back inside store to process card...again. 135 gallons. I really wanted to take a pic of the total amount on the pump and post it here. Didn't work. Thought I was going to have to give them my first born. Rats, cuss, darn.
Got to my mountain home and fuel was $4.37 per gallon.
I'm gonna ask why you need to run 77 mph with a F450 and 4.88's and what RPM's are you seeing at that speed. My 4.30's in my F350 at 75 are around 2500, and I don't keep it wound up that tight for long.
Those 4:88's are killing you. As I see it you have 2 options, Gearvendors overdrive or change out those 4:88's for something higher say 4:10's. Remember the 450's are HEAVY, its not really comparable to a one ton. The other option is reducing your speed, I don't know what your results may be but I see a huge difference between 55 and 75mph. But I still cant drive 55...
The F450's (2008-2010) weigh in around 10k curb, vise the 8k of an F350 Pickup. You are definitely geared to tow heavy, and have the hardware to bring such a load to a stop. Before you consider re-gearing or throwing the overdrive set up behind the transfer case consider a few things.
What is the overall cost of that project going to be (Both the Overdrive set up, with a new driveshaft, and the re-gearing)?
How many miles do you drive a year with your F450, and how long will it take to make up those expenses with fuel economy?
Ultimately, are you required to be running the speeds of the OP, and do you need the F450 in the first place?
I'm not trying to insult anyone here, but these re-gear projects never seem to report back the numbers they had intended, and always seem to be a waste of money in the long term. (Expample, would be additional repairs on the drive train after dropping to something like 3.73's or 4.10's and trying to pull a full load of the 4.88's.) Maybe in the end, you can get the job done in an F350?