I am getting a new truck. The old 2006 6.0 is getting pretty tired with 250k on her. I am looking to get a long box F350 Crew Cab. If I get 3.31 gear ratio, will that give me the best MPG. I do pull a trailer that is about 4k when fully loaded and do a lot of highway driving.

 

I think 3.31's would be fine in your application. 4K is a light load for the truck, I have the same gears and it's been great for pulling 10K.

 

Whatdo you average for highway mileage for your truck with the 3.31 rear.

 

The truck is a 2011 F250 CC, I keep track of every fill up and empty is 19.9mi/Imp gal (that would be 16.6 mile/US gal) and towing is 14.3 mi/Imp gal (12 mi/US gal). That's over 85,000 km (~53,000 miles)

 

I have a big 5er with combined weight of 23,000. Plenty of power, takes a big, big hill to slow it down and driving at 65 highway I get 10 mpg.

 

I have the 3.31s in town I get about 15 mpg. On the highway I get about 20 mpg at 70 to 75 mph.

 

my understanding is that the higher the gear ratio (numerically lower) the better the fuel economy. great article on it somewhere...lemme find it

 

That's what I was thinking when I decided to go with the 3.31 rear. With only pulling about 4k, I figured the 3.31's would be ok with the load and give a little better fuel efficiency.

 

I went with the 3.55 and the electronic locking differential. My 6.4 is 3.73

 

I might have this backwards as i often get confused over it. Anyone care to chime in?

 

Sounds like you could get away with an F150 6cyl but you will love the superduty!

 

I have a F250 6.0 and have just under 250k on her. The truck before that was a 2000 F150 manual transmition with the 4.6 v8 and I got 334k on it before she died. That's when I made the move and went with the diesel. the repairs are starting to add up with the 6.0 so I am looking to upgrade to a new F350 6.7. I do a lot of driving so thats why i think the 3.31's will be a little easier on the fuel bill.

 

I have an F250 with 3:55 with 20" wheels and see 20+ on the highway at 65-70 mph

 

The 3.31 comes with the 18" tires. Most folks getting the 3.55 are also getting the 20" rims. The bottom line is the final drive ratio is not that much difference, The tall tires on the 20" rims make up for the 3.55 gears. So you are almost Sixes.

I've driven both a F250 with 3.31 and 18's and my F350 with 3.55 and 20's and have gotten almost exactly the same fuel mileage on 80 mph cross country interstate trips. I get 18 to 18.5 on the interstate at 80 mph, I get 14-16 mpg around town depending on how much stop and go and I get 10 mpg towing. And I tow a lot. My fuelly in the signature below probably reads around 14 for my avg for 50,000 miles. But there are a LOT of towing miles with big 5th wheels, GN Horse trailers and flatbeds with skid loaders in that mileage.

There are more choices in 18: rims, Tires are slightly cheaper. But unless you go to a slightly taller 18" tire, I think you will get almost the same mpg as the 3.55 and the 20s

 

So if I'm connecting the dots correctly the 331s with 18" wheels are similar in gearing/mpg as 355s with 20s. Why does Ford offer these two seemingly similar setups if you end up with essentially the same gearing. Are there any towing differences? Is one setup better ( faster, more torque) for pulling a 10K 5er? the 20s look cool though - maybe that's the reason.