3.31 rear end gears - Diesel Forum

wreed · 03-15-2010, 10:46 PM

Man 3.31 rear end gears are gonna get some good MPG.

Blackdawg · 03-17-2010, 10:30 AM

That's the beauty of having 6 gears and I really like the way Ford spaced them:
3.970 2.320 1.520 1.150 0.860 0.670

I've always been one to switch to a numerically higher axle gear to compensate for the tall 1st and 2nd automatic gear ratio's of the past. But it comes at a highway fuel economy price. Now that this new tranny has a 3.97 first gear followed by a 2.32 second there's really no reason to have to do that. The previous torqueshift and current GM allison tranny start at something like 3.1 first gear.

Look at GM's Allison gear ratio's:

3.100 1.810 1.410 1.000 0.710 0.610

Do the math and you will see that assuming identical od tire size the Ford with 3.31 axle gear has a lower 1st (and 2nd) yet higher 6th gear combined ratio. The Allison stacks up 5/6 gear at .71 and .61, respectively so it looks like a tight spread but it's really not, it's a funky spread imo from 1st to 2nd and 5th to 6th. To me the ford spread is better since you can have a very relaxed 6th for cruising or light work yet lock it out and have a strong 5th gear when towing heavier.

Geared correctly I think this tranny sets up for good highway mileage AND pulling power. I don't think this setup necessarily needs to use a 3.73 or 4.1 axle gear to get the job done unless the truck is a dedicated heavy hauler.

tu3712 · 03-17-2010, 12:38 PM

"Geared correctly I think this tranny sets up for good highway mileage AND pulling power. I don't think this setup necessarily needs to use a 3.73 or 4.1 axle gear to get the job done unless the truck is a dedicated heavy hauler."

What would be the ideal setup on the reared for the best MPG? I am getting a new SD after I see what the dealers get for their stock. If I don't care for what they have I will order one. I just want to know what the best setup for the best MPG is. It will be my daily driver and I won't be using it to tow anything other than a quad.

Blackdawg · 03-17-2010, 01:03 PM

tu, with 735 ft.lbs and almost 400 hp you can go with the tallest axle gear available (3.31?) if you are only towing light. All that power and the low early gears are still way more than enough to get you going without hardly any load on the engine so it shouldn't hurt your city mileage. Unless you plan on adding a significantly larger tire down the road. Plus with the lockout feature and all those gears you can always lock out a gear or two and find a strong towing gear if you ever do pull heavy on the highway.

John Rambo · 03-17-2010, 05:39 PM

Is there a major difference between a 35" and 37" tire in terms of rolling resistance and mileage loss? I know the 37 will weigh more too.

lowerd96 · 03-17-2010, 05:52 PM

So if you put on a leveling kit with say some 35's, your effective ration is going to possibly in the 2.9x area? Maybe 3.05...(I dunno the exact conversion on that. Someone probably does though.) Even with a short 1st and 2nd gear ratio, that seems like not the ideal setup for moving things of any substance around....granted it has a ton of power to make up for gearing. But I am from the days of yore where a lot of gearing made up for lack of power...I feel now that my 3.73 geared 6.4L could use some 4.10's when I hook the heavy trailer up to it...but that's just me.

I'm awaiting the first few towing reports to filter out on these new beasts!

mont91 · 03-18-2010, 10:17 AM

One thing to consider is this engine does not derate its self in 1st to protect the tranny like the 6.4 did. Most people do not know this.The new tranny is designed to take full power from the engine in 1st gear. This info is from Ford.

Dak · 03-18-2010, 06:40 PM

Until I drive them with different gear ratios it is hard to make a good comment on this, much less a decision if I were to order one.

However, IMHO at the moment, unless you do a lot of non-towing, long distance high speed (highway) driving the 3.31 is probably out.

jasonmt · 03-18-2010, 08:26 PM

Quote:

Originally Posted by lowerd96

So if you put on a leveling kit with say some 35's, your effective ration is going to possibly in the 2.9x area? Maybe 3.05...(I dunno the exact conversion on that. Someone probably does though.) Even with a short 1st and 2nd gear ratio, that seems like not the ideal setup for moving things of any substance around....granted it has a ton of power to make up for gearing. But I am from the days of yore where a lot of gearing made up for lack of power...I feel now that my 3.73 geared 6.4L could use some 4.10's when I hook the heavy trailer up to it...but that's just me.

I'm awaiting the first few towing reports to filter out on these new beasts!

The difference in effective ratio between a truck with the factory 275-70-18's and a set of typical 35's is ~.952% so 3.31's go to 3.15...

wreed · 03-18-2010, 10:38 PM

I never tow and drive all day long.

dzzz · 03-22-2010, 11:06 AM

There's a 3.55 option too. That's nice for the trucks that don't tow that will get bigger tires. With the electric locker changing gears later is a lot more expensive.

jgoff78 · 03-22-2010, 03:27 PM

The new motor has unbelievable power not really comparable to my 05 but I think I would still want the 3.73, I don't feel any low end problems and with more power it would be pointless but I still like this setup. If you really want BEST fuel econ you should get the 3.31 in a 250 sc swb drive it like an old lady and you will be set. I wouldnt sacrifice my superduty even if I didnt tow but I want to know why you want a large truck like this if you will only tow a quad every once in awhile. Regardless get the lightest truck, highest gears, stock tires (large rim low rubber if possible like 18 or even 20s) keep the tires inflated and it will be fine. I got 12-13 mpg in my truck for the first 3 years and thought that was the best I could do but after alot of time in the shop ($15000) I'm getting 17-18 when I drive it gently to and from school an hour and a half per day each way. The new emissions garbage will hurt you but if everything is done right I think you could get 17. Ford says 18% increase and my buddies 6.4 350srw ccswb is around 13.5. 18% increase is 16, higher gears should get you another MPG. Good luck

tu3712 · 03-22-2010, 03:39 PM

Quote:

Originally Posted by jgoff78

I wouldnt sacrifice my superduty even if I didnt tow but I want to know why you want a large truck like this if you will only tow a quad every once in awhile.

Short answer why I want one and don't have anything to tow is because I like them and they are comfortable. I am a big guy and had a '05 and loved it. My wife totaled it in Jan. and I planned on going and getting an F-150 because I know I don't need a 3/4 ton truck to hunt out of. I drove my in-law's F-150 ten minutes and said I am getting another 250. I then found out they are coming out with this beast and now I am patiently waiting. As I don’t have anything to tow now I plan on getting a large boat and 5th wheel in the future just don't have one as of yet or see me getting one in the next year or so. I agree that getting the 3.31 would be over kill just for MPG so I will probably go with the 3.73 again. Thanks for the input.

zcellis3 · 03-22-2010, 07:42 PM

The only way you can get the 3.73 rearend is if you go with the 6.7L DRW F-350 or the 6.2L gas engine in any configuration. For the F-250/350 SRW with the 6.7L you can get the 3.31 (Standard for 17"-18" wheels) or 3.55 (Standard for 20" wheels) non limited slip or the 3.31 or 3.55 electronic locker.

Rutcutter · 03-22-2010, 07:43 PM

The 3.73's are no longer an option,in place is a 3.55 with an Electronic Lock rear diff.. With the gearing in the new 6 speed tranny thats more than enough rear gear..