I'm getting ready to order a 2WD, Reg Cab EcoBoost with a locking rear diff and I'm struggling with which gears to get. For what I tow, either is adequate but I'm leaning toward the 3.15's to get the best mpg possible. However, I live in a hilly terrain and I hate when the transmission jumps back and forth between 6th and 5th gear going up and down hills. I drove a 3.55 truck and it has no problem at all staying in OD over the hills but my dealer doesn't have a 3.15 to try. Anyone have any experiences to share?

 

I personally think if you are going to do any type if towing the 3.55's would be ideal. You will have that little bit extra pull when needed especially in the hills/mountains. I don't think there will be too much of a difference between the two gas mileage wise IMO.

 

My past experience with gear ratio's tells me there will be 2 mpg difference between the two so I'd like to get the 3.15's if they have the guts to tackle the hills in OD...

 

If you live in a hilly area, get the 3.55's. My buddy and I have identical trucks, except he has the 3.15's and I have the 3.55's. We live in a hilly area and I average about 1-1.5mpg better than him. I've noticed his truck tends to downshift just a little more than mine.

 

90% of my driving is on Interstate 70 (about 30k miles per year)

 

90% on 70 - Then go with the 3.15's The gearing on the 6 speed with the 3.15's should work just fine in that situation.

 

With a truck that light and that much power, you'll have no problem with the stock gears.
Mine is a Screw 4wd EB 3.31 gears and it rarely downshifts.It's curb weight is close to
6000 lbs. You're probably 1200 lbs under that.
At 65, my truck is taching 1500 rpm, and it happily chugs up the mountains where I live. If it does downshift, it's nowhere as obnoxious as the 4th to 3rd in a 4 speed auto.

 

My 3.31's work effortlessly in the mountains. I'd go with the 3.15's considering all the driving you do each year.

 

Why do people think that it is some kind of weakness if their truck can't stay in the highest gear all the time? It is not like the old days of Powerglides or C6's where dropping down a gear means the engine is screaming--5th gear is still a pretty tall economy gear, and 6th is there to give ultra mileage when you are easily cruising along.

I ride bicycles that have between 18 and 27 speeds, and I shift to the gear I need to maintain a 90 rpm leg speed. The only time I use the highest gears is if I want to pedal while going 45 mph down a mountain. Likewise, engines have a power band, and if you had a manual 6 speed, there would be times you would want to downshift to 5th or 4th.

Now if your truck could not go over 40 mph on an uphill regardless of what gear it was in, THAT would be a problem, but shifting down to 5th or 4th occasionally is not a *bad* thing.

If the 3.15 will handle your towing needs, I would really be inclined to go that way unless you are gonna upsize your tires, in which case you would be trashing your mileage somewhat regardless of your rear end gear, but probably less so with the 3.55's.

Happy New Year with a new truck!
George

 

^^^ Great post as usual George! Tried to rap ya, got the message.

Let us all remember the sheer volumes of half tons built over the decades that had 300 I-6's and 3.08 axles. These trucks were not fast, in fact a little doggy. But, they worked hard and they built America. I had two of them.

An Ecoboost mated to a six speed with 3.15's will feel like a rocket compared to anything built prior to 1997.

 

Got him for ya Tim

 

Thank you, gents! Just doing my best explaining...and in 1978 I ordered an F100 with the 300 inch six and a 2.75 axle AND the 4 speed manual OD, giving a final drive of 2.19. THAT was overgeared and I should have ordered a 3.25 axle. But I bought it for a commuter for a 60 mile round trip to work and for that it did great. 1600 RPM at 60 mph (in the days of the 55 mph limit). That was the old toploader 4 speed with 3rd and 4th gears reversed, turning 3rd into an O/D gear, and the countershaft bearings eventually died. But it literally got 20-22 mpg on the road, when most pickups got 12-14 mpg. There are days I wish I had that truck (a short bed flareside) back--about 3600 lbs empty, Ranger trim package, ordered with AC, but manual windows...silver paint that fell off after a number of years...still a tough truck. Love the old dentside pickups.

Likewise I had an '86 GMC 2500 van with a wimpy carbed 305, Turbo 350 with lockup, and a factory 2.73 axle. What a dog on even little hills. Changed over to 3.42 when I had an Auburn posi put in, lost NO mileage and gained a lot of performance--those were the days of overgearing for EPA standards. But a 3.15 on an engine with the grunt of an EB coupled with a 6 speed auto is gonna be fine unless it spends its life towing.

In the old days, an Overdrive was a gear you used when you were gonna be humming along for hours on the Interstate, to cut down engine noise and wear, and kick up gas mileage. My first car was a '65 Sunbeam Alpine (English sports car with about 65 hp) that had an electric O/D behind the 4 speed manual trans. Never used it in daily driving, but worked fine on the road...

Happy Holidays to all,
George

 

I agree great post, and it annoys me too when people think its some kind of achievement that a truck can hold top gear pulling a 6% grade at 70 mph. My 5.0L with 3.55 gears will pull a 6% grade better than any other truck I've ever owned besides my old '05 Cummins Diesel Ram, but it will kick down from 6th to 5th gear to do it.

Although it must kick down, I don't see it as inferior or struggling. The engine is never screaming or racing to tackle the hill, just smooth, linear and solid power.
I had a rental 3.31 EcoBoost 4x4 while my truck was in for warranty work. Im not going to lie that EcoBoost did not kick down a gear to pull the same grade as my 5.0L but I don't consider it a handicap on the 5.0l part because it has to. IMO and I could be wrong as I've done no scientific research on the two, but I would wager my 5L was using less fuel than the EcoBoost as even though my V8 has dropped a gear, the EcoBoost is using full turbo which is equals more fuel in place of dropping a gear.

 

Thanks everyone for your help. I ordered the 3.15's. By the way, shifting in and out of OD doesn't annoy me, it just prematurely wears out transmissions. I drive vehicles for several hundred thousand miles and have never had a failure when driven and geared properly.

 

These transmissions are a whole different animal compared to the old school E4OD, 4R100, 4R70W, etc. My '09 is pushing 115K and still shifts flawlessly - fluid is changed every 60K (severe service). My father's '10 is pushing 150K, same deal.

These trucks are subjected to massively varying loads... from cruising down the road with one person and no other gear, to pulling 10Klbs. The most efficient configuration is one that will maximize efficiency for every load - with that in mind, a truck that will happily cruise in 6th but might need to drop a gear or two every so often when pulling 10K is exactly what you want. Tractor/trailers have quite a number of gears for exactly this reason!

Hunting for gears is bad... but the occasional downshift to pull a hill is no problem whatsoever.