An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[Olive Green 5.7]

My apologies if this has already been mentioned, but I'm curious to know what the 1/4 mile difference would be in similar equipped trucks with one having the 3.21 gearing vs the 3.91?

 

[Calsun]

Most of the horsepower with normal use is used to overcome air drag. The old Citroen sedans were so aerodynamic that they could cruise at 70 mph using only less than 10 horsepower from the engine. With a truck you have a brick and adding an air dam helps as does smaller mirrors but it is really putting lipstick on the pig. Air drag increases exponentially with speed so there is 4x as much drag to overcome at 80 mph as at 40 mph.

Even towing a trailer it is the air drag that reduces fuel economy so very much and this is compounded by the distance of the front of the trailer from the rear of the cab of the truck.

With tires there is energy required to keep their mass rotating and the heavier the tire the more gas will be burned to turn them. If you want the worst possible fuel economy then lift the truck and put on the largest tires that will fit and drive at 80mph on the freeway. But if you do this then don't whine about the price of gas or how poor the fuel economy is with the truck.

 

[Rossum]

With tires there is energy required to keep their mass rotating and the heavier the tire the more gas will be burned to turn them.

I respectfully disagree with this. Tire mass is only relevant when accelerating. Once the tires are rotating at a steady speed, their mass is no longer a factor. A body in motion tends to remain in motion, and it doesn't matter if that motion is linear or rotational. What causes the most losses in a steadily rotating tire is the deflection at the bottom; this loss is what heats up the tire at speed and can be reduced somewhat by increasing tire pressure. But the main reason bigger tires cause a loss of fuel economy on the highway is again aerodynamics; bigger tires increase frontal area of the tires under the truck and lift the truck (even w/o any changes to the suspension). Both are no bueno for fuel efficiency. In addition, I suspect people also tend to run their big tires at lower pressures than they would smaller/stock tires in order to maintain acceptable ride quality, and this increases losses from the deflections I mentioned earlier.

 

[Jack]

In "City" or "Combined" driving, yes, very much so. But at a steady speed on the highway, the effects of extra mass become close to negligible, and aerodynamic drag becomes the biggest factor. Yes, an air suspension might add a few pounds (but it's not a lot), and if it can put the truck in at attitude that reduces aerodynamic drag on the highway, it is entirely plausible for it to increase fuel economy under such conditions.

1. Heavier trucks suffer more rolling resistance. 2. Even when eliminating acceleration from a stop, roads are not perfectly flat and there are constant incline and decline sections in which the engine will burn extra fuel to maintain speed, and the heavier the truck the more fuel it burns.

 

[silver billet]

My apologies if this has already been mentioned, but I'm curious to know what the 1/4 mile difference would be in similar equipped trucks with one having the 3.21 gearing vs the 3.91?

This is an interesting question, I don't know if anybody here has posted anything regarding this. My guess if the trucks are empty and in 2WD... either the 3.92 wins by a few fractions of a second, or they are even. Quite sure my truck is traction limited. If 4 auto is used, or pulling a load, 3.92 will have a bigger advantage than when empty and in 2wd. Just guessing though.

 

[should_have]

So the point you're suggesting is that since the 3.92 revs higher in 8th, and makes more power due to revving higher, MDS can stay active longer. That's a possibility. However, does it offset the fact that you're revving 400 to 500 RPMS (or whatever amount it is) higher at the same speed (mph)? My belief is no, lower RPMs from the 3.21 is still more effective overall, even if MDS is used less often.

For example: I have gotten 24 to 25 mpg consistently on a 3 hour trip (several times making the same trip), with MDS disabled the entire time (I permanently disable it because I hate how it works and jerks and sounds). Assuming MDS is somewhat effective, the same trip might have given me 25 to 26 mpg with it enabled. I have yet to see anybody with a 3.92 post those kinds of numbers.

Silver Bullet, I guess I'm one of the fortunate one's who is quite happy with my gas mileage with the 3.92 gears. The MDS certainly increases my gas mileage. This past week I was riding from Rehoboth Beach Delaware to Fenwick Island Delaware and could consistently see my Lie-O-Meter jump from 28 Mph to 31 Mph cruising @ 57 Miles Per Hour. So the ride home which included some back roads, Highway with lots of Red Lights, ran into Rush hour traffic, stopped at a Outlet store and the last third of my trip without red lights. I tried staying between 57 and 60, I was able to achieve 25 Mpg. this was using Reg/87 Octane Gas, I'm sure I can get higher if It would have been all highway traffic. I only use 89 when towing, 10Mpg, 65 Mph, for 5K TT Dry Weight. The next time I head down to the beach I will take the long way and record my gas mileage while stay under 60 Mph for long as possible because my wife hated me driving that slow in addition to being in the slow lane.

 

[Rossum]

1. Heavier trucks suffer more rolling resistance. 2. Even when eliminating acceleration from a stop, roads are not perfectly flat and there are constant incline and decline sections in which the engine will burn extra fuel to maintain speed, and the heavier the truck the more fuel it burns.

All true, but a very minor factor compared to aerodynamic drag at highway speeds.

 

[silver billet]

Silver Bullet, I guess I'm one of the fortunate one's who is quite happy with my gas mileage with the 3.92 gears. The MDS certainly increases my gas mileage. This past week I was riding from Rehoboth Beach Delaware to Fenwick Island Delaware and could consistently see my Lie-O-Meter jump from 28 Mph to 31 Mph cruising @ 57 Miles Per Hour. So the ride home which included some back roads, Highway with lots of Red Lights, ran into Rush hour traffic, stopped at a Outlet store and the last third of my trip without red lights. I tried staying between 57 and 60, I was able to achieve 25 Mpg. this was using Reg/87 Octane Gas, I'm sure I can get higher if It would have been all highway traffic. I only use 89 when towing, 10Mpg, 65 Mph, for 5K TT Dry Weight. The next time I head down to the beach I will take the long way and record my gas mileage while stay under 60 Mph for long as possible because my wife hated me driving that slow in addition to being in the slow lane.

Those are great numbers! I don't know man, I just have a theory that MDS isn't effective, but perhaps I should test again if you're getting 25 in a 3.92. But that sort of begs the question; do I put up with the (IMO) annoying behaviour of MDS to gain another MPG... or just drive and enjoy it?

 

[Jack]

All true, but a very minor factor compared to aerodynamic drag at highway speeds.

The ride height adjustment offset to aerodynamic drag is minute compared to added weight.

 

[bwsRam19]

I respectfully disagree with this. Tire mass is only relevant when accelerating. Once the tires are rotating at a steady speed, their mass is no longer a factor. A body in motion tends to remain in motion, and it doesn't matter if that motion is linear or rotational. What causes the most losses in a steadily rotating tire is the deflection at the bottom; this loss is what heats up the tire at speed and can be reduced somewhat by increasing tire pressure. But the main reason bigger tires cause a loss of fuel economy on the highway is again aerodynamics; bigger tires increase frontal area of the tires under the truck and lift the truck (even w/o any changes to the suspension). Both are no bueno for fuel efficiency. In addition, I suspect people also tend to run their big tires at lower pressures than they would smaller/stock tires in order to maintain acceptable ride quality, and this increases losses from the deflections I mentioned earlier.

I disagree with you on this, I have had many vehicles that I have put heavier 10 ply tires on, and the mileage on the highway at speed always went down.

 

[BowDown]

The ride height adjustment offset to aerodynamic drag is minute compared to added weight.

I disagree, once you're moving, weight isn't as big a factor. Punching a huge hole through the air gets exponentially more expensive (horsepower and fuel) the faster you travel. A truck that lowers itself decreases the frontal area and reduces drag

 

[BowDown]

I disagree with you on this, I have had many vehicles that I have put heavier 10 ply tires on, and the mileage on the highway at speed always went down.

Tires and wheels are unsprung weight so thats true, they will impact the fuel consumption but what he said is also correct

 

[Jack]

I disagree, once you're moving, weight isn't as big a factor. Punching a huge hole through the air gets exponentially more expensive (horsepower and fuel) the faster you travel. A truck that lowers itself decreases the frontal area and reduces drag

While that is true when roads are perfectly flat, in reality roads have constant incline and decline sections. While maintaining a fixed speed in the real world, the greater the weight the higher the fuel cost. A few cm lower ride height is not going to off set the weight gain. You can easily test this by loading your truck with weights and do the same section of the freeway drive at the same speed and measure the difference.

 

[BowDown]

While that is true when roads are perfectly flat, in reality roads have constant incline and decline sections. While maintaining a fixed speed in the real world, the greater the weight the higher the fuel cost. A few cm lower ride height is not going to off set the weight gain. You can easily test this by loading your truck with weights and do the same section of the freeway drive at the same speed and measure the difference.

Lol, yeah it will. Plays a huge part in runway 1/2 mile and mile events, something that we pay attention to when setting the car up.
Look how a Trackhawk runs up to about 60 then the acceleration slows. Frontal area and drag are huge. Rolling resistance from heavy tires will impact acceleration but not constant speed driving as much. A lifted truck will act as a parachute at highway speeds due to the frontal area.

Ram has about 38' frontal area (height x width)

2025 Ram 1500 Specs | Dimensions, Weights & More

Explore specifications of the 2025 Ram 1500. Discover fuel economy, GVWR, curb weight, lengths & more on this pickup truck here today.

www.ramtrucks.com

Drag CD of .357
(https://ramtrucklakeelmo.furymotors...00-aerodynamic-improvements-weight-reduction/)
Weight, 5200-5500, mines 52xx

Sultanslot

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www.apexgarage.com

At 38' frontal area and a cd of .357, it takes 34hp to maintain speed.

Add height, changing the cd to conservatively .457 requires 41 hp to maintain speed, nothing else changed and a lifted trucks cd is probably uglier than .457.

Vehicle cd numbers

Automobile drag coefficient - Wikipedia

en.m.wikipedia.org

Now, that same truck weighing 5500 lbs has a power to weight ratio of 13.9 : 1 hp per pound.

Add tires 20lbs heavier x's 4 = 80lbs.
5500 + 80 = 5580lbs and a power to weight ratio of 14.1 : 1 a change of about 1/2 hp.

As you can see, aero plays a bigger role in power loss than tire weight would

 

[Jack]

Lol, yeah it will. Plays a huge part in runway 1/2 mile and mile events, something that we pay attention to when setting the car up.
Look how a Trackhawk runs up to about 60 then the acceleration slows. Frontal area and drag are huge. Rolling resistance from heavy tires will impact acceleration but not constant speed driving as much. A lifted truck will act as a parachute at highway speeds due to the frontal area.

Ram has about 38' frontal area (height x width)

2025 Ram 1500 Specs | Dimensions, Weights & More

Explore specifications of the 2025 Ram 1500. Discover fuel economy, GVWR, curb weight, lengths & more on this pickup truck here today.

www.ramtrucks.com

Drag CD of .357
(https://ramtrucklakeelmo.furymotors...00-aerodynamic-improvements-weight-reduction/)
Weight, 5200-5500, mines 52xx

Sultanslot

SULTANSLOT Link Login Game Terbaru Sultan Slot

www.apexgarage.com

At 38' frontal area and a cd of .357, it takes 34hp to maintain speed.

Add height, changing the cd to conservatively .457 requires 41 hp to maintain speed, nothing else changed and a lifted trucks cd is probably uglier than .457.

Vehicle cd numbers

Automobile drag coefficient - Wikipedia

en.m.wikipedia.org

Now, that same truck weighing 5500 lbs has a power to weight ratio of 13.9 : 1 hp per pound.

Add tires 20lbs heavier x's 4 = 80lbs.
5500 + 80 = 5580lbs and a power to weight ratio of 14.1 : 1 a change of about 1/2 hp.

As you can see, aero plays a bigger role in power loss than tire weight would

0.6 inches lower results in drag coefficient from .457 - > 0.357? Is the air suspension made of snake oil? That math does not add up.

0.360 is F-150's drag coefficient without any air suspension. https://www.motortrend.com/news/2015-ford-f-150-is-most-aerodynamic-f-series-ever/

.45 is like a 05 super duty. https://www.powerstroke.org/threads/2005-f-250-drag-coefficient.266518/
So with air suspension, you're saying Ram 1500 went from 05 super duty to the current gen F150?

 

[BowDown]

0.6 inches lower results in drag coefficient from .457 - > 0.357? Is the air suspension made of snake oil? That math does not add up.

0.360 is F-150's drag coefficient without any air suspension. https://www.motortrend.com/news/2015-ford-f-150-is-most-aerodynamic-f-series-ever/

.45 is like a 05 super duty. https://www.powerstroke.org/threads/2005-f-250-drag-coefficient.266518/
So with air suspension, you're saying Ram 1500 went from 05 super duty to the current gen F150?

Weren't we talking about a lifted truck and or tires vs a stock truck and air ride? Also, that link is a forum where they guessed the cd, not fact. Lastly, the air ride on the ram impacts cd as lifted trucks have a larger frontal area, did you not watch? Even if you cut the cd down, its still more power loss than 1/2 hp

 

[silver billet]

I think we already know what is worse: weight vs height. Loaded limiteds with air ride seem to get far worse highway MPG than lightly speced tradesman/big horns with coil springs. To me that suggests the air ride and lowered height is less important than reducing weight.

 

[BowDown]

I think we already know what is worse: weight vs height. Loaded limiteds with air ride seem to get far worse highway MPG than lightly speced tradesman/big horns with coil springs. To me that suggests the air ride and lowered height is less important than reducing weight.

IDK about that, too many factors in how everyone drives.
There's people here stating 12-14 mpg and others stating 20-23 with the same truck. My limited has gotten 23-25 on the highway and through 3800 miles of predominantly city driving, Dallas area, is at 17.9 mpg. The only real way to compare vehicles is using math and vs everyone's varying driving styles.

 

[Skidowntown]

So are there happy 3.21 owners out there? Seems most are 3.92 guys.

Mine will be 85-90% or more daily driver, so I thought I'd go that direction...in addition, I have the ORG and may consider the bigger Rebel Wheels on the Laramie...all of which could hurt my mpg. I don't drive like Mario Andretti (sp?), so figure I can keep the mileage reasonable - hoping 18+

Listen, I know you don't buy a truck for mileage, and I don't think the one should pick the Diesel solely to save dollars over time, because you probably wont...anyway...I've been picky on this, because it's my first half-ton.

 

[BowDown]

So are there happy 3.21 owners out there? Seems most are 3.92 guys.

Mine will be 85-90% or more daily driver, so I thought I'd go that direction...in addition, I have the ORG and may consider the bigger Rebel Wheels on the Laramie...all of which could hurt my mpg. I don't drive like Mario Andretti (sp?), so figure I can keep the mileage reasonable - hoping 18+

Listen, I know you don't buy a truck for mileage, and I don't think the one should pick the Diesel solely to save dollars over time, because you probably wont...anyway...I've been picky on this, because it's my first half-ton.

I'm a happy 3.21 owner, previous truck was a 3 92 gear and I liked it for a year or two then the fuel mileage got old.
I chose 3.21s for better mileage as the ZF trans is very good and can mitigate the advantage 3.92s after 2nd gear.
I bought a truck because I wanted a truck, I bought the most fuel efficient option for me there was excluding diesel, mileage was not the 1st priority but it was 2nd or 3rd.

The bigger joke is people buying the truck equipped with 3.92s and thinking they are going to run low 13.9x 1/4 miles. This truck is not quick and it will never be quick, the 3.92's aren't going to fix that, when I want fast, I drive my car; I wanted a luxury truck that got better than 15-16 mpg in the city and 22-23 on the highway and I got it. My car gets 12-14 mpg, I don't need another thirsty mouth to feed