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

[theblet]

m

Direct drive. No gears.

Yes I know lol

 

[theblet]

Electric motors not crippled by connecting rods and valve train, they are probably running more like 6.13's.

They are just crippled by everything else

 

[Rick3478]

They are just crippled by everything else

Cybertruck has a nice big bed for your extension cord. Just play it out as you go and no worry about running out of charge.

 

[Rick3478]

Direct drive. No gears.

Actually, I think most of them do have gear reduction.

Direct drive is possible, but it's hard to get the electro-magnetics down to practical size at low speed. That thing out of Lordstown actually had wheel motors, I guess, but that's a lot of unsprung weight.

With high frequency electronic phase commutation, you can easily get the same power from a small motor spinning rapidly and only need one sturdy reduction gear to match road speed. No clutch, no shifting, so it can be pretty simple and light.

 

[theblet]

Cybertruck has a nice big bed for your extension cord. Just play it out as you go and no worry about running out of charge.

The cord and the generator?

 

[Ramroo]

The cord and the generator?

Need to carry lots of gas for the generator also.

 

[Rick3478]

The cord and the generator?

Nah, leave the generator running in your driveway and use long extension cord. Hurts efficiency too much to tow generator around all the time, only tow it to the truck stop when it needs refueled.

 

[E.Hands]

Back on the gear ratio topic:

If you changed to taller tires but did NOT correct the calibration of your speedometer, then consider this:

If larger tires slowed down your speedometer then it also slowed down your odometer by the same percentage.
If 70 MPH on the speedo is really 75 MPH on the road then you have gone more than 1 mile when the odometer registers 1 mile.
You are getting more miles out of every full tank of fuel than what you are being shown on the dash.

Therefore your calculated mileage, be it on the dash or hand calculated, is now showing less than the true MPG that you are achieving.

We all agree that 3.21 gears will get better mileage on the highway than 3.92, right? But why exactly?
Well, a taller tire just slightly raises (numerically lowers) your gearing. Your crankshaft turns less RPMs for every mile driven.

Of course, this does NOT take into account the increased rotational mass or unsprung weight... but since lowering your speed/RPM whilst in high gear makes such a large impact on MPG, it stands to reason that the higher "effective ratio" of taller tires should theoretically improve efficiency at a particular speed on the highway.
Albeit, so long as you are below the threshold of diminishing returns from the increased wind resistance load or at an RPM to far below peak torque (volumetric efficiency) of the engine.

 

[Mirowpl]

Huh?

 

[E.Hands]

Huh?

In summery, just fill it up & drive ur truck.

 

[MN-Ram]

I hope this post will help to end the debate with facts and not opinions, and become THE post people refer to those who are having a hard time deciding.

You already know that 3.92 is better for towing, and 3.21 gets better fuel economy, so I will talk about what you might not know

Bottom line up front:

In layman's terms, to conceptualize the difference, imagine

1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92.

2) 3.21 has "an extra" overdrive gear.

3) 3.21 has 2 unique lower gears for towing.

4) 3.92 has 3 unique lower gears for towing.

5) Speed range that 3.21 is better at towing: 31-38 MPH, 48-57 MPH.

6) Speed range that 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH.


Explanation


1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92:

Here's the gear ratio for the 8 speed transmission:
1) 4.71:1 2) 3.14:1 3) 2.10:1 4) 1.67:1 5) 1.29:1 6) 1.00:1 7) 0.84:1 8) 0.67:1 Reverse) 3.30:1

Final drive ratios with 3.21

1st. 15.12, 2nd. 10.10, 3rd. 6.74, 4th. 5.36, 5th. 4.14, 6th. 3.21, 7th. 2.70, 8th. 2.15, R 10.6

Final drive ratios with 3.92

1st. 18.46, 2nd. 12.31, 3rd. 8.23, 4th. 6.55, 5th. 5.06, 6th. 3.92, 7th. 3.29, 8th. 2.62, R 12.94

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

2) 3.21 has "an extra" overdrive gear:

The 8th gear in 3.92 is the 7th gear in 3.21, thus effectively mean the 8th gear in the 3.21 is an extra gear to the 3.92.

Meaning, when you go test drive the 3.21 you will have to downshift to 7th to get the same acceleration at 3.92's 8th on freeways. That is why some people complain about how "sloppy" the 3.21 is, because the 3.21 has an extra overdrive gear for fuel economy. If you shift 3.21 in 7th gear, you will get the same acceleration as the 3.92 in 8th on the freeway. No, 3.21 isn't sloppy, you're just in a gear that 3.92 does not have.

3) 3.21 has 2 unique lower gears for towing:

As we know from 1), 5 gears have the same final drive ratio.
You "gain" an overdrive gear, but you "lose" one towing gear.
Here's the final drive ratio for the 2 towing gears.
1st. 15.12, 2nd. 10.10,

4) 3.92 has 3 unique lower gears for towing:

Same logic as the last
Final drive for 3 towing gears.
1st. 18.46, 2nd. 12.31, 3rd. 8.23.

5) Speed range where 3.21 is better at towing: 31-38 MPH, 48-57 MPH,
AND
6) Speed range where 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH:

Calculated towing shift point to be 6000 rpm, if I'm off the logic is the same but the speed will vary.

For towing,
From the speed 0-30 MPH, 3.92 has higher final drive ratio over 3.21 (18.46 vs 15.12) until it has to shift to 2nd gear at 30MPH.

From the speed 31-38 MPH, 3.21 has higher final drive ratio over 3.92 (15.12 vs 12.31) until it has to shift to 2nd gear at 38MPH.

From the speed 39-47 MPH, 3.92 has higher final drive ratio over 3.21 (12.31 vs 10.10) until it has to shift to 3rd gear at 47 MPH.

From the speed 48-57 MPH, 3.21 has higher final drive ratio over 3.92 (10.10 vs 8.23) until it has to shift to 3rd gear at 57 MPH.

From the speed 58-70 MPH, 3.92 has higher final drive ratio over 3.21 (8.23 vs 6.74) until it has to shift to 4th gear at 70 MPH.

The key takeaway here is that towing heavier trailers uphill with 3.21 might never reach the desired speed within the 58-70 MPH range (typical highway towing speed) because 3.21 jumps from 10.10 to 6.74 without the 8.23 final drive ratio found in 3.92 that really help maintaining highway towing speed at max load.

Do you value the "extra" overdrive gear for fuel economy? or do you value the extra towing capability that you tell yourself you might one day need? That's up to you.

What’s tow haul change then in both comparatively? That’s what should be compared for towing not regular drive and over drive

 

[theblet]

Larger tires will lower your gear ratio. So if you are putting 35"+ tires, get 3.92s.

 

[Rick3478]

We all agree that 3.21 gears will get better mileage on the highway than 3.92, right? But why exactly?
Well, a taller tire just slightly raises (numerically lowers) your gearing. Your crankshaft turns less RPMs for every mile driven.

True, but so what? You didn't quite answer your own question so ...

1) Pumping loss. Your engine is actually using some power to create a vacuum in the intake. Heavy throttle opening at low RPM is relatively more efficient than light throttle at high RPM.

2) Friction. At least to a first order approximation, friction is proportional to speed, and for whatever reasons, car and truck engines are built with sleeve bearing technology that has seen only marginal improvement in millennia. In spite of the fact that just about every other rotating part of the vehicle has ball or roller bearings. Motorcycle and marine outboard motors are routinely built with roller bearings. Go figure.

... since lowering your speed/RPM whilst in high gear makes such a large impact on MPG, it stands to reason that the higher "effective ratio" of taller tires should theoretically improve efficiency at a particular speed on the highway.

I wouldn't characterize it as a large impact. To a first order approximation, It still takes a certain amount of fuel to be burned to accelerate a couple tons and maintain speed against air resistance. Marginal improvements come from burning that fuel more efficiently.

In my own unscientific, non-double-blind testing, the difference between 6th, 7th, and 8th gear at a fixed speed on the same stretch of level road falls out to be about 20, 19, and 18 MPG respectively. Tire size achieves basically the same thing.

- EXCEPT -

Taller tires will hurt aerodynamics, letting more air pass under the truck, creating turbulence and drag. You could offset this by lowering the suspension to cancel out tire size.

Just stuff for discussion...

 

[Rick3478]

What’s tow haul change then in both comparatively? That’s what should be compared for towing not regular drive and over drive

Tow haul is discussed extensively in other threads, mostly raises shift points and adds engine braking, equally useful/useless with any final drive ratio IMO.

 

[E.Hands]

True, but so what? You didn't quite answer your own question so ...

1) Pumping loss. Your engine is actually using some power to create a vacuum in the intake. Heavy throttle opening at low RPM is relatively more efficient than light throttle at high RPM.

2) Friction. At least to a first order approximation, friction is proportional to speed, and for whatever reasons, car and truck engines are built with sleeve bearing technology that has seen only marginal improvement in millennia. In spite of the fact that just about every other rotating part of the vehicle has ball or roller bearings. Motorcycle and marine outboard motors are routinely built with roller bearings. Go figure.



I wouldn't characterize it as a large impact. To a first order approximation, It still takes a certain amount of fuel to be burned to accelerate a couple tons and maintain speed against air resistance. Marginal improvements come from burning that fuel more efficiently.

In my own unscientific, non-double-blind testing, the difference between 6th, 7th, and 8th gear at a fixed speed on the same stretch of level road falls out to be about 20, 19, and 18 MPG respectively. Tire size achieves basically the same thing.

- EXCEPT -

Taller tires will hurt aerodynamics, letting more air pass under the truck, creating turbulence and drag. You could offset this by lowering the suspension to cancel out tire size.

Just stuff for discussion...

Interesting... thank you.

By "6th, 7th, and 8th gear at a fixed speed on the same stretch of level road falls out to be about 20, 19, and 18 MPG respectively."

What speed were you maintaining? Was it the same speed in all gear ratios? Are you basing this on the mileage/vacuum gauge in the cluster?
Clearly, better MPG can be attained by driving at 45 MPH than will be attained at 65 MPH. (I'm kinda assuming that is because load & drag increase exponentially?)
But my real world experience driving from Louisiana to New Mexico many, many times showed that maintaining 80 or even 75 MPH burned far more fuel than maintaining 65. That is why I characterized it as a "large impact"
I found that even slower speeds in 8th gear proved even more efficient but I lack the patience to burn a full tank at 55 MPH

 

[CalvinC]

Back on the gear ratio topic:

If you changed to taller tires but did NOT correct the calibration of your speedometer, then consider this:

If larger tires slowed down your speedometer then it also slowed down your odometer by the same percentage.
If 70 MPH on the speedo is really 75 MPH on the road then you have gone more than 1 mile when the odometer registers 1 mile.
You are getting more miles out of every full tank of fuel than what you are being shown on the dash.

Therefore your calculated mileage, be it on the dash or hand calculated, is now showing less than the true MPG that you are achieving.

We all agree that 3.21 gears will get better mileage on the highway than 3.92, right? But why exactly?
Well, a taller tire just slightly raises (numerically lowers) your gearing. Your crankshaft turns less RPMs for every mile driven.

Of course, this does NOT take into account the increased rotational mass or unsprung weight... but since lowering your speed/RPM whilst in high gear makes such a large impact on MPG, it stands to reason that the higher "effective ratio" of taller tires should theoretically improve efficiency at a particular speed on the highway.
Albeit, so long as you are below the threshold of diminishing returns from the increased wind resistance load or at an RPM to far below peak torque (volumetric efficiency) of the engine.

I pretty much confirmed this in real life myself.
Went up to 34.3” tires, but in doing so
- reduced weight at each tire by 2 lbs
- only increased width 20mm (275 to 295)
- went from aggressive to mild AT tread pattern (DuraTrac to Revo3)
- made no other suspension changes (minimal ride height increase - corrected for tire size in both cases to be spot on vs GPS

Tracked years of use in both tires in Fuelly.
Slight uptick in mpg going with bigger tire, but most importantly no downtick.

Usually people going to a bigger tire introduce too many variables to attribute mpg changes solely to tire size - lifts go on, spoilers come off or get trimmed, and usually the first go-round in the brodozer-ization of a new truck calls for the most aggressive tires in. 12.5” width.

 

[E.Hands]

Simple arithmetic:
>Large displacement engine + >more RPMs per mile = more fuel burned.

 

[Rick3478]

Interesting... thank you.

By "6th, 7th, and 8th gear at a fixed speed on the same stretch of level road falls out to be about 20, 19, and 18 MPG respectively."

What speed were you maintaining? Was it the same speed in all gear ratios? Are you basing this on the mileage/vacuum gauge in the cluster?

That's at 55, a speed that all three gears are pretty happy with. And yeah based on the lie-o-meter in the dashboard, which really isn't all that bad, and it's relative anyway.

Clearly, better MPG can be attained by driving at 45 MPH than will be attained at 65 MPH. (I'm kinda assuming that is because load & drag increase exponentially?)

Yeah friction is probably linear, but aero drag may be geometric. 45 is about max for mileage, much slower insists on downshifting. I find myself wishing for lower shift points sometimes. But on the other end of that, I find myself driving around in 7 (or even 6 in town), as it's more quiet, smooth, and responsive.

And all these numbers are well and good, but at the end of the day, I'm gonna drive it.

 

[2021EcoDiesel]

Simple arithmetic:
>Large displacement engine + >more RPMs per mile = more fuel burned.

What we need is an engine that can run at high RPM but low load in a very lean condition. Sipping on fuel and creating good power. But then we need to strangle it with emissions equipment and EGR’s. Ahhh that would be a dream. (Please note the sarcasm here)

 

[Mountain Whiskey]

You guys really believe that engine rpm is the key to milage? My 4000 lb car has a 6.4l, bigger cam, runs 500 rpm higher than my little 5.7l truck. The car gets 4 mpg better. Why? It does not work as hard at a higher rpm to move. Kind of like the truck with proper 3.92 gears as opposed to the truck with underpowered 3.21 gears lugging along. It's not all about rpm. If it were the 6.4 spinning faster would burn more fuel.