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

[flyfingers]

Can't say that I'm actually "worried" about drivetrain strain but rather curious is all. If the 3.21's cause the trans to hunt more often then it could potentially affect trans temps (and therefore fluid longevity) and clutch pack wear. Anyone know if the ZF trans has the ability to lockup the torque converter in all 8 gears? This is what I get for being retired - too much time to think about non-existent potential problems that aren't aren't even in the wheelhouse of ever being a problem.

 

[Rossum]

Having read every post in this topic my question would be which rear end ratio would put less of a strain on drive line components overall? I see the 3.92's seem run approx 21% higher in rpms at hwy speeds than the 3.21's and I wonder if, over say 100k highway miles would that engine (geared with 3.92's) technically have 21% more wear / tear than one geared to 3.21?

Depends what you mean by "strain" and "wear / tear". 3.92 gears will result in higher engine, transmission, and driveshaft speeds; more rotations per mile on the highway. They are also capable (if you press hard on the loud pedal) of putting more torque into the axles in 1st and 2nd gear. 3.21 gears will spin less, but at the same road speed, with the same load, that will mean higher cylinder pressures in the engine and more torque being pushed through the transmission and the driveshaft. The strains and wear/tear will be somewhat different, but I don't think it's possible to say one is worse than the other.

 

[flyfingers]

Ahhhhh.....Thanks Rossum, that's the kind of answer I was looking for. Upon thinking more about it, I suppose it's really a moot point due to the endless variables. Sure wish they offered 3.55's so this wasn't even a topic.

 

[BowDown]

Having read every post in this topic my question would be which rear end ratio would put less of a strain on drive line components overall? I see the 3.92's seem run approx 21% higher in rpms at hwy speeds than the 3.21's and I wonder if, over say 100k highway miles would that engine (geared with 3.92's) technically have 21% more wear / tear than one geared to 3.21?

Neither will cause an appreciable wear, my 3.92 geared truck has 277k miles on it and still ~165psi in all cylinders (174-165). The 3.92 geared truck will turn about 22% higher rpm on the highway though

 

[bwsRam19]

3.92 FTW........LOL

 

[BowDown]

3.92 FTW........LOL

Nah, never again

 

[bwsRam19]

I see your location is Frisco, TX, so I understand why you don't need the 3.92, I live in OK, near the arkansas border, which has hills/mtns.

 

[silver billet]

I see your location is Frisco, TX, so I understand why you don't need the 3.92, I live in OK, near the arkansas border, which has hills/mtns.

Yeah my truck just sits down and whimpers when it sees mountains

 

[Buz]

Ahhhhh.....Thanks Rossum, that's the kind of answer I was looking for. Upon thinking more about it, I suppose it's really a moot point due to the endless variables. Sure wish they offered 3.55's so this wasn't even a topic.

Another way to look at the difference between 3.92vs 3.21 is this:
Assuming you’ve ridden a 10 speed bicycle, the 3.92 is like having the chain always on the smaller front sprocket. Remember how easy it was to climb hills and easily glide along with the chain on the small front sprocket? That’s the 3.92. You didn’t have to muster up as much energy in your legs (torque/driveline strain) than if the chain was on the bigger front sprocket.
The trade off of course is speed, at any given RPM (the rate your legs are spinning the pedals) you aren’t going as fast as your friend whose chain is always on the bigger front sprocket. Aka the 3.21.
You always need to pedal faster to keep up with your friend who’s on the bigger front sprocket. You’ll always be peddling faster (using more gas) to go the same speed as your friend.

 

[HSKR R/T]

Another way to look at the difference between 3.92vs 3.21 is this:
Assuming you’ve ridden a 10 speed bicycle, the 3.92 is like having the chain always on the smaller front sprocket. Remember how easy it was to climb hills and easily glide along with the chain on the small front sprocket? That’s the 3.92. You didn’t have to muster up as much energy in your legs (torque/driveline strain) than if the chain was on the bigger front sprocket.
The trade off of course is speed, at any given RPM (the rate your legs are spinning the pedals) you aren’t going as fast as your friend whose chain is always on the bigger front sprocket. Aka the 3.21.
You always need to pedal faster to keep up with your friend who’s on the bigger front sprocket. You’ll always be peddling faster (using more gas) to go the same speed as your friend.

Not "always" going to use more fuel. As it doesn't take as much energy to get up to speed and maintain. So the RPMs might not be as high at any given speed, with 3.21, but will require more pedal input to get up to and maintain that speed.

 

[BowDown]

Not "always" going to use more fuel. As it doesn't take as much energy to get up to speed and maintain. So the RPMs might not be as high at any given speed, with 3.21, but will require more pedal input to get up to and maintain that speed.

Thats the way rear gears work, the numerically higher gear requires more rpm. More rpm requires more fuel and in this case its 500-600 more rpms. Now, if you think that doesn't make a difference, drive your truck and maintain 1600 rpm, not the fuel economy. Now drive your truck at 2200 rpm and tell e the fuel economy is the same. As for needing more pedal, the 8 spd ZF pretty much negates that theory

 

[HSKR R/T]

Thats the way rear gears work, the numerically higher gear requires more rpm. More rpm requires more fuel and in this case its 500-600 more rpms. Now, if you think that doesn't make a difference, drive your truck and maintain 1600 rpm, not the fuel economy. Now drive your truck at 2200 rpm and tell e the fuel economy is the same. As for needing more pedal, the 8 spd ZF pretty much negates that theory

Not true. I'd be willing to bet around town, stop and go driving the 3.92s would eek out slightly better mileage than the 3.21s. the is if you can keep the same easy application of throttle. and especially if there is a load in the bed or a trailer.

 

[silver billet]

Another way to look at the difference between 3.92vs 3.21 is this:
Assuming you’ve ridden a 10 speed bicycle, the 3.92 is like having the chain always on the smaller front sprocket. Remember how easy it was to climb hills and easily glide along with the chain on the small front sprocket? That’s the 3.92. You didn’t have to muster up as much energy in your legs (torque/driveline strain) than if the chain was on the bigger front sprocket.
The trade off of course is speed, at any given RPM (the rate your legs are spinning the pedals) you aren’t going as fast as your friend whose chain is always on the bigger front sprocket. Aka the 3.21.
You always need to pedal faster to keep up with your friend who’s on the bigger front sprocket. You’ll always be peddling faster (using more gas) to go the same speed as your friend.

This is very wrong, and it's why the 3.92 with the 8 speed in particular is so very horribly misunderstood.

What you wrote is only true if and as long as both trucks are always in the same numerical gear at the same speed. That's not the case. There are enough gears in these trucks so that at any speed (beyond 2n'd gear), both trucks will use the gear that gives identical gear ratios. So at any given speed, both trucks will be at the same (almost identical) RPM at the same speed, which means they are in the same "gear". The 3.92 might be in 7th, but the 3.21 will be in 6th, at the same RPM, at the same speed. Therefore power is identical, RPMs are identical, speed is indentical, just the 3.92 is in 1 gear ahead (which is completely irrelevant).

Only in first/second does the 3.92 have a big enough difference where there is no approximately equal gear to be found in the 3.21. After that, there is an almost identical match between both transmissions, just offset by 1 gear. Power is identical for those gears, just not in first and second.

And even in first and second, the story is not that simple. The 3.92 has the advantage from a stop, until it shifts into second, while the 3.21 is still in first. During that period, the power advantage actually is to the 3.21 in first, not the 3.92 in second (do the math, calculate the final gear ratios; 3.21 in first is higher than 3.92 in second). Until the 3.21 shifts to second where power swings back to 3.92. But once we're in third and beyond, the 3.21 is just in a numerically lower gear, but identical gear ratio at the wheels which is what matters; who cares if you are in 8th or 20'th, what matters is the gear ratio at that point, and since there are 8 gears to choose from, both transmissions will use the gear ratio that gives optimum RPMS for the throttle you are requesting.

 

[BowDown]

Not true. I'd be willing to bet around town, stop and go driving the 3.92s would eek out slightly better mileage than the 3.21s. the is if you can keep the same easy application of throttle. and especially if there is a load in the bed or a trailer.

A, the amount of pedal needed to accelerate is totally dependent upon how fast the driver wants to accelerate, not the gear requirement
B, After 1st and 2nd gear, the differences are minimal at best, see 1st post in this thread, 15.xx vs 18.xx and you'll be out of 2nd gear before you get 1/2 way across the intersection. After 2nd. gear, they are essentially numerically the same so unless you're spending a whole lot of time in 1st and 2nd, its the same until 7th and 8th

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 --

My around town with 3.21 is high 17-18. I have very little highway time

 

[BowDown]

This is very wrong, and it's why the 3.92 with the 8 speed in particular is so very horribly misunderstood.

What you wrote is only true if and as long as both trucks are always in the same numerical gear at the same speed. That's not the case. There are enough gears in these trucks so that at any speed (beyond 2n'd gear), both trucks will use the gear that gives identical gear ratios. So at any given speed, both trucks will be at the same (almost identical) RPM at the same speed, which means they are in the same "gear". The 3.92 might be in 7th, but the 3.21 will be in 6th, at the same RPM, at the same speed. Therefore power is identical, RPMs are identical, speed is indentical, just the 3.92 is in 1 gear ahead (which is completely irrelevant).

Only in first/second does the 3.92 have a big enough difference where there is no approximately equal gear to be found in the 3.21. After that, there is an almost identical match between both transmissions, just offset by 1 gear. Power is identical for those gears, just not in first and second.

And even in first and second, the story is not that simple. The 3.92 has the advantage from a stop, until it shifts into second, while the 3.21 is still in first. During that period, the power advantage actually is to the 3.21 in first, not the 3.92 in second (do the math, calculate the final gear ratios; 3.21 in first is higher than 3.92 in second). Until the 3.21 shifts to second where power swings back to 3.92. But once we're in third and beyond, the 3.21 is just in a numerically lower gear, but identical gear ratio at the wheels which is what matters; who cares if you are in 8th or 20'th, what matters is the gear ratio at that point, and since there are 8 gears to choose from, both transmissions will use the gear ratio that gives optimum RPMS for the throttle you are requesting.

Exactly, these gears are only really different in 1st gear and again in 8th

 

[silver billet]

Not true. I'd be willing to bet around town, stop and go driving the 3.92s would eek out slightly better mileage than the 3.21s. the is if you can keep the same easy application of throttle. and especially if there is a load in the bed or a trailer.

I doubt it. My guess is that in city driving, the 3.21 and 3.92 return identical MPG. MPG is a function of engine RPMS and MPH, not gear ratio. As long as both transmissions can find a gear such that the RPMS are as low as possible for that speed, then MPG is equivalent. So for a given speed (holding say exactly 30 mph for hours on end), one gear ratio will have a lower RPM for exactly 30 RPM). But at another MPH, maybe 20, the other gear ratio might be putting the transmission in a lower RPM. So it's a complete wash, because speed goes up and down in city traffic.

3.21 is only better on the highway, because eventually the 3.92 runs out of gears and can't upshift to drop lower RPMS, whereas the 3.21 has one more gear left.

 

[HSKR R/T]

I doubt it. My guess is that in city driving, the 3.21 and 3.92 return identical MPG. MPG is a function of engine RPMS and MPH, not gear ratio. As long as both transmissions can find a gear such that the RPMS are as low as possible for that speed, then MPG is equivalent. So for a given speed (holding say exactly 30 mph for hours on end), one gear ratio will have a lower RPM for exactly 30 RPM). But at another MPH, maybe 20, the other gear ratio might be putting the transmission in a lower RPM. So it's a complete wash, because speed goes up and down in city traffic.

3.21 is only better on the highway, because eventually the 3.92 runs out of gears and can't upshift to drop lower RPMS, whereas the 3.21 has one more gear left.

You are missing the part about the amount of effort needed during acceleration from a stop. 3.92 won't require the engine/trans to work as hard to get up to speed, meaning less fuel used.

 

[bwsRam19]

Yeah my truck just sits down and whimpers when it sees mountains

running at higher rpms in 8th gear means less downshifting

 

[HSKR R/T]

A, the amount of pedal needed to accelerate is totally dependent upon how fast the driver wants to accelerate, not the gear requirement
B, After 1st and 2nd gear, the differences are minimal at best, see 1st post in this thread, 15.xx vs 18.xx and you'll be out of 2nd gear before you get 1/2 way across the intersection. After 2nd. gear, they are essentially numerically the same so unless you're spending a whole lot of time in 1st and 2nd, its the same until 7th and 8th



My around town with 3.21 is high 17-18. I have very little highway time

You are assuming the transmission will shift differently because of the rear gears. While, granted, it will have an effect on transmission shifting, with the same amount of throttle input, the truck with the 3.92s would theoretically have an easier time reaching speed. Your "essentially numerically the same" comment assumes that the truck with 3.21 will hold gears longer than the truck with 3.92 and always be a gear behind.

 

[bwsRam19]

Wish they had a 4.10 option