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

[bwsRam19]

I wonder why they didn't use 3.21 gears in the 2500s

 

[silver billet]

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.

I'm not missing it, it's intentionally ignored due to being next to irrelevant. Technically speaking an obese driver requires more effort to accelerate to 30 than the same truck with a skinny driver, but it's so minimal that it will overshadowed by all the other variables. There are other variables like this, such as some gears in the transmission are more direct/efficient than other gears, but it's pointless to talk about them. Driving style, weight of the truck, octane in use etc are probably far more worth worrying about then the insignificant extra effort that the 3.21 requires.

 

[silver billet]

running at higher rpms in 8th gear means less downshifting

Yes it means less downshifting; and if that is a requirement/preference, the 3.21 can be locked to 7th at which point RPMS will be literally identical to that of the 3.92 in eight; voila, no more downshifting!

 

[silver billet]

I wonder why they didn't use 3.21 gears in the 2500s

Probably because the 2500 is a work truck, not a family hauler like the 1500 has become. Gas mileage is not a priority in the 2500, so they focus on towing/hauling. If the 2500 was driven unloaded for most of it's life, absolutely correct, the 3.21 would be a perfectly sane option to pick. But nobody buys the 2500 to run around empty, to commute to work, to haul kids to school etc etc. They drop a trailer on it and pull 12000 pounds.

 

[bwsRam19]

Yes it means less downshifting; and if that is a requirement/preference, the 3.21 can be locked to 7th at which point RPMS will be literally identical to that of the 3.92 in eight; voila, no more downshifting!

I like the fact that i don't have to play around with the gear limiter

 

[bwsRam19]

Probably because the 2500 is a work truck, not a family hauler like the 1500 has become. Gas mileage is not a priority in the 2500, so they focus on towing/hauling. If the 2500 was driven unloaded for most of it's life, absolutely correct, the 3.21 would be a perfectly sane option to pick. But nobody buys the 2500 to run around empty, to commute to work, to haul kids to school etc etc. They drop a trailer on it and pull 12000 pounds.

ok, but if it did have the 3.21, couldn't they just drop the gear limiter to 5 or 6 and it be the same using your theory?

 

[silver billet]

I like the fact that i don't have to play around with the gear limiter

I like the fact that I have more options than you; if I absolute need/want to prevent an extra few downshifts, I can do that and get same RPM/power as you guys in the 3.92. And when I don't, I have the option to optimize for MPG.

"I want less options", said no one, ever.

 

[silver billet]

ok, but if it did have the 3.21, couldn't they just drop the gear limiter to 5 or 6 and it be the same using your theory?

The difference is in first/second gear, not in 5th/6th. You need the 3.73/4.10 for taking off from a start hauling heavy weights. But yes, once you're at highway speed, as long as two trucks with two different rear ends are driving the same speed, with the same RPM, then they are putting down equivalent power using the same gear ratio, regardless of what numerical gear you are in. It's a mathematical certainty.

 

[bwsRam19]

I like the fact that I have more options than you; if I absolute need/want to prevent an extra few downshifts, I can do that and get same RPM/power as you guys in the 3.92. And when I don't, I have the option to optimize for MPG.

"I want less options", said no one, ever.

The difference is in first/second gear, not in 5th/6th. You need the 3.73/4.10 for taking off from a start hauling heavy weights. But yes, once you're at highway speed, as long as two trucks with two different rear ends are driving the same speed, with the same RPM, then they are putting down equivalent power using the same gear ratio, regardless of what numerical gear you are in. It's a mathematical certainty.

Im perfectly happy with my 20 mpg avg, and not having to **** around with the gear selector and having more towing power off the line, looks like i have more options.

 

[Redtrucks]

So the truck that I’m looking at has a 3.21 and is good for pulling 8500 lbs. I live on flat ground but once or twice a year I will go out to MT or WY pulling a sled trailer which roughly weighs 4500-5000 lbs. With the ZF transmission I will be able to lock 8th gear out when I get to the mountains and the truck should pull just fine?

 

[BowDown]

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.

Trans shifts based on TPS location and engine load MAP sensor. I can drive with a light throttle and be in 4th gear before crossing an intersection. Trans only shifts by speed at WOT so it will not hold the gear longer due to the rear gear, its all load based

 

[BowDown]

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.

That would only play a role if heavily loaded or the ruck was far heavier. Gears are a lever, yes the 3.92 is a bigger lever but the 8 trans negates that by 2nd gear

 

[HSKR R/T]

Trans shifts based on TPS location and engine load MAP sensor. I can drive with a light throttle and be in 4th gear before crossing an intersection. Trans only shifts by speed at WOT so it will not hold the gear longer due to the rear gear, its all load based

Which the load will be different for different gears. But still assuming the trans will always be in different gears to "match" rpm

 

[BowDown]

Im perfectly happy with my 20 mpg avg, and not having to **** around with the gear selector and having more towing power off the line, looks like i have more options.

Towing power difference from a dead stop is minimal if both trucks have similar weights. The gear selector lock is an option, not a necessity

 

[BowDown]

Which the load will be different for different gears. But still assuming the trans will always be in different gears to "match" rpm

Based upon drivers throttle input, not the gear, thats how MAP sensors work.
If you think the load is significantly different, connect a scan tool to the OBD2 port and look at the real time data, specifically vacuum, ignition timing and MAP. A higher engine load will cause timing to go up slightly and MAP and vacuum to drop while TPS is increasing. As I said, I can go through 4 gears with minimal load on the engine and really, 1st would be the only one that mattered because again, after 1st gear, the trans equalizes the difference in the rear gears.
Your theory would be accurate or more importantly be more measurable if say we were talking about a 6500lb truck and 2.71 gears vs 3.92 in an old style trans.

 

[silver billet]

Im perfectly happy with my 20 mpg avg, and not having to **** around with the gear selector and having more towing power off the line, looks like i have more options.

You tell yourself that and you might end up believing it. For a start, nobody in the 3.21 fiddles around with the gear limiter to limit to 8th, because the downshift bothers literally nobody (just like it doesn't bother you when you start slowing down and downshifts occur naturally). BUT, if you're the irritable sort, you still have the option to do that in the 3.21.

Secondly; you're telling me that if you had the option to push a button every time you engage drive, to enable a 9th gear, you wouldn't press it everytime? BS. You would, and you would be happy doing it. You would tell yourself, I'd prefer not to have to push a button, but look at all the gas I'm saving by having this extra gear.

Thirdly; many of us (3.21 and 3.92 alike) hate and disable MDS by using gear selector. We do it everytime we pop into drive. So we're already fiddling with the gear selector anyway.

 

[HSKR R/T]

Based upon drivers throttle input, not the gear, thats how MAP sensors work.
If you think the load is significantly different, connect a scan tool to the OBD2 port and look at the real time data, specifically vacuum, ignition timing and MAP. A higher engine load will cause timing to go up slightly and MAP and vacuum to drop while TPS is increasing. As I said, I can go through 4 gears with minimal load on the engine and really, 1st would be the only one that mattered because again, after 1st gear, the trans equalizes the difference in the rear gears.
Your theory would be accurate or more importantly be more measurable if say we were talking about a 6500lb truck and 2.71 gears vs 3.92 in an old style trans.

I think you are overestimating the whole "equalizing" of the gears of the trans because it was posted that there is a way that the final drive ratio would be similar between the two sets of rear gears

 

[bwsRam19]

You tell yourself that and you might end up believing it. For a start, nobody in the 3.21 fiddles around with the gear limiter to limit to 8th, because the downshift bothers literally nobody (just like it doesn't bother you when you start slowing down and downshifts occur naturally). BUT, if you're the irritable sort, you still have the option to do that in the 3.21.

Secondly; you're telling me that if you had the option to push a button every time you engage drive, to enable a 9th gear, you wouldn't press it everytime? BS. You would, and you would be happy doing it. You would tell yourself, I'd prefer not to have to push a button, but look at all the gas I'm saving by having this extra gear.

Thirdly; many of us (3.21 and 3.92 alike) hate and disable MDS by using gear selector. We do it everytime we pop into drive. So we're already fiddling with the gear selector anyway.

We will just agree to disagree, because we both like what we have

 

[Buz]

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).

Well I’ll be dammed. 3.92 is just a gimmick.

 

[silver billet]

We will just agree to disagree, because we both like what we have

Don't get me wrong, the 3.92 is a great option to have. My issue is more due to the reasons/logic some people are using to justify it, rather than the gear itself. It definitely has a purpose and makes the most sense in some cases.

So if you say: "The 3.92 makes the most sense because the 3.21 is downshifting a lot", then I'm going to say "no, it doesn't downshift a lot, and if a few extra downshifts here and there do annoy you, the gear limiter is an option."

But if you say: " I tow 9500 pounds, and have the payload to support it", to me that is a perfectly valid reason.