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

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I have to ask about some statements Jack made in his analysis in the first post:
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.
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My truck (3.21 axle) has never ever stayed in 1st gear up to 38 MPH regardless of what I'm towing. Jack also said:
Calculated towing shift point to be 6000 rpm, if I'm off the logic is the same but the speed will vary.
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6000 rpm is a much higher shift point than my truck has ever shifted at (I assume 1st gear at 6000 rpm is 38 mph on the 3.21). I'll have to pay attention but it seems like shifts typically happen at 3000 rpm or less for me. As he said, if that shift point changes, the optimal towing speeds change, but the ideal towing gears for each axle remain the same.

So the question - what's everyone else's experience? Do you guys get up anywhere close to 6000 rpm before the shift happens?
 
give more right foot and it will shift at a higher rpm.

Those are "calculated" numbers though. So real world experiences will vary due to load, incline, throttle position.
 
trouserpants said:
I have to ask about some statements Jack made in his analysis in the first post:

My truck (3.21 axle) has never ever stayed in 1st gear up to 38 MPH regardless of what I'm towing. Jack also said:

6000 rpm is a much higher shift point than my truck has ever shifted at (I assume 1st gear at 6000 rpm is 38 mph on the 3.21). I'll have to pay attention but it seems like shifts typically happen at 3000 rpm or less for me. As he said, if that shift point changes, the optimal towing speeds change, but the ideal towing gears for each axle remain the same.

So the question - what's everyone else's experience? Do you guys get up anywhere close to 6000 rpm before the shift happens?
Click to expand...

You're seeing part throttle shifting, go WOT and see what happens, mine shifts at 6000 at WOT. My previous truck, 2003 1500 5.7 3.92 also shifted at 6000
 
Anybody on here running 3.21 with 35's and a lift? I am in the ordering process and going back and forth between the 3.21 and 3.92. I hardly tow anything and don't care much about the "sporty" feel. Would mainly be for the strain on transmission or severe pedal lag with heavy tires and lift. Appreciate any feedback!
 
Steveng8 said:
Anybody on here running 3.21 with 35's and a lift? I am in the ordering process and going back and forth between the 3.21 and 3.92. I hardly tow anything and don't care much about the "sporty" feel. Would mainly be for the strain on transmission or severe pedal lag with heavy tires and lift. Appreciate any feedback!
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The severe pedal lag is just programed into the truck. Various pedal commander style devices out there for the lag.

There is no real strain to worry about. I mean it is a truck and meant to haul stuff no matter if you have a proper ratio or not.

You touched on the real difference. The size and weight of the tires and you will also have extra drag from the lift needed to accommodate the 35s. Many here think milage and all is just about rpm. Your truck will need a little help to push the extra resistance. The 3:92 will help with that and get you up the road better.

And let's be honest with yourself, you say you are not concerned with the sporty feel but you bought a gas hog 5.7? You could have done much better with the V6 and saved some bucks to boot. Nah, what fun would that be? 😄
 
Mountain Whiskey said:
The severe pedal lag is just programed into the truck. Various pedal commander style devices out there for the lag.

There is no real strain to worry about. I mean it is a truck and meant to haul stuff no matter if you have a proper ratio or not.

You touched on the real difference. The size and weight of the tires and you will also have extra drag from the lift needed to accommodate the 35s. Many here think milage and all is just about rpm. Your truck will need a little help to push the extra resistance. The 3:92 will help with that and get you up the road better.

And let's be honest with yourself, you say you are not concerned with the sporty feel but you bought a gas hog 5.7? You could have done much better with the V6 and saved some bucks to boot. Nah, what fun would that be? 😄
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Thanks for the reply! Yeah I'm leaning towards the 3.92's. I'm looking at the 3.0 eco diesel, will be a big upgrade from my Tacoma! My current truck moves like a slug so either way will be a huge upgrade haha
 
Steveng8 said:
Anybody on here running 3.21 with 35's and a lift? I am in the ordering process and going back and forth between the 3.21 and 3.92. I hardly tow anything and don't care much about the "sporty" feel. Would mainly be for the strain on transmission or severe pedal lag with heavy tires and lift. Appreciate any feedback!
Click to expand...

It can be done, but if you havent purchased yet get the 3.92s. They will help your acceleration. . You’ll just be turning a few extra rpm on the Hwy
 
Steveng8 said:
Anybody on here running 3.21 with 35's and a lift? I am in the ordering process and going back and forth between the 3.21 and 3.92. I hardly tow anything and don't care much about the "sporty" feel. Would mainly be for the strain on transmission or severe pedal lag with heavy tires and lift. Appreciate any feedback!
Click to expand...
I did some calculations for the difference between a standard 32” wheel and a 35” wheel, without showing all the math calculations, it turns out to be very close to 10% bigger, if your speedometer is not corrected it changes everything, your miles per hour and your mpg will be off by 10%.

Going to a 35” wheel your speedometer will be off by just under 6 mph.

I also calculated the amount of revolutions per mile for a 32” wheel and by going with a 35” wheel with the same amount of revolutions your truck will have travelled an extra 500 feet, so when your odometer shows that you travelled 10 miles your truck will actually have travelled almost 11 miles.
1 mile difference added every 10 miles does not seem like much but when you calculate your mpg and if it says 20 mpg your truck will have actually travelled 22 miles. So in reality you did 22 mpg.

Many people say “I lost 2 mpg or so by going to a bigger wheel” without taking in the consideration of the extra distance travelled.

I realize the extra weight of the tire makes a difference, it’s the extra diameter that will make a bigger difference, specially 3” taller.

It is a very wise decision to go with the 3.92 gear ratio if your getting bigger wheels, you will not notice much torque loss and it will slightly drop your rpm at highway speed, you would have to go with a much bigger wheel to achieve the rpm of the 3.21, more like 38”

My Rebel diesel comes standard with the 33” wheels, it’s my first diesel and extremely satisfied with the performance and efficiency.

Let us know what you end up buying.
 
Steveng8 said:
Anybody on here running 3.21 with 35's and a lift? I am in the ordering process and going back and forth between the 3.21 and 3.92. I hardly tow anything and don't care much about the "sporty" feel. Would mainly be for the strain on transmission or severe pedal lag with heavy tires and lift. Appreciate any feedback!
Click to expand...

Money well spent to get 3.92 with going 35’s. No brainer for only $95
 
Rebelguy2020 said:
I did some calculations for the difference between a standard 32” wheel and a 35” wheel, without showing all the math calculations, it turns out to be very close to 10% bigger, if your speedometer is not corrected it changes everything, your miles per hour and your mpg will be off by 10%.

Going to a 35” wheel your speedometer will be off by just under 6 mph.

I also calculated the amount of revolutions per mile for a 32” wheel and by going with a 35” wheel with the same amount of revolutions your truck will have travelled an extra 500 feet, so when your odometer shows that you travelled 10 miles your truck will actually have travelled almost 11 miles.
1 mile difference added every 10 miles does not seem like much but when you calculate your mpg and if it says 20 mpg your truck will have actually travelled 22 miles. So in reality you did 22 mpg.

Many people say “I lost 2 mpg or so by going to a bigger wheel” without taking in the consideration of the extra distance travelled.

I realize the extra weight of the tire makes a difference, it’s the extra diameter that will make a bigger difference, specially 3” taller.

It is a very wise decision to go with the 3.92 gear ratio if your getting bigger wheels, you will not notice much torque loss and it will slightly drop your rpm at highway speed, you would have to go with a much bigger wheel to achieve the rpm of the 3.21, more like 38”

My Rebel diesel comes standard with the 33” wheels, it’s my first diesel and extremely satisfied with the performance and efficiency.

Let us know what you end up buying.
Click to expand...
No need to do calculations.
tiresize.com

Tire Size Comparison

Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.
tiresize.com tiresize.com
 
Mountain Whiskey said:
Or you could just put all the theories and nonsense aside. Go with cold hard facts. 3:21 = wussie gears, 3:92 = real truck gears. Hmmm. Wussie truck, Real truck.

Well, facts are facts. Stick with 3:92 for Real Truck. Easy.
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"According to my calculations, your truck will be 392% more manly."
 
I have 3.21 gears and just put on 295/65r20 Yokohama Geloandar MT's. Wanted 3.92 gears but couldn't find the truck I wanted with it.

Went back and forth for months trying to decide on tire size, but ultimately I am impressed with how little the tires have affected things. I have the Bighorn "Lonestar" with etorque, 4wd. It's always been fast, off the line and while rolling. Not sure if's lighter than higher trims or I got some kind of freak factory 5.7, but I've recorded 0-60 times faster than i've seen advertised online.

With the 35's I've lost about 0.5 second from my rough 0-60 runs. You can tell takeoff is a little bit slower to respond, but once your rolling its just as quick as before.

I still see 8th gear at highway speeds, eco still kicks in although a little less frequently. My speedometer was way off before getting tire size recalibrated. Speedometer showing 70mph yet google gps showing 82. Now been fixed and my gas mileage also isn't as terrible as I planned for.
 
Louhound88 said:
I have 3.21 gears and just put on 295/65r20 Yokohama Geloandar MT's. Wanted 3.92 gears but couldn't find the truck I wanted with it.

Went back and forth for months trying to decide on tire size, but ultimately I am impressed with how little the tires have affected things. I have the Bighorn "Lonestar" with etorque, 4wd. It's always been fast, off the line and while rolling. Not sure if's lighter than higher trims or I got some kind of freak factory 5.7, but I've recorded 0-60 times faster than i've seen advertised online.

With the 35's I've lost about 0.5 second from my rough 0-60 runs. You can tell takeoff is a little bit slower to respond, but once your rolling its just as quick as before.

I still see 8th gear at highway speeds, eco still kicks in although a little less frequently. My speedometer was way off before getting tire size recalibrated. Speedometer showing 70mph yet google gps showing 82. Now been fixed and my gas mileage also isn't as terrible as I planned for.
Click to expand...
No way would 35's cause your speedometer to be off by 12 mph...more like 3-4 mph. Was that a typo?
 
Idahoktm said:
No way would 35's cause your speedometer to be off by 12 mph...more like 3-4 mph. Was that a typo?
Click to expand...
@Idahoktm is much closer. Looking up tire calculations fron a stock 32.1 inch tire to a 35 you should be doing about 76 mph when the speedo is reading 70. I don't think Google GPS is all that accurate.

This page does the math, not me.... https://tiresize.com/

Either way, once again.... PROOF! ......... 3:92 gears are superior. 😁
 
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