Gearing and tire size

[Cuppedup]

Based off the numerous threads I’ve read regarding gearing, 3.21 will yield better fuel economy than the 3.92 assuming all is kept stock. If I were to put on a level and 33” AT tires (maybe wildpeaks or similar), would 3.21 gearing still be the way to go?

I know these tires are only an inch Or so bigger than stock. I would try to keep them light as well (not E rated).

 

[dts828]

I have had rams with both gears. My latest is a rebel with 3.92/33"tires /2" level. In my opinion after owning both I would go with the 3.92. Fuel saving is minor between the two. Even more minor if you don't always drive with a lead foot.

Sent from my GM1917 using Tapatalk

 

[TruckDriver]

a couple questions to consider

Are you towing with regularity at all? 3.92

If you bought either gearing, is that the setup you'll go with? likely 3.21 would still be better, but if you think about it, bigger tires = harder to get moving. That's where 3.92 would do better, but once you're at highway speed, 3.21 is taller gearing... so do you drive more long highways or more city?

and ultimately - the difference is likely minimal. You can extrapolate the math, but it's not going to be a bank-breaker in fuel costs.

 

[Cuppedup]

I’m looking for a CC standard box with either anti slip or e-locker. I want to keep decent mpg so I won’t go above a 33” AT tire or bigger than a 2” level. I do a mix 50/50 city/hwy. Down the road, I’d like to get a liteweight travel trailer, around 20’. But this isn’t a factor as I won’t be using it often.

 

[Cuppedup]

Down the road, hwy driving might increase as I will likely have to travel further for work.

 

[Cuppedup]

If no trailer is involved in the mix, strictly commuting. 33” tires and 2” level. I’ve got a vote for 3.21 and a vote for 3.92. Anyone else? 50/50 cit/hwy.

 

[stevj]

With 3.21 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.11.
With stock tires, 70 MPH in 8th will put your engine at about 1500RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1450RPM.

With 3.92 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.79.
With stock tires, 70 MPH in 8th will put your engine at about 1850RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1800RPM.

Steve

 

[Firebird]

3.92 gears all the way. 3.21’s were a deal breaker for me.

 

[ksn240]

That's my one regret on my truck. I wish I had the 3.92 gears instead of the 3.21s I have. I am looking at potentially swapping them out. I can get a full front differential for 700-800.

I am running 33s and the truck handles it just fine for what its worth.

 

[5thGenRebel_AMK]

3.92’s are def better. I had the same 35’s on my last truck that had 3.21’s, and while the truck still drove fine and I didn’t have any issues with the set up, the 3.92 rear end makes the truck feel a lot peppier after putting such a bigger and heavier tire on it. My fuel mileage stayed pretty much the same after the tire size increase with 3.92’s (around 14 highway) but it really changed when I put 35’s on my 3.21’s. It went from around 18 highway to around 14. Take this for whatever it’s worth, I know you’re considering 33’s, not 35’s

 

[Cuppedup]

With 3.21 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.11.
With stock tires, 70 MPH in 8th will put your engine at about 1500RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1450RPM.

With 3.92 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.79.
With stock tires, 70 MPH in 8th will put your engine at about 1850RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1800RPM.

Steve

So with the 3.21 gears and 33” tires, I will still have lower RPMs at freeway speeds, therefore better fuel economy than the 3.92?

 

[Willwork4truck]

With 3.21 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.11.
With stock tires, 70 MPH in 8th will put your engine at about 1500RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1450RPM.

With 3.92 gears, going from stock 32" tires to 33" tires, would give an effective gear ratio of 3.79.
With stock tires, 70 MPH in 8th will put your engine at about 1850RPM.
With 33" tires, 70 MPH in 8th will put your engine at about 1800RPM.

Steve

Based on what Steve has shown on gearing above, I’d go 3.92. Note that I have a 3.21 on my personal truck, but any bigger tire, any lift and any much towing I’d be running the 3.92‘S.
Since we don’t have/do any of that, we accept the 3.21 gearing and get the better mileage.

 

[Cuppedup]

Based on what Steve has shown on gearing above, I’d go 3.92. Note that I have a 3.21 on my personal truck, but any bigger tire, any lift and any much towing I’d be running the 3.92‘S.
Since we don’t have/do any of that, we accept the 3.21 gearing and get the better mileage.

His explanation states that the 3.21 would still have lower rpms at 70MPH with the 33” tires. Doesn’t that equate to better MPG? I’m clueless at this so I’m trying to learn prior to purchasing. Keep in mind towing as of now is not a factor. Just 33” tires

 

[Neurobit]

So with the 3.21 gears and 33” tires, I will still have lower RPMs at freeway speeds, therefore better fuel economy than the 3.92?

Absolutely.

 

[Willwork4truck]

I’d not recommend continuing to lower the gearing (numerically) from the 3.21. The 8 speed will pull it however there’s a point of no return where you’ll be dropping gears going up grades, having lackluster acceleration etc.

This is coming from a guy who likely never would lift or run bigger tires and feels mpg’s are important even on a Limited truck, yet I was really on the fence about the gears.

Your choice but I’d not go lower on the effective gear ratio. You aren’t buying a truck for mpg’s. If you want to use it as a truck you sometimes need the gearing.

 

[stevj]

Doesn’t that equate to better MPG? I’m clueless at this so I’m trying to learn prior to purchasing. Keep in mind towing as of now is not a factor. Just 33” tires

Not necessarily.
There comes a point of diminishing returns, where lower engine RPM creates more work for the engine when trying to accelerate, or even just to maintain speed.
Try going 30MPH in a manual transmission vehicle in top gear. Pushing the accelerator to the floor will lug the engine, waste fuel and likely not achieve the desired result.
This is why we downshift transmissions: to get the engine RPM closer to torque peak, and add the mechanical advantage of the higher ratio of a lower transmission gear.
The same logic can be applied to the effective rear end gear ratio created by a taller tire. Your longer acceleration times and lower fuel economy are more a result of the reduced mechanical advantage created by the larger diameter tires than by their additional weight. Mechanical advantage is lost with taller tires, and the engine usually ends up having to work harder.

Steve

 

[Scram1500]

33's will weight more as well, the mileage gain might be a wash in the end
Edit: stevj pretty much covered it

 

[Cuppedup]

What are the weights of the standard 275/55r20 tires and the 275/65r18 tires that come with the 1500 bighorn package? I can’t seem to find the exact 20” tire or the 18” either I

 

[Hadit55]

What are the weights of the standard 275/55r20 tires and the 275/65r18 tires that come with the 1500 bighorn package? I can’t seem to find the exact 20” tire or the 18” either I

What did you end up going with and how are your MPG's?