Diff Gears: Looking to Learn

[Mr Dubya]

Hi Folks,



Been absent for a bit since school and work started again. Hoping to get back to tinkering with the ride once the weather warms up and school ends.

Looking to see if someone has the answer to a question I keep getting in conversations with people about. Trying to educate myself.

To those knowledgeable: What is the advantage/relationship to increasing my differential gears? I have a lift and larger tires (in the sig), and heard that bringing my 3.73 gears up to 4.56 will help restore alot of my lost mileage as well and give the ride more 'kick' with acceleration - a lot like the old days before my mods. I've also heard that it'll only give me more torque and not better mileage or acceleration.

Can anyone shed some light real quick? Are larger gears ONLY going to benefit offroading by increasing torque? Or will it help restore the old ride by compensating for the tires. Or something else?

I'm perfectly fine with the answer 'what's done is done - no practical mod (minus an engine swap) can bring it back to the way it was'. I wouldn't trade the ride quality and looks for anything anyways. This project wouldn't happen anytime soon. Just like to plan ahead.



Thanks gents,
-Dubya

[Kazer]

You're best bet is to copy and paste that post into a private message to Ronin8002. He'll tell you everything you need to know

[Ronin8002]

Think about it like riding a bicycle that has gears. Shift it into the highest gear and your pedal speed (RPMs in the case of the truck) will be lower, but you'll have to work really hard to push the pedals (engine will have to work harder in the case of the truck). This is what it's like when you go with tires that are larger than the manufacturer planned for. So to use the bike example again, what if you shifted into the lowest gear possible...you'd be able to pedal really easily, but you have to pedal like crazy to get any forward motion on the bike. For your truck, this means that at highway speeds your engine RPMs will be way higher than needed.

You don't have to go extreme...though some people with more dedicated trail trucks do. IMO the best answer is to pick a gearset for your truck that will give you a similar or slightly higher RPM range as you had with the stock tires. That way it will feel at least as fast as it did with stock tires and stock gears.

For 265's, go with 4.10 gears...that will be like 3.73 gears on stock tires. For 285's, go with 4.30 or 4.56. 4.56 will be a little high, so 4.30 is probably better for a daily driver. You have the V-8 so I'll bet 4.30 would be a good balanced option.

If you want to crunch the numbers yourself and get RPM comparisons side by side, use this calculator:

http://www.grimmjeeper.com/gears.html

for the transmission, pick Ford 5r55w from the drop down, and use the BW1354 for the transfer case (I know, it's not the same as the one we have, but the gear ratios are, and it's not a factor at highway speed anyway).

[941Explorer]

I have a 4.0 v6 and 315/75r16 "35s" what gears would bring me close to stock thank you all very much ! Thomas

[Ronin8002]

Originally Posted by 941Explorer

I have a 4.0 v6 and 315/75r16 "35s" what gears would bring me close to stock thank you all very much ! Thomas

What is your current gear ratio and what was your stock tire size?

[941Explorer]

I want to say they are 3.55 and my tires were 235/75r16

[Ronin8002]

Here's a couple formulas you can use:

If you want to find out what your effective gear ratio is now with the larger tires:

current gear ratio / (new tire size in inches / old tire size in inches) = effective gear ratio

So using the numbers you gave me and converting the p-metric tire sizes to diameter in inches:

3.55 / (34.6 / 29.9) = x

3.55 / 1.15719 = x

x = 3.067

So basically, when you put on the larger tires, your gear ratio effectively dropped from 3.55 to 3.06.

To get back to the stock gearing (3.55), here's what you do:

(new tire size in inches / old tire size in inches) x existing gear ratio = new gear ratio needed

(34.6"/29.9") x 3.55 = x

1.15719 x 3.55 = x

x= 4.108

So to get back to an effective gear ratio of 3.55, you would install 4.10 gears.

Personally I would go higher, like 4.56, especially since you have a V-6.

To see what effective gear ratio that will result in with your size tires and a given gear ratio, you use the first formula I listed, but you use the planned axle ratio instead of the current one you have:

planned gear ratio / (new tire size in inches / old tire size in inches) = new effective gear ratio

So I'll use 4.56 for example:

4.56 / (34.6/29.9) = x

4.56 / 1.15719

x = 3.94

So 4.56 gears with your size tires will give an effective gear ratio of 3.94. This would feel a lot more quick off the line than stock, but not be bad at highway speeds either. It would also be helpful since you have the 4.0.

[941Explorer]

Thank you so much I will be ordering some soon any good company's to look at ?

[Ronin8002]

Originally Posted by 941Explorer

Thank you so much I will be ordering some soon any good company's to look at ?

Rockauto.com or ringpinion.com, you can get a discount at rockauto with the forum discount code. Remember that you will also need a 3.73 carrier for the front diff since the 3.55 one won't work for anything higher than 3.55. You need master kits for each axle too. Check the "regearing the front diff" sticky at the top of this forum.

[941Explorer]

I can't do 4.56s in the front ?

[Ronin8002]

Originally Posted by 941Explorer

I can't do 4.56s in the front ?

Yes you can, but you need a 3.73 carrier...the 3.55 one won't work. The carrier is the piece that goes inside the ring gear and holds the spider gears. Not an expensive part.

The one you have in the rear axle will work fine as it is not specific to any ratio.

[Mr Dubya]

All excellent information gents. I can see there are even more varying opinions out and about on the Interwebs. Still sifting through it all to get a clear picture.

I maybe re-asking someone to state the obvious, but for me to get a good mental image, how would one describe the change I'd under go with a gear swap?

As in: I currently have 285/70s with 3.73s. It has less-than-desirable MPGs (which I'm okay with), not much pick-up from a dead stop, and a comfortable ride and city and highway speeds. RPMs don't poke past the 2K mark at 65-70mph.

The above I said before I know, but how would those factors be altered if my ratio was lowered (higher numerically)?

Trying to get something less abstract, even though I understand the gearing analogies and math perfectly well.


Thanks folks.


-Dubya

[Ronin8002]

Provided you select a gearset that provides a good effective ratio, you will see a substantial improvement in acceleration from stops, and a reasonable rpm range on the highway. Also expect that hill climbs at highway speeds will be accomplished with less downshifting than you have now. Fuel economy improvements will depend on how high you go on the ratio...I saw an improvement on my old truck when I went to 265/75/16 tires and 3.55 gears, to 4.10 gears. Gave me an effective gear ratio of around 3.73. Don't expect miracles though. Also keep in mind that you'll probably want to get a tuner and correct your Speedo for both the tire and gear change, as well as making sure the trans shifts properly (ie, at the correct rpms).

in summary, if you end up with an effective gear ratio that is at or a bit better than stock, you'll get the benefits of better accelleration and a bit better fuel economy while keeping the highway rpms reasonable (highway rpms too high can negatively impact fuel economy).

Hope that answers your question.

[Mr Dubya]

Originally Posted by Ronin8002

Hope that answers your question.

That's exactly what I was looking to hear. Can't thank you enough for spelling it out.

Tuning it correctly and everything else is coming far before I even budget out gears, so I don't plan on embarking on that journey for some time.

Thanks for the info, I've got a clear picture now.

[3rdGenExploder]

Originally Posted by 941Explorer

I have a 4.0 v6 and 315/75r16 "35s" what gears would bring me close to stock thank you all very much ! Thomas

may i ask... how did you fit these 315/75/16?? what lift setup(s) are you running? i currently have 285/75/16 but have 4.56 gears comming soonish and wouldnt mind getting 35s when i need new tires

[Ronin8002]

Originally Posted by 3rdGenExploder

may i ask... how did you fit these 315/75/16?? what lift setup(s) are you running?

I was wondering the same thing, looked around and he said in another post it is a 3" BL and Truxxx spacers (Truxxx are 1.625" front/1.5" rear unless he has something different).