Axle Ratio Help

[Car'n'Guitars]

Okay my question is what is the gear ratio 3.08 used for and what to expect from it? I'm new to gearing info but learn quit. Thanks.

[WarBeard]

All I know is a 3.08 was popular with the 6.2 and 700r4 combo.

[Keith Seymore]

Lower numerical ratios (2.xx and 3.08) are supposed to be better for fuel economy, all else being equal. (ie, assuming the engine power band is unaffected, etc).

Higher numerical ratios (4.xx and 5.xx, etc) are better for low end launch, as they provide more torque multiplication. They are also worse for fuel economy simply because the engine has to spin faster and more times when going down the road.

As a lay person, I like to start with 3.42:1, and then move from there (ie, lower ratios for better fuel economy - or Pontiacs; higher ratios for more launch or racing applications).

If you are asking about availability in Chevy trucks specifically, they are usually found in those models more designed as daily drivers or light duty work trucks where you want to try to eek some fuel economy out of them.

Powertrain charts (like the one shown in this link http://www.gmheritagecenter.com/gm-h...olet-Truck.pdf (page 13) ) are used to track the usages. In this case the base RPO GQ1 is 2.73:1; Optional GU4 is 3.08.

Does that answer your question?

K

[Car'n'Guitars]

Originally Posted by Keith Seymore

Lower numerical ratios (2.xx and 3.08) are supposed to be better for fuel economy, all else being equal. (ie, assuming the engine power band is unaffected, etc).

Higher numerical ratios (4.xx and 5.xx, etc) are better for low end launch, as they provide more torque multiplication. They are also worse for fuel economy simply because the engine has to spin faster and more times when going down the road.

As a lay person, I like to start with 3.42:1, and then move from there (ie, lower ratios for better fuel economy - or Pontiacs; higher ratios for more launch or racing applications).

If you are asking about availability in Chevy trucks specifically, they are usually found in those models more designed as daily drivers or light duty work trucks where you want to try to eek some fuel economy out of them.

Powertrain charts (like the one shown in this link http://www.gmheritagecenter.com/gm-h...olet-Truck.pdf (page 13) ) are used to track the usages. In this case the base RPO GQ1 is 2.73:1; Optional GU4 is 3.08.

Does that answer your question?

K

Yep that is exactly what I wanted to know, thanks.

[piecesparts]

You have to think about what kind of driving that you will be doing. Also what type of transmission are you running and where do you want your RPMs at on the engine for aligning with the power band of that engine. Also consider your tire diameter in this evolution. If you get the combination to the point that you are running at 1600 RPMs on the highway, then if there are no hills, you should have no probelms. However if you have any kind of hills or even a lot of intersections that you have to accelerate from, then you will be in trouble with your fuel economy. The engine will have to work harder to get up to speed and then stay there.

My son's truck ended up with a 700 R4 and a 3.08 gear ratio and his fuel mileage went into the dump, at approximately 10 MPG. We put a 3.73 gear ratio behind the 700 R4 and he now gets around 14 MPG. The engine runs better and will be in the power band when needed and does not have to overcome the drag of being geared wrong.

I use a set of formulas to calculate what I want to achieve and that lets me get my gearing/tranny outpu and tire sizes where I want them for performance.

RPM =GEAR RATIO x MPH x TRANS OUTPUT RATIO x 336
TIRE DIAMETER

MPH = RPM x TIRE DIAMETER
REAR AXLE RATIO x TRANS OUTPUT RATIO x 336

[Car'n'Guitars]

Originally Posted by piecesparts

You have to think about what kind of driving that you will be doing. Also what type of transmission are you running and where do you want your RPMs at on the engine for aligning with the power band of that engine. Also consider your tire diameter in this evolution. If you get the combination to the point that you are running at 1600 RPMs on the highway, then if there are no hills, you should have no probelms. However if you have any kind of hills or even a lot of intersections that you have to accelerate from, then you will be in trouble with your fuel economy. The engine will have to work harder to get up to speed and then stay there.

My son's truck ended up with a 700 R4 and a 3.08 gear ratio and his fuel mileage went into the dump, at approximately 10 MPG. We put a 3.73 gear ratio behind the 700 R4 and he now gets around 14 MPG. The engine runs better and will be in the power band when needed and does not have to overcome the drag of being geared wrong.

I use a set of formulas to calculate what I want to achieve and that lets me get my gearing/tranny outpu and tire sizes where I want them for performance.

RPM =GEAR RATIO x MPH x TRANS OUTPUT RATIO x 336
TIRE DIAMETER

My truck has a 305 with a 700R4 running 32" tires.

[Edahall]

Originally Posted by Car'n'Guitars

My truck has a 305 with a 700R4 running 32" tires.

What that setup (oversize tires and overdrive), you'll be lugging that 305 with 3.08's. For best fuel economy, you want to select a gear ratio that'll keep your rpm's at around 2000 at cruising speed with tires you intend to use.

[Edahall]

Originally Posted by Car'n'Guitars

My truck has a 305 with a 700R4 running 32" tires.

What that setup (oversize tires and overdrive), you'll be lugging that 305 with 3.08's. If fuel economy is what you're after, you want to select a gear ratio that'll keep your rpm's at around 2000 at cruising speed with the tires you intend to use.

[Car'n'Guitars]

Originally Posted by Edahall

What that setup (oversize tires and overdrive), you'll be lugging that 305 with 3.08's. For best fuel economy, you want to select a gear ratio that'll keep your rpm's at around 2000 at cruising speed with tires you intend to use.

Okay what do you suggest is the best gearing to use? Just so everyone knows this is the setup that came factory on this truck except it had 31" tires.

[Edahall]

Originally Posted by Car'n'Guitars

Okay what do you suggest is the best gearing to use? Just so everyone knows this is the setup that came factory on this truck except it had 31" tires.

Well, that depends on what you intend to do with the truck. Gearing is definitely not something that fits all purposes.

However, if you're just going to use the truck for solo driving and want the best fuel economy at 55-60 mph, your best bet would probably be 3.73's.

Here's the numbers for 3.73's.
Speed = 60 mph
Tire Diameter = 32"
RPM's = 1645

If fuel economy is what you're looking for and you carry no heavy loads, I would suggest going to P235/75/15's tires. You'll gain a 1-2 mpg with just this.
Speed = 60 mph
Tire Diameter = 28.9
RPM's = 1821

This would be you put you into a more efficient RPM band and the tires would produce less wind drag and less rotational mass (which kills fuel economy). Also, keep your speed down. Any faster than 60 mph and fuel economy plunges with these vehicles and even more so with a 4x4.

[Car'n'Guitars]

Originally Posted by Edahall

Well, that depends on what you intend to do with the truck. Gearing is definitely not something that fits all purposes.

However, if you're just going to use the truck for solo driving and want the best fuel economy at 55-60 mph, your best bet would probably be 3.73's.

Here's the numbers for 3.73's.
Speed = 60 mph
Tire Diameter = 32"
RPM's = 1645

If fuel economy is what you're looking for and you carry no heavy loads, I would suggest going to P235/75/15's tires. You'll gain a 1-2 mpg with just this.
Speed = 60 mph
Tire Diameter = 28.9
RPM's = 1821

This would be you put you into a more efficient RPM band and the tires have less wind drag and less rotational mass (which kills fuel economy).

This truck will used for a daily driver and for working with all types of stuff. The smallest tire size I will go is 31" because I will also use it for a lot of offroad stuff too and I like the stance. Thanks for your help so far.

[Edahall]

Originally Posted by Car'n'Guitars

This truck will used for a daily driver and for working with all types of stuff. The smallest tire size I will go is 31" because I will also use it for a lot of offroad stuff too and I like the stance. Thanks for your help so far.

What axle ratio is in the truck right now?

[Car'n'Guitars]

Originally Posted by Edahall

What axle ratio is in the truck right now?

3.08 that's why I was wondering, about how I want to go with this truck.

[Edahall]

Originally Posted by Car'n'Guitars

3.08 that's why I was wondering, about how I want to go with this truck.

That's an insane axle ratio behind a 305. At 60 mph with that setup in O/D, the engine would only turn over at 1358 rpm's. That's way too low rpm's for that engine at that speed. Does the engine even have enough power to use 4th gear? However in 3rd gear (direct drive) the rpm's would be 1940 rpms which is perfect. I guess you could just cruise in 3rd gear and not worry about changing axles.

[Car'n'Guitars]

Originally Posted by Edahall

That's an insane axle ratio behind a 305. At 60 mph with that setup in O/D, the engine would only turn over at 1358 rpm's. That's way too low rpm's for that engine at that speed. Does the engine even have enough power to use 4th gear? However in 3rd gear (direct drive) the rpm's would be 1940 rpms which is perfect. I guess you could just cruise in 3rd gear and not worry about changing axles.

I could tell you it hasn't been road driven in years, it was handed down to me from dad so it is now my project truck. I am getting close to finishing the mechanical stuff and I was really wondering about this because I have noticed not too many people in here favor the 3.08 ratio and I was wondering why and how it would affect my truck.

[Edahall]

Originally Posted by Car'n'Guitars

I could tell you it hasn't been road driven in years, it was handed down to me from dad so it is now my project truck. I am getting close to finishing the mechanical stuff and I was really wondering about this because I have noticed not too many people in here favor the 3.08 ratio and I was wondering why and how it would affect my truck.

Well 3.08's or even 2.73's work fine behind a non overdrive transmission. Having 3.08's is like having an overdrive with 3.73's.

[Car'n'Guitars]

But isn't a 700r4 an overdrive transmission or am I wrong?

[Edahall]

Originally Posted by Car'n'Guitars

But isn't a 700r4 an overdrive transmission or am I wrong?

Yes it is. It has a final drive ratio of 0.70 Also, 3.08 can work well behind a high torque engine like a 454 or diesel engine.

[Car'n'Guitars]

So what is my best option for my driving needs?

[Edahall]

Originally Posted by Car'n'Guitars

So what is my best option for my driving needs?

Please define your driving needs.
How much are you willing to spend?
How much weight are you planning to tow?
How many miles are you planning to drive it?
Mainly City or Highway driving?
Do you want it performance oriented or fuel economy oriented?

[Car'n'Guitars]

Originally Posted by Edahall

Please define your driving needs.
How much are you willing to spend?
How much weight are you planning to tow?
How many miles are you planning to drive it?
Mainly City or Highway driving?
Do you want it performance oriented or fuel economy oriented?

Willing to spend whatever I might have to, weight wise is various things but if it is something too heavy I will use my dads truck. But overall nothing too heavy, like cleaning out houses and that type of stuff. Miles is another thing I can't really determine or it be definite. Mainly though it will be driven in city. I would like power but would rather have fuel economy. Hope this helps.

[Edahall]

A set of 3.73's would give you a little better fuel economy around town and you'll actually be able to use O/D when cruising.

Or you could rip out that 305 and put in a 6.2L diesel and keep the 3.08's. It would get significantly better fuel economy around town.

Or if money is no issue, you could put in a Cummins 6BT and enjoy both power and good fuel economy.

[Car'n'Guitars]

Originally Posted by Edahall

A set of 3.73's would give you a little better fuel economy around town and you'll actually be able to use O/D when cruising.

Or you could rip out that 305 and put in a 6.2L diesel and keep the 3.08's. It would get significantly better fuel economy around town.

Or if money is no issue, you could put in a Cummins 6BT and enjoy both power and good fuel economy.

I would love a 6BT in my truck but changing gearing would cost a lot less. If I really need the 6BT power I will just use our Dodge. What all is needed to switch rearend gears? And do the front need switched too?

[86c20]

i have 3.42 in my c 20 with a sm 465 and a 350. i find it a good all around gear. i dont remember what size my tires are 265 85 16?? stock size i think. i get good milage 21 mpg at 70 14.5 in the city where i live. can still pull any trailer that i should have to. i have driven these trucks with the 3.08 and i think they suck ass. the 4.10 that is in my diesel good for city and pulling crap for the highway. just my .02

[lap15]

Here's my .02

My 86 C10 (HD half ton 6100 GVW) has been a work truck all it's life with a 305, SM465 & 3.08 rear axle. It's handled everything you could throw at it- city, highway, heavy loads, light loads and towing- while providing 15mpg. (Top speed is about 65mph)
I would leave it alone, unless it breaks.