What Gear ratio is good for me

[GmtGmt]

i cracked open my Dana 70HD rear end to look it over and drain the old oil.
sadly not a posi

i pretty sure i have 4.10 Gears i dont have any RPO codes and the tag on the case is gone so i can confirm it. but most duallys came with that.

i have been looking for posi for mine and found a place that sells new ring and pinions for this axles cheap and i have been thinking if i should change it out
my truck will mostly be used on the highway and with a light car trailer
no heavy towing.
its a 6.2 diesel, 4 speed and stock 235/85/R16 wheels
what gears would be great for me

3.54, 3.73, 4.10 or 4.53?

[Monte Carlo Man]

A guy I know, who teaches auto mechanic's at the local Vo Tech school, who worked at a lead technician at a local GM garage. Owned a 1978 Chevrolet 1 ton crew cab x state truck with dual wheels.

It had 4.56 gears in it.

His idea was to move back to 4.11 gears to get better gas mileage.

He claims that when he swapped the gears, he lost 2 mpg.

I don't think that it is just any one thing that can magically make a vehicle get better gas mileage, but a combination of things. The fuel delivery system, the exhaust system, the camshaft, the compression and all the things that it has to move determines which combination of things will work to make things more efficient.

Lets say your combo is designed to make peak horsepower at say 3500 rpms at 65 mph in high gear.

If you change the gear ratio to 4:11 and now the truck is only running 3000 rpm's, you would think that it would get better gas / fuel mileage. But because the engine is lugging and is not producing the same amount of horsepower at a given speed, the transmission has to shift more and as it is hunting for gears - it uses more fuel.

So if you change the camshaft, increase the compression, add some type of water injection, put a shift kit in the transmission, change the gear ratio, put some lighter wheels on the truck and some lighter / smaller tires, the fuel economy and performance should increase -eh.

I forgot to mention that the term "Shift Kit" is copywrited and is a exclusive term refered to by the TransGo company and not to be used as a generic term to be applied to the use of a transmission reprogramming kit manufactured by other companies.

[GmtGmt]

Originally Posted by Monte Carlo Man

A guy I know, who teaches auto mechanic's at the local Vo Tech school, who worked at a lead technician at a local GM garage. Owned a 1978 Chevrolet 1 ton crew cab x state truck with dual wheels.

It had 4.56 gears in it.

His idea was to move back to 4.11 gears to get better gas mileage.

He claims that when he swapped the gears, he lost 2 mpg.

I don't think that it is just any one thing that can magically make a vehicle get better gas mileage, but a combination of things. The fuel delivery system, the exhaust system, the camshaft, the compression and all the things that it has to move determines which combination of things will work to make things more efficient.

Lets say your combo is designed to make peak horsepower at say 3500 rpms at 65 mph in high gear.

If you change the gear ratio to 4:11 and now the truck is only running 3000 rpm's, you would think that it would get better gas / fuel mileage. But because the engine is lugging and is not producing the same amount of horsepower at a given speed, the transmission has to shift more and as it is hunting for gears - it uses more fuel.

So if you change the camshaft, increase the compression, add some type of water injection, put a shift kit in the transmission, change the gear ratio, put some lighter wheels on the truck and some lighter / smaller tires, the fuel economy and performance should increase -eh.

I forgot to mention that the term "Shift Kit" is copywrited and is a exclusive term refered to by the TransGo company and not to be used as a generic term to be applied to the use of a transmission reprogramming kit manufactured by other companies.

mine is not an Auto its a Stick shift with Granny low

[Shawn5488]

3.73's w\Detroit soft locker it what I would use. Even if the gears are cheap you will never drive enough miles to get your money back, just add the posi, take a loss at the pumps and move on.

[STOCKISH]

With the diesel you will be better off leaving in there what was in there originally. GM spent tone of R&D money determining what ratios worked best with the diesel depending on which trans it has. If you use your truck like a truck, the gears that are in it are probably the best all around gears already.

[tucsonjwt]

All things being equal, you would have to think a low number rear axle ratio would yield better gas mileage. 4:56 and 4:11 are both bad for gas mileage, but a 1 ton dually stake truck is very heavy and probably takes a lot of umph just to get going, so maybe a 4:10 wasn't enough gearing to get started, causing the engine to rev longer to get to a given speed. In city driving, I can see how that might be worse. I just bought my first truck with a 3:23, and it really seems like it takes a long time to get up to normal driving speeds, even compared to a 3:42 or 3:73.

[clinebarger]

Originally Posted by GmtGmt

i cracked open my Dana 70HD rear end to look it over and drain the old oil.
sadly not a posi

i pretty sure i have 4.10 Gears i dont have any RPO codes and the tag on the case is gone so i can confirm it. but most duallys came with that.

i have been looking for posi for mine and found a place that sells new ring and pinions for this axles cheap and i have been thinking if i should change it out
my truck will mostly be used on the highway and with a light car trailer
no heavy towing.
its a 6.2 diesel, 4 speed and stock 235/85/R16 wheels
what gears would be great for me

3.54, 3.73, 4.10 or 4.53?

Dana 60, 70 & 80s require a special tool called a "case spreader" to remove & reinstall the carrier.

You can count the teeth on the ring gear then the teeth on the pinion, divide the two together to get the ratio.

Dana 70HD carrier split.......4.10 and down (3 series), 4.56 and up (4 series).

I whould go with 3:54s on that truck.

[gchemist]

In 84, GM rated the 6.2 diesel = OHV Diesel Eight-cylinder 379 cid. (130) @ 3600 rpm
What we need to know is max torque RPM. I have a factory manual at home. I can check tomorrow. Keep our cruise RPM at the max torque output for max milage.

[GmtGmt]

Originally Posted by gchemist

In 84, GM rated the 6.2 diesel = OHV Diesel Eight-cylinder 379 cid. (130) @ 3600 rpm
What we need to know is max torque RPM. I have a factory manual at home. I can check tomorrow. Keep our cruise RPM at the max torque output for max milage.

my truck has 145HP and 330 lb·ft

[Matt Man]

Originally Posted by GmtGmt

my truck has 145HP and 330 lb·ft

i would go with the lowest gear ratio possible if its going to be used on the highway a lot, unless your gonna tow with it.i would go with 3:54

[gchemist]

Originally Posted by GmtGmt

my truck has 145HP and 330 lb·ft

How do you know that information?

[GmtGmt]

Originally Posted by gchemist

How do you know that information?

its says that on my Air Cleaner it also has all the specs like idle speed fast/normal, Emission diagram.

theres alot of diffrent HP and TQ rates on these it all depents on what they where used in N/A engines has from 130-160 HP cant at the moment remember tqs

[gchemist]

Does it give RPMs on the label?

[GmtGmt]

Originally Posted by gchemist

Does it give RPMs on the label?

idle 650 rpm + - 50
TQ 2,100 rpm
max 3,600 rpm

[67_C-30]

I'm running 3.21's in my '67 c-30 the engine loves it. I had 4.10's in it when I first put it together, and it ran so far above its torque peak that it actually scrubbed off more speed on grades than it does now when towing. My M1009 Blazer has 3.08's and it handles hills better at 70 mph than a M1008 with 4.56's. The M1008's are actually ridiculously high geared for a 6.2 truck. They really don't pull well at all above 50 mph, which is what they were designed to do, but that are not practical for anything other than a 50-55 mph military convoy with the factory sized tires. A lot people told me that I wouldn't be able to tow if I put in the 3.21's, but that's simply wrong. Most people are used to dealing gas engines that need the RPM. I based my assumption that my truck would fine off the fact the 4L80E with 4.10's is the most popular set-up for 6.5TD trucks. Well, if you do that math going with higher gears with 4 speed puts you closer to the final drive of the 6.5TD/4L80E/4.10 set-up.

Here's my set-up.





Here's 4L80E/4.10 set-up.




If you went with the 3.54's (assuming you're running a 31" tire) you'd be running 65 MPH at 2500 RPM vs 2900 RPM at 65. Its pretty much universally said that 6.2/6.5's lose efficiency and ecomony above 2400 RPM, so with your 4.10's, you're just making noise anything much over 55 mph.

[gchemist]

Use this calculator: http://wahiduddin.net/calc/calc_speed_rpm.htm

Using a a 31" inch tire, 1:1 tranny gear ratio, 3.54 gears, 2600 RPMs gets you to 67.7 MPH. Not bad.

[nelsonadam84]

I have a 3+3 C30 with 454 and 4 speed. I plan on using 3.54 area gears. I dont have to worry about take off with a BBC and super low first gear. If I could get lower with my 70hd axle I would and not worry about the take off. Also if you have your axle open you can turn the ring gear and the should be a number like 43:12 set up. That is the tooth count for the ring and pinion.