Opinion's wanted

[Andy4639]

Thinking about swapping out the 4:10 gears for a Pos trac 3:38 are 3:08 to gain some gas mileage. I don't hot rod my truck so I'm not real concerned with loosing some torque since I have the 4L80E in it also. It has a good 1 and 2 gear.
I want better than 15 MPG!
What do all of you think, lets hear what you got.

[Kennuck]

I wouldn't assume that lowering you gear ratio that much will give you better mileage. I know people will tell you that lower numerically gears will give better mileage but this is dependant on many factors especially with an overdrive transmission. You need to keep your engine in its peak operating range to deliver the best mileage. A lugging engine will give poor mileage as much as an over revving engine.

I'm not saying that the 4:10 will give the best mileage but you may find a 3:73 or 3:55 will be a better solution with the overdrive trans. Tire size also plays a roll in overall gear ratio.

I see you have a LS engine so a higher ratio may be fine in a relatively light truck compared to today's behemoths? Most modern trucks no longer offer a 3:08 ratio for good reason as they are not necessarily good in a truck.

I can tell you I changed from 3:31 to 3:73 in my 2007 F150 and my mileage went up.

[67ChevyRedneck]

Originally Posted by Kennuck

I wouldn't assume that lowering you gear ratio that much will give you better mileage. I know people will tell you that lower numerically gears will give better mileage but this is dependant on many factors especially with an overdrive transmission. You need to keep your engine in its peak operating range to deliver the best mileage. A lugging engine will give poor mileage as much as an over revving engine.

I'm not saying that the 4:10 will give the best mileage but you may find a 3:73 or 3:55 will be a better solution with the overdrive trans. Tire size also plays a roll in overall gear ratio.

Agreed 100%

3.73 will help a bit, but you're not going to jump to 18-20 mpg, but you might see a 1+mpg improvement. For power/economy, the 3.73 is the sweet spot for these trucks.

3.08 and OD are a NO NO in these trucks. Just search on here how many times guys tried that combo and the trucks lugged so bad in OD they went to a lower ratio.

A 3.42 might be an OK option, but if you're used to a 4.11, your truck will probably feel like a dog to you with that swap. If I remember right, the trucks with the 5.3L came with a 3.42 standard and the 3.73 and lower gears were optional (my Z71 had the 3.73 G80).

[davepl]

I run 4.88s with overdrive! The overdrive makes it about the same as 3.08s with a 1:1 transmission like the TH400, and that's about as low numerically as you'd ever want to go on these trucks I think...

Best thing to do is do the math as to what your engine RPM would be in overdrive at 60mph; decide what the lowest number (1250?) your engine makes good solid torque at and go from there.

There's no super-magic difference in mileage with gears, it's the same amount of work being done. Steep gears waste friction, tall gears pull your engine down out of its efficiency band. Hence my comment about where your engine (more accurately, your cam selection) makes good torque.

It might even be the case that you are best cruising at your peak torque RPM since that's where your volumetric efficiency is highest, but that puts you at a higher RPM than needed and you lose efficiency to ring drag and other items that don't matter at WOT. So I figure as long as your engine is making efficient torque, that's a good cruise RPM. But too tall of gearing can pull you down out of that.

Another point to consider is that if you have very tall gearing and want to speed up a little or climb a hill without downshifting you might use so much throttle that your vacuum dips and the power enrichment circuit kicks in; that'll ruin your mileage.

[1972RedNeck]

Originally Posted by davepl

I run 4.88s with overdrive! The overdrive makes it about the same as 3.08s with a 1:1 transmission like the TH400, and that's about as low numerically as you'd ever want to go on these trucks I think...

Best thing to do is do the math as to what your engine RPM would be in overdrive at 60mph; decide what the lowest number (1250?) your engine makes good solid torque at and go from there.

4.88's with an ~.7:1 overdrive = ~3.41:1

It seems to me that engines get the best mileage at their peak torque.

[davepl]

Originally Posted by 1972RedNeck

4.88's with an ~.7:1 overdrive = ~3.41:1

It seems to me that engines get the best mileage at their peak torque.

Overdrive is 0.67:1 in mine, so 3.26:1 effective ratio. I'd meant to say 3:23 actually (still wrong but closer).

They can't get their best mileage at peak torque because peak torque is at an RPM that has -great- volumetric efficiency (good) but a lot of friction (bad). If you have 30 hp of frictional losses it may not matter relaive to 450ftlbs but it sure does relative to the 40hp it probably takes to push a truck through the air at highway speeds.

That's why lower RPMs are usually better. If you had a graph that included frictional losses, torque, and the amount of power it takes to move our truck through the air you could plot the ideal RPM, but I don't have that kind of data.

My stock 402 peaks torque at 420@3350, but you sure wouldn't want to cruise at 3350.

So, we generally wind up at "the lowest RPM at which the motor makes good, efficient torque". Sadly we didn't dyno my own motor that low, but if the curve is at all symmetrical, 1500 would probably be ideal. That's about the lowest RPM where I think it makes 350ft lbs.

[special-K]

You want 3.42s. Noticeably taller than the 4.10 while keeping the grunt to pull that 3.08s lack. I think that is the sweet combo for light trucks. 3.73s are a small step away from 4.10s in straight drive, but more noticeable when you're talking O/D, if you go with those. That's the easy one for these years, plenty out there to swap-in.

[Boog]

Andy you have the 6.0 engine and it makes good power but I don't see it getting you to the high teens mpg. If you are already in the mid teens, 14 or 15 mpg you may not see much more than that with a gear change. Like others I think a good gear choice is in the 3.42 to 3.55 range. JMO

[68shortfleet]

Originally Posted by special-K

You want 3.42s. Noticeably taller than the 4.10 while keeping the grunt to pull that 3.08s lack. I think that is the sweet combo for light trucks. 3.73s are a small step away from 4.10s in straight drive, but more noticeable when you're talking O/D, if you go with those. That's the easy one for these years, plenty out there to swap-in.

I would have to disagree with wanting 3.42's.

My truck is currently running a LS1(5.7)/4L60e combo with 3.42 gears and 275/60/15 rear tires. I feel like it is "lugging" when cruising at 55 to 60 mph. I too, am pulling right at 15mpg consistently.

For this issue, I have a large order from Randy's ring and pinion sitting on the shelf waiting for me to install. Eaton posi, 3.73 gears, and bearings to do the job. Also being installed at the same time will be rear disc brakes.

Good luck on your gas mileage quest, I would like a little better too. I'm sure mine will do better if I wasn't driving a crooked mountain road to work.

[PanelDeland]

I haven't looked to see what the Ratio is on a new truck but up to the end of the 700R4 era 3.42:1 was standard and 3.73:1 was optional w the 700. I figure GM hired and paid people a lot better versed than me in this to determine what was best all around gear. So I would look into matching what the factory did. I'm assuming it was a 3.42:1 or so. The 3.73:1 isn't a lot of difference but you would notice it some.

[Andy4639]

Well not dead set on the numbers of gears yet, just want to find out what everyone likes. If the 3.73 is not much help MPG wise and only 1 MPG differance like Jesse states I'll stay where I'm at!
Right now running 60MPH I'm turning 2300RPM's on flat ground. But once I get to 70MPH I'm up around 2800RPM's and I drive a lot on the interstate anywhere from 65-80 MPH. I'd like to get the truck down below 2500RPM's at those speeds.

I checked my MPG last night when I filled up.

36 gallons @ 510 miles =14.16 MPG. I have a 42 gallon tank. That is a mixture of miles also City,interstate and country driving!
Yes it's not wrong.
The tune was done professionally!
The motor combo is all stock.

[redbaron]

Originally Posted by Andy4639

Well not dead set on the numbers of gears yet, just want to find out what everyone likes. If the 3.73 is not much help MPG wise and only 1 MPG differance like Jesse states I'll stay where I'm at!
Right now running 60MPH I'm turning 2300RPM's on flat ground. But once I get to 70MPH I'm up around 2800RPM's and I drive a lot on the interstate anywhere from 65-80 MPH. I'd like to get the truck down below 2500RPM's at those speeds.

I checked my MPG last night when I filled up.

36 gallons @ 510 miles =14.16 MPG. I have a 42 gallon tank. That is a mixture of miles also City,interstate and country driving!
Yes it's not wrong.
The tune was done professionally!
The motor combo is all stock.

Hey Andy,

What is the size of the tire you are running on it? Metric numbers please. I'll do some calculating for you with this info.

Thanks

[Andy4639]

Originally Posted by redbaron

Hey Andy,

What is the size of the tire you are running on it? Metric numbers please. I'll do some calculating for you with this info.

Thanks

Tires are 29.5" tall which =. 29.5 inches is 749.3 millimeters

[mooseknuckles]

6.0/4l80e with 3:42 posi, 28" tires, I can roast the tires at will and cruise along 65-70 mph 1900-2200 rpm.

[redbaron]

Originally Posted by Andy4639

Tires are 29.5" tall which =. 29.5 inches is 749.3 millimeters

Ok Andy here goes the various RPM your engine pulls with the 4L80E and 29.5" tires in 4th gear or overdrive.

4:10 @ 80 is 2800 RPM, 70=2450 and 60=2100.

3:73 @ 80 is 2550 RPM, 70=2230 and 60=1900

3:42 @ 80 is 2340 RPM, 70=2045 and 60=1750

3:08 @ 80 is 2100 RPM, 70=1840 and 60=1580

Hope this helps, Mike

[Dustytrix]

I agree with 68, higher is not going to help much, I have 6.0/ 4l60e/ 373 in my camaro with a 228/584 cam and get 21 miles to gallon highway, 17 city. I think maybe get a good tune will and you will get the best results.

[67ChevyRedneck]

That's about right. My LS 00 Z71 got the same mileage. 13.5-14mpg around town, maybe 14-15mpg average. It only got 18mpg if it was 100% highway and the cruise was set on 70mph.

Yes, these trucks are lighter than the LS trucks their motors come from, but you can negate the lighter bodies with the "aerodynamics of a brick" and they about cancel each other out. Seriously, next time your up to 75mph lift your foot off the gas and watch how fast the truck slows down... now do the same thing in a modern truck... they slow down MUCH slower when you let off. They're heavier buy way more aerodynamic.

My 67 with a 350/700R/3.73 gets 11-12 mpg, but I drive around town and leave it in 3rd most of the time (mostly under 45mph driving). If I take her out on the interstate and keep it around 65-70 I can get about 15mpg with OD.

[Shon]

I feel ya!
I got 13ish with my set up. Swapped to the T56 and did not pick up what I hoped, now at 15.
I just don't think I will see HIGH Teens or LOW 20s.
5.3/60 does though....

[XwalkerX]

i just swapped out my 3.07 rear end with an open diff for a 3.42 with an eaton posi.

no complaints here...

miles per gallon went down but smiles per gallon went up

[toolboxchev]

After doing the math myself and listening to a lot of people on here I think this is spot on:

That's why lower RPMs are usually better. If you had a graph that included frictional losses, torque, and the amount of power it takes to move our truck through the air you could plot the ideal RPM, but I don't have that kind of data.

From there my thought are to actually DYNO your actual setup you have now. Both HP and TORQUE.

The cam has to figure into this somewhere, so does actual vacuum. (Vacuum at actual cruising speed and load.) Then decide on those 3.08, 3.54, or 3.73.

Right now with my 150k motor, older turbo 350, I am at roughly 3300 rpms at 65mph. I aint liking it.

[BigRed76]

My 71 with a 5.3/4L60E has 3.73's with a 28" tire. It gets 18 mpg, and cruises at about 2500 rpm at 75 mph, in OD with the torque converter in lockup. It is a blast to drive, but I really do feel it would do better with a 3.42 or 3.55 gear. I would like to be around 2000-2200 rpm at 75. And I run 91 octane 10% ethanol (that's all we can get around here).

[redbaron]

Originally Posted by BigRed76

My 71 with a 5.3/4L60E has 3.73's with a 28" tire. It gets 18 mpg, and cruises at about 2500 rpm at 75 mph, in OD with the torque converter in lockup. It is a blast to drive, but I really do feel it would do better with a 3.42 or 3.55 gear. I would like to be around 2000-2200 rpm at 75. And I run 91 octane 10% ethanol (that's all we can get around here).

With your 4L60E and 28" tire at 75 MPH the engine RPM would be at 2155 running a 3:42 gear ratio in overdrive.

[BigRed76]

Originally Posted by redbaron

With your 4L60E and 28" tire at 75 MPH the engine RPM would be at 2155 running a 3:42 gear ratio in overdrive.

That is good to know. Even with the power of my stock 5.3 it doesn't need to be running 2500 rpm or more on the interstate just cruising.

[72MARIO]

I dont have a ls in the suburban but i did switch to a 3:42 gear and gear vendors from a 3:73. Its great on the freeway but the gv is off unless its on the freeway now. I kinda liked the 3:73 better....

What year is your 6.0 ? i am considering using a 2004 truck as a doner for. 4wd chassis i want to put under the 1967 C20

[XwalkerX]

this is what i went with

http://www.summitracing.com/parts/et...-010/overview/

dont forget you will want a series 3 carrier for the 3.42 combo