Gas Mileage 5.3 3.42 vs 3.73

Nanotech Environmental · April 16, 2017

That is not necessarily true. It is all in how one operates the vehicle and the RPM band the motor is in. Actually, an engine running below the optimum RPM is going to get worse mpg than another one running a bit higher. Rotational inertia and other factors come into play on how well the engine develops power with a given amount of fuel. So, 3.42 will not always get better mpg than 3.73. My 2013 1500 5.3 with 3.42 only got an average 1 mpg better than my current 2015 2500 6.0 with 4.10 that weighs probably 1000 lb more than the 1500. And that is even running the same general speeds and same roads. I find the GM engines seem to really have a nice balance of power and fuel economy when running in the 1800-2200 RPM band. For my 2500 4.10, it is about 1900 RPM at 65 mph. For 3.73 and stock rubber, it would be about 1800 at 65. When an engine is running lower than optimum, it takes more fuel to develop enough power to over come resistance, and it also leads to more frequent gear hunting by the trans, which also has a detrimental effect on fuel economy. Of course, too high of RPM is detrimental also.

Over 6 million miles of commercial trucking has taught me a lot about gear ratio selections.

I'm going to have to respectfully disagree with this.

When apples to apples are compared exactly, in every which way, under any condition, the 3.73 will ALWAYS get worse mileage than a 3.42, by a factor of approx. ~9%. Always.

It's just simple math and physics. It has nothing to do with the points you've made, and I've seen all kinds of guys try to use that type of reasoning. Unfortunately they seem to miss the obvious:

The OP has said he is only towing a trailer sometimes, which means the truck is probably going to be under (relatively)low load conditions most of the time. It's in his best interest to figure out what his overall usage will be and what would be best for him.

Any modern v8 pickup with a 3.42 gear going down a flat highway, with no headwind, at 65 mph is producing a large excess of power at that speed. You need roughly 50-75 hp and a certain amount of torque to maintain that speed in a fullsize truck. At 1500 rpm, a 5.3 is putting out a lot more power than that. That means the surplus is actually wasted and is why GM can have these engines drop into v4 mode in those conditions. Turning more rpm in those conditions will only waste more fuel and create extra wear in the engine and provide zero benefit. It has nothing to do with 'optimal engine RPM' or any other catch phrase or term.

This is why GM can get the 450 hp Vette to get exceptionally good highway mileage; very low rpm at hwy cruising speeds. The less rpm you turn at any given speed, under any given load, the better for mileage.

The same argument holds true for any load condition and can be extrapolated to demonstrate that, which is why the automakers are going to transmissions with more and more speeds; the goal is actually to keep the rpm as low as possible for any given load condition.

It's the same thing for class 8 trucks with 80,000 lb GVW. There is no way that you can get better fuel economy turning 1500 rpm, when an identical truck, with higher gearing, under identical conditions, with engine loads in the normal expected ranges, is doing the same hwy speeds with the same engine, but only turning 1100 rpm. The guy turning 1100rpm, is going to get a lot better mileage. Always.

To the OP- If it was my truck, for the usage you mentioned, I'd consider getting a lower numerical rear end gear and get the transmission with the most speeds I can (8 speeds in this case, if I could). If all I was doing was pulling a 7000lb load all day long, then I'd look at getting a higher numerical rear end gear....

I have a 3.42 in my current truck, a 2006. For my usage, my next one will likely have a 3.23, or something close to that.

Edited April 16, 2017 by 3beejay3

Bobber · April 17, 2017

I'd stick with the 3.73's if I were you... I certainly wish I had someone tell me this when I was buying mine; if I did mine all over again I wouldn't go with the 3.42's.

One thing that everyone talking here/now (about fuel economy and higher numerical ratios) seems to forget about is the AFM system... I certainly hadn't really taken it into account when I bought mine. I was thinking, just like everyone else, that lower rpm is better - especially with all of the highway miles I drive. Sure 200rpm faster consumes more fuel - but ONLY if you are thinking V8 vs V8. If you can get the similar torque from the engine in V4 mode (when spinning 200rpm faster) as you get from the V8 mode 200rpm less, then you can operate in V4 mode instead of V8 - and that is where the fuel savings comes from.

Tthe axle ratio would actually help by putting the rpm at a point where torque is high enough in V4 mode to maintain cruise without bumping out of AFM (back to V8 mode) at the slightest grade or headwind buffet. The longer you can spend in V4 (AFM) mode, the better your fuel economy will be.

And I've pulled my 4000# 7x16 enclosed (7' rear door / 9' OAH) V-Nose cargo trailer several times now... that trailer wasn't something I had (or even considered) when I bought the truck. It definitely would benefit from the 3.72 gearing when I'm trying to pull that much of a brick through the wind!

Cowpie · April 17, 2017

There are a lot of dynamics involved. On the surface, lower RPM's should give better mpg. But exhaust flow dynamics can screw that up. There has to be a balance of proper exhaust flow so that exhaust does not cool in the pipe too quickly to form a bottleneck, which will cause more pressure up stream and also elevate EGT's. All of which will have a negative effect on economy and efficient power. One has to look at the rear ratio, combined with the trans ratios, and factor tire revolutions per mile to determine what RPM the engine will be turning and if that is appropriate for overcoming low RPM rotational inertia and restrictive exhaust flow.

To that end, we really have no choice in trans ratios. So that pretty much leaves tires and diff ratios, the latter only a couple of choices. After many years, I have found 4.10 is a great ratio in pickups. 3.73 being next in line. Only the taller ratios if pickup is primarily general auto transportation and not called upon to do any substantive work.

Nanotech Environmental · April 17, 2017

There are a lot of dynamics involved. On the surface, lower RPM's should give better mpg. But exhaust flow dynamics can screw that up. There has to be a balance of proper exhaust flow so that exhaust does not cool in the pipe too quickly to form a bottleneck, which will cause more pressure up stream and also elevate EGT's. All of which will have a negative effect on economy and efficient power. One has to look at the rear ratio, combined with the trans ratios, and factor tire revolutions per mile to determine what RPM the engine will be turning and if that is appropriate for overcoming low RPM rotational inertia and restrictive exhaust flow.

To that end, we really have no choice in trans ratios. So that pretty much leaves tires and diff ratios, the latter only a couple of choices.

Well, I got to hand it to ya.

In this thread, you've taken a very simple premise and managed to not only do a great job of totally & arcanely Rube Goldberg-ing it, but also almost sounding kinda impressive doing it. LOL

I used to see a bunch of guys do this on the VW diesel forums, but never quite as good as you did in this thread.

Back then, I always got better fuel economy than all of them, in the diesel Jetta I had(comparing apples to apples) All I wanted then was a 6th gear so I could drop cruising RPMs even more and pick up another 4-5mpg.

For anyone else reading, this is the shortcut way to good real world fuel economy;

- Run the lowest RPM as possible for any given speed.

-Keep speeds below 65, if possible. The lower the cruising speed, the better, as long as you're in top gear and the TC is locked. Fuel mileage will improve all the way down to whatever speed the TC unlocks in top gear.

-Use momentum in hills.

-Be easy on the accelerator.

-Be very steady on the accelerator.

-Go with as high a rear gear (low numerical) as you can get away with for most of your driving and use transmission gears to make up the difference when loads increase. You can always easily change what gear the transmission is in, when loads lighten, but rear end gears are a lot harder to change. In other words, heavier loads are often temporary, too low of a rear end gear(high numerical) is much more permanent. No point in spinning that engine faster than necessary, as it just wastes fuel.

"After many years, I have found 4.10 is a great ratio in pickups. 3.73 being next in line. Only the taller ratios if pickup is primarily general auto transportation and not called upon to do any substantive work."

Maybe for your specific circumstances, but for most other owners, I couldn't disagree more-

A 3.42 is very adequate for most late model GMFS V8 pickups that see typical general residential duty and most work duty. In fact the hp and torque levels have gotten high enough, especially in the 6.2, that most residential users will be fine with a 3.23 or possibly even a 3.08. Especially with the new 8 and 10 speed(coming soon) transmissions.

Maybe 10-20% of buyers would really need a 3.73. A 4.10 is insane for general use in an average truck and should only go in trucks with large oversized tires, plow trucks that see heavy use for 4-6 months, or heavy offroad use, etc. This ratio will use ~20% more fuel than a 3.42(apples to apples) and over a 300k mile life span will use roughly $6-10,000 more in fuel at todays prices. That's nuts.

Edited April 17, 2017 by 3beejay3

Cowpie · April 18, 2017

Yeah, All full of it I am. Guess that is why I can average 17-18 mpg on road trips with my 2015 2500 6.0 4.10 tipping the scales at 7200 lb. And how I can beat the EPA fuel economy numbers on my 2006 CTS 3.6 by 2 mpg, both city and highway estimates. And how I can consistently beat the nationwide class 8 dry van trucking industry average mpg by 20% with a truck pulling a 53' trailer with a average gross combination weight of 69000 lb that is geared so that the engine runs 200-300 RPM higher than OEM recommendations.

Nanotech Environmental · April 18, 2017

Yeah, All full of it I am. Guess that is why I can average 17-18 mpg on road trips with my 2015 2500 6.0 4.10 tipping the scales at 7200 lb. And how I can beat the EPA fuel economy numbers on my 2006 CTS 3.6 by 2 mpg, both city and highway estimates. And how I can consistently beat the nationwide class 8 dry van trucking industry average mpg by 20% with a truck pulling a 53' trailer with a average gross combination weight of 69000 lb that is geared so that the engine runs 200-300 RPM higher than OEM recommendations.

Hey, as long as you believe it.......

Imagine how much lower your fuel bill would be in that big truck if you beat the average by 30 or 35%, by say, running 2-300 rpm lower on the highway. Just imagine.

Cowpie · April 18, 2017

Tried that. Fuel economy lower. 3.36 rear ratio on last semi. On this engine, replaced the goofy wastegated turbo with a BW 171702 assymetric turbo, put a PTP turbo blanket on it, replaced stock exhaust manifold with ported, polished, ceramic coated one. With 2 Walker megaflow mufflers up the back. 3.55 ratio diffs on this one. Same loads on same routes. 6.36 lifetime mpg average on last semi, 7.92 mpg avg with this one. Darn near a 25% improvement in fuel economy with a engine that even has more power than previous one.

Edited April 18, 2017 by Cowpie

Dnt · April 18, 2017

I guess it is all a balance of having the right engine coupled to the right transmission and then coupled to the right rear end then finally the right driver that knows how to massage the setup to get great mileage. This was taken by a friend of mine who was really impressed by how the 6.2 8spd 3.23 just seemed to be loping along at 2000 rpm at 90 MPH and showing an average gas mileage of well over 20mpg and looking dang good doing it LOL

Edited April 18, 2017 by MyFavTruck

Nanotech Environmental · April 18, 2017

Tried that. Fuel economy lower. 3.36 rear ratio on last semi. On this engine, replaced the goofy wastegated turbo with a BW 171702 assymetric turbo, put a PTP turbo blanket on it, replaced stock exhaust manifold with ported, polished, ceramic coated one. With 2 Walker megaflow mufflers up the back. 3.55 ratio diffs on this one. Same loads on same routes. 6.36 lifetime mpg average on last semi, 7.92 mpg avg with this one. Darn near a 25% improvement in fuel economy with a engine that even has more power than previous one.

So,.... you're comparing apples to oranges then, and using completely unrelated data and facts to jump to a wrong conclusion.......

OK......

Edited April 18, 2017 by 3beejay3

tnchevy · April 18, 2017

The sad thing to me is that I got the same if not better gas mileage out of my 2002 4 speed with 4.10 gear ration than I do my 2014 6speed 3.42 rear gear. I know my 2014 has about 30% more power but my 2002 had plenty of power for what I needed it for. I am OCD about keeping up with gas mileage in all my vehicles, including my motorcycles and after 12 years of data on my 2002 it averaged 18.5 mpg, while my 2014 has a lifetime avg of 18.4 mpg (hand calculated)

Nanotech Environmental · April 18, 2017

I guess it is all a balance of having the right engine coupled to the right transmission and then coupled to the right rear end then finally the right driver that knows how to massage the setup to get great mileage. This was taken by a friend of mine who was really impressed by how the 6.2 8spd 3.23 just seemed to be loping along at 2000 rpm at 90 MPH and showing an average gas mileage of well over 20mpg and looking dang good doing it LOL

2000 rpm 3.23.JPG

This is essentially how the Corvette can get such good hwy mileage; a good spread of gears with a fat, wide torque band allow it to loaf along at very low rpm in top gear. Aero helps a bit as well in that case.

It also shows what I was explaining in a post earlier in this thread.

Edited April 18, 2017 by 3beejay3

Nanotech Environmental · April 18, 2017

The sad thing to me is that I got the same if not better gas mileage out of my 2002 4 speed with 4.10 gear ration than I do my 2014 6speed 3.42 rear gear. I know my 2014 has about 30% more power but my 2002 had plenty of power for what I needed it for. I am OCD about keeping up with gas mileage in all my vehicles, including my motorcycles and after 12 years of data on my 2002 it averaged 18.5 mpg, while my 2014 has a lifetime avg of 18.4 mpg (hand calculated)

What has changed? Something has changed in order for your data to go like that & it isn't the trucks, unless there is something mechanically and/or electronically wrong with the newer one. The correct comparison would be to drive both side by side on the same trip, using the same driving methods, speeds etc. and compare fuel usage after that trip.

Edited April 18, 2017 by 3beejay3

TxTruckMan · April 18, 2017

The sad thing to me is that I got the same if not better gas mileage out of my 2002 4 speed with 4.10 gear ration than I do my 2014 6speed 3.42 rear gear. I know my 2014 has about 30% more power but my 2002 had plenty of power for what I needed it for. I am OCD about keeping up with gas mileage in all my vehicles, including my motorcycles and after 12 years of data on my 2002 it averaged 18.5 mpg, while my 2014 has a lifetime avg of 18.4 mpg (hand calculated)

I have the opposite experience. My 1996 4x4 with 5.7L, 4 speed, and 3.73 gears got horrible gas mileage. I averaged 14 or so. My new 6.2L with 8 speed and 3.23 gears averages 17.5 mpg over same commute.

Like I said from the start - ALL OTHER THINGS BEING EQUAL, taller gears will get you ~9% worse mileage in most cases. The 8 speed will help put the engine in its sweet spot for mileage. Sure, if you tow a lot, taller gears will help when towing. But unloaded, lower gears are better.

mookdoc6 · April 18, 2017

That is why GM offers 3.42's as the bread and butter! It's just right with pull/efficiency! I really like how the 8speed handles the hills and what not....I really want to get out of my truck and choke the guy out in front of me after driving it in HEAVY TRAFFIC! It's the Biopolar Trans baby get you one

Donstar · April 18, 2017

This is an interesting discussion with some interesting perspectives. I would choose a truck with the towing mirrors! Whether it is a 3.42 or a 3.73 it will be fine with a 7,000 lb. trailer. A V6 with a 3.42 is rated and capable of pulling a 7,000 lb trailer. The extra torque and hp of the 5.3 is a bonus. Your powertrain choice will be reflected in your mpg when towing. If you are only towing for a small percentage of your driving time then mirrors are more of a concern than stock gear ratio choices, imo!