Gas Mileage 5.3 3.42 vs 3.73

TxTruckMan · April 11, 2017

Hey Everyone

I'm new here, and do not currently own a GM truck. I currently own a 2006 Nissan Titan and I'm starting to look to get into something newer. I do tow a 28 foot 7000 pound travel trailer from time to time in the summer. I like my Titan, but want to go to something different. My father in law and brother in law both have Sierra's a '15 All Terrain and a '16 Kodiak.

What I want to know from those that are in the know is what kind of fuel economy are you seeing in your 3.73 truck vs those that have the 3.42 trucks. I know my father in law has a 3.42 truck and gets steady 21 MPG on the highway.

I'm well aware that these are trucks and are not going to be great, but considering my Titan gets a stead 13 MPG just hauling itself around and 9 MPG hauling my TT I am looking to do better. The 3.73 truck is the one I am looking at for the added towing ability, but the 3.42 is no slouch.

Anyway if you could shoot your number and gear ratio as a reply, as well any guys with the 3.42 towing anywhere near the 7000 pounds what are your thoughts, feelings and such with that setup?

Thanks

Well, the 3.73 gears are ~9% taller, so one would expect ~9% worse gas mileage with them on everyday driving. I averaged 17.5 in mixed driving on my 2014 Denali with 5.3L and 3.42 gears. Around 20-21 on highway trips.

Billy346 · April 11, 2017

If you are going to use it for towing, I would go with the 3.73 regardless of fuel economy. Even with the 5.3, the max trailer edition with 3.73 gears can pull your trailer without struggling.

The gas mileage that your truck gets is going to depend largely on where you drive it and how it is driven. My truck has the 3.42 gears, but I drive it around town a good amount. The roads in my area are high speed (about 45 mph to 55 mostly) with lots of red lights and plenty of hills. Around town, my truck gets about 12 MPG. However, if I get out on the highway I get anywhere between 19 and 23 MPG. (I should add that I have bigger tires with a level kit, which probably takes away one or two MPG's.)

Cowpie · April 11, 2017

Well, the 3.73 gears are ~9% taller, so one would expect ~9% worse gas mileage with them on everyday driving. I averaged 17.5 in mixed driving on my 2014 Denali with 5.3L and 3.42 gears. Around 20-21 on highway trips.

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.

Edited April 11, 2017 by Cowpie

CO_2014LTZ · April 12, 2017

2014 LTZ 4x4 with 3.73 ratio, pulling 6000lbs trailer around Colorado getting as much as 14 mpg, but that's sort of on level roads at 45 to 50 mph. Less on the Interstates, but around 11 to 12 mpg. I get a lot less mpg going up hills / passes. Without trailer, I've had 22 to 23 mpg on the highway doing 65 to 75 mph. I was impressed but it helps when the truck downshifts to 4 cylinders and the roads are level.

TxTruckMan · April 12, 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.

True, a lot of other factors come into play. BUT, if the lower gears don't get better mileage, then why have them? I guess I should have started by saying "all other things being equal"...

Of course, having more gears (8) in the transmission helps the higher gears make up the difference - there's less gaps in the "efficiency" bands, so the engine/tranny can compensate for the different ratios. So at lower speeds, it'll be a wash. Actually, one may be able to argue that the higher gears could get you better mileage as they have an easier time revving up. But the biggest differences will be on the highway. Once you hit the top gear in the tranny, the lower gears have the advantage. That's why the lower gears are there - highway fuel economy.

Cowpie · April 12, 2017

More trans gear do help in regards to keeping engine RPM spreads between gear changes more tight, but the top gear ratio of the 8 spd and 6 spd are virtually identical. With tall rear ratios, a lot more trans gear shifting goes on to overcome variations in terrain under loads. Negating the fuel economy benefits.

You have to keep in mind that a major portion of why GM does this sort of thing pertains to appeasing the government gorilla CAFE nonsense.actual work and various aspects of what pickups are made for don't play as much as we would like into it. Ever wonder why a lot of this nonsense like AFM and super tall ratios are not part of the 3/4 ton game? It is because they do not fall into CAFE regulations. The tall ratios like 3.08 do deliver better MPG, but only on flat roads, no loads, no winds, etc. The CAFE testing is done inside on a platform, not even on a test track. In the real world, things are not so ideal.

Edited April 12, 2017 by Cowpie

mookdoc6 · April 12, 2017

More trans gear do help in regards to keeping engine RPM spreads between gear changes more tight, but the top gear ratio of the 8 spd and 6 spd are virtually identical. With tall rear ratios, a lot more trans gear shifting goes on to overcome variations in terrain under loads. Negating the fuel economy benefits.

You have to keep in mind that a major portion of why GM does this sort of thing pertains to appeasing the government gorilla CAFE nonsense.actual work and various aspects of what pickups are made for don't play as much as we would like into it. Ever wonder why a lot of this nonsense like AFM and super tall ratios are not part of the 3/4 ton game? It is because they do not fall into CAFE regulations. The tall ratios like 3.08 do deliver better MPG, but only on flat roads, no loads, no winds, etc. The CAFE testing is done inside on a platform, not even on a test track. In the real world, things are not so ideal.

Now, Look at the announced GM what is it 10L80 vs the 8L90 top Gear ratio's....VERY SMALL difference......10 is the end my friend!

Bowtiejunkie · April 12, 2017

On my 2014 CC 5.3 3.73 and 4x4, I can squeak out 20 mpg on 100% freeway, no hills going 70 mph using 87 octane. The DIC mpg has always been off 1 mpg, so I manually calculate it. I nearly always use E-85 though, and it's 17.5 mpg same freeway drive I just stated. Towing a 5,000 boat/trailer, with E-85, I barely get 11 mpg. on freeway at 60-62 mph. I like the increased performance using E-85, so don't have any solid data on towing while running 87 octane.

flattop · April 12, 2017

I have a 17 5.3 Max tow with the 3.73's and get an average of 16.9 mpg with city and highway driving. I do have a fiberglass cap and carry about 500 lbs of tools and equipment in back. My driving is slightly aggressive, as I like to accelerate briskly and run slightly over the speed limit.

The best i have seen is on the thruway with the cruise set around 73mph is around 19 mpg. Not to bad in my opinion. The gears are well worth the slight penalty in fuel mileage.

Flattop

I have a 17 5.3 Max tow with the 3.73's and get an average of 16.9 mpg with city and highway driving. I do have a fiberglass cap and carry about 500 lbs of tools and equipment in back. My driving is slightly aggressive, as I like to accelerate briskly and run slightly over the speed limit.

The best i have seen is on the thruway with the cruise set around 73mph is around 19 mpg. Not to bad in my opinion. The gears are well worth the slight penalty in fuel mileage.

Flattop

phabejhmoob · April 12, 2017

average 16 winter (city/hwy), 18 summer/fall (city/hwy) when weather is warmer. I got 22MPG at one time on cruise, but average 21 in summer/fall on trips 3hrs each way.

5.3 / 3.42

Edited April 12, 2017 by cs2016lt

Grumpy Bear · April 12, 2017

I have experienced more than once going to deeper gearing and getting better mileage. No not because I failed to calibrate the speedo/odometer. Deeper gears do indeed spin her faster but...they also reduce the load on the motor. Mileage is about LOAD, not speed or rpm. Will it always do that? Nope. But sometimes it will. More than you might think.

Cowpie · April 12, 2017

Now, Look at the announced GM what is it 10L80 vs the 8L90 top Gear ratio's....VERY SMALL difference......10 is the end my friend!

Again, small steps between gears, but the same problem of being too low of engine RPM in top gear so that even on rolling hills with anything in the box or on the hitch and the trans need to go into gear hunting mode to get RPM's up for the pull.

The new 10 spd, as proposed, has a top gear ratio of .64. The 6L90 currently in the 2500 has .667, like I stated previously, virtually identical. Let's see how that looks.... as I stated earlier, 4.10 at 65 on stock 18" rim rubber would have the engine turning at 1900 RPM. With the 10 spd .64 top gear and 3.08 rear ratio, at 65 the engine RPM would be at roughly 1350. We all know that is well outside any working performance band and almost immediately the trans would have to downshift on so much as an elevated overpass with any kind of serious load to bump RPM's up to generate the power to maintain the pull. Get into hills, and the trans will be all over the place trying to keep the engine in an effective RPM band for the pull.

The only reason for more gears is due to the much taller 3.08 ratio and needing to overcome harder start off, as the 8 and 10 speeds do have deeper reductions. Still, drivetrain stresses are magnified tremendously doing it that way.

Commercial heavy trucks have been playing in this tall ratio sandbox far longer than GM ever caught on. And there is a wealth of data out there. Taller rear ratios, when used with top gear direct drive transmissions do provide some fuel economy benefit, but they also have higher maintenance costs and require some pretty beefed up driveline components. And they have the same problem... any little mole hill and they drop out of effective RPM band and shifting comes into play. But a setup that keeps the engine RPM's in the top of the effective band of the engine usually requires less shifting and better fuel economy when actually doing something more than running down a flat, straight interstate highway.

Some of these commercial trucks, as promoted by the OEM's, have rear ratios clear down to 2.21 and engines turning 1100 RPM at 65 mph. Mine is set up to be turning 1500 RPM at 65 mph, and I can pull a lot of hills, fully loaded, and never need a downshift. And for the last 20 years, I have averaged 20-25% better fuel economy than the industry average for Class 8 trucks in the same freight type category. That is bucking everything the OEM's are trying to shove at the trucking industry. GM is drinking at the same well.

Edited April 12, 2017 by Cowpie

Diver6 · April 13, 2017

with the 6.2 are you running premium (as recommended), or only when towing, or never?

I live close to the US so I go there and fill up with 93 its a lot cheaper, but when I fill up in Canada I run mid or reg just driving around and premium only when towing. The guys in the US think they have expensive fuel, its nothing compared to here!! 1.29 per liter = 4.51 per gallon. As far as gas mileage goes you have to remember most guys on here live in the US were the speed limit is 75 to 80 MPH and here its only 60 so you will get a lot better mileage than your seeing here.

Edited April 13, 2017 by Diver6

Darth_Vader · April 14, 2017

4.56 gears and I average 19 MPG on the highway.

5RWill · April 14, 2017

Maybe i'm a bad driver...but my 3:42 6spd with my 5.3 is averaging 12.5-14mpg in town and 16 on the highway. With exhaust, MIT, and 295/70/17 RTs. I've got long tubes ready to go i just need to get them installed and then i'm having it tuned and AFM deleted.

So yeah my 5.3L has never been on the efficient side, even with stock tires it was 14-16 in town, and 18 on the highway. Which truthfully is what the sticker advertised. Having said all this i run cheap 87 octane most of the time. Those numbers are better with pure gas or 91/93 octane.

I actually want to move to 4.11 or higher once i upgrade the suspension.

If it were me personally i'd go 3:73 or 3:42 with full intention of re-gear.

Edited April 14, 2017 by 5RWill