Gas mileage hit 3.73 vs 3.42?

[kstruckcountry]

9.1% difference in gearing. That isn't going to translate into 9.1% better mileage. While you gain some efficiency since the motor isn't spinning as fast, you have to remember that you're also LOSING 9.1% of your mechanical advantage, which will affect how much work the engine has to do. Gearing isn't the only thing that affects mileage, even unloaded on a flat road with no wind. It's not a linear relationship. So you'll never see a 9% bump on the road. 3.08 gears would provide MUCH better economy than they do if that relationship were true.

[rdnkcpa]

I'm kind of curious what kind of roads you run on? For me, 17 highway is easily doable, even with some wind. And that's with everything in my sig.

Limited access highways, some with traffic, some with none. But for the most part a fair amount of traffic. I live in the Washington DC/Baltimore Metro Area, about 45 miles from DC and 30ish highway miles from B-More. Would not say flat, would not say hilly.

[Nanotech Environmental]

9.1% difference in gearing. That isn't going to translate into 9.1% better mileage. While you gain some efficiency since the motor isn't spinning as fast, you have to remember that you're also LOSING 9.1% of your mechanical advantage, which will affect how much work the engine has to do. Gearing isn't the only thing that affects mileage, even unloaded on a flat road with no wind. It's not a linear relationship. So you'll never see a 9% bump on the road. 3.08 gears would provide MUCH better economy than they do if that relationship were true.

In an unloaded situation at typical cruising hwy speeds(ie say 65mph), there is little, if no gains to be had via mechanical advantage(up to a point either way). For example, with the 3.42/5.3 package, the truck isn't using anywhere near the available power at the RPM req'd to maintain normal speeds in most, or all of those situations, especially on flat terrain. Therefore going with lower gearing won't help, but will just make the engine spin faster, producing more unused power and waste gas. Infact, it could actually use more fuel than the %age difference in gearing, depending on the situation.

The mechanical advantage only comes into play once you run out of torque at a given gear ratio vs RPM&load. On the flip side, if you go with too high of a gear, it may not produce all the desired gains in economy, as you may over load the torque available at cruising speeds.

It seems to bear out in real life, as stock trucks with 5.3 seem to be in the 15-18mpg range with 3.73 and in the 17-20 range with 3.42.....

[kstruckcountry]

I've got 32,000 miles worth of driving with the 3.73's, high A-B-A trip over 1000 miles of 18.5mpg. 18 is very doable at 72 on the interstate. I guess I need to swap to 3.42's and find out for sure.

 

 

It's worth noting that with 285/70/17 tires, the effective gear ratio is somewhere in between 3.73 and 3.42. Turning that size of tire, I wouldn't be surprised if mileage actually dropped some with 3.42 gears. Big tires and fuel efficiency aren't buddies.

 

I prefer the way the pickup runs with 3.73's, it rarely downshifts, and climbing steep grades it's easy to keep it in the sweet spot. That 18.5mpg trip included driving up and down WC pass multiple times. I realize that others may have different results from the same gearing based on how and where they drive.

[bgbdwlf2500]

I kinda forgot where I was going with what I said. If you've towed with a 3.73 towing with a 3.42 is night and day. I don't think id hook a trailer to mine with the 3.08

[Grumpy Bear]

I just drove over 1000 miles and 15 was the highest I got (cruising at 77-80). If I got 17 I could live with it.

 

 

I have these on as well... could they be robbing me?

 

Count you blessings at that speed on those tires your not getting 10-12 mpg.

[barry G]

interesting thread revival.

 

I have 3.73 and the 5 speed.. 5.0 (306)

16 mpg seems to be its norm.

77-80 on the cruise.

21 years old this year.

 

highway, city, no difference.

 

seems i could tug at my dads 17000 pound rig in a rutted driveway easy enough.

 

I guess i can accept my own mileage.. I seem to be in the average of this thread.

[KARNUT]

Long ago I had a 73 impala. I changed the 273 out for a 373. I went down 2 mpg at 60 mph on the HWY. Pick up 2 mpg in town which was good trade off for that's where it was the most, being my wife drove it the most.

[Dnt]

Don't know the answer for sure but it would seem that even though the engine is turning a faster RPM with the 3.73 that the same HP is being utilized to keep the truck at speed. Doubt if the mileage would vary much at all?

 

I can tell you that a 2016 6.2 8spd 3.23 gets over 20mpg consistently over 16,000 miles in mixed driving and does a little better maybe 25mpg or so on highway cruises.

[redvett]

My 20014 EXT cab "LT" 5.3L with 3:73 gears and OEM Bridgestone tires 255-70R16 and highway I get 19.5 to 20.3 MPG.

If you have larger/heavier tires mileage will be lower.

[MaverickZ71]

If you ever pull or haul anything, you'll want 3.73's in a GMT800 or 3.42's in a GMT 900 or newer. If fuel mileage is the biggest concern, go with a diesel Colorado or a diesel Cruze.

[Gurrzt]

The RPM difference between the two is negligible, case in point the difference between 3.73:1 and 3.23:1 is only 500 rpm.

 

If you want to feel a seat of the pants difference from a dead stop, have a 3600 stall installed with the stock gears.

[Grumpy Bear]

The RPM difference between the two is negligible, case in point the difference between 3.73:1 and 3.23:1 is only 500 rpm. If you want to feel a seat of the pants difference from a dead stop, have a 3600 stall installed with the stock gears.

 

I calculate the difference at 60 mph to be 224 rpm. At 134 mph 500 rpm.