3.31 vs 3.73 Highway MPG Difference

[v8440]

As I recall, an engine's lowest bsfc (brake specific fuel consumption, a direct measure of how much fuel an engine is using to produce a given amount of power) occurs at peak torque. That may well explain the axle ratio selections of many different vehicles.

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[roadPilot]

I don’t consider a window sticker an explanation.

The estimated MPGs are identical. That means there's very little difference between the two. Again, not rocket science.

 

[roadPilot]

Rear gear ratio is the multiplication of torque so at a 1 to 1 ratio transmission ratio (7th) 3.31 x 410 =1357, 3.55 x 410=1,455 , 3.73 x 410=1529. 410 is the torque output of the 5.0

Your moving a plus 6000 pound brick on wheels one can struggle with a short lever or one can move the same thing easier with a longer lever. It's your choice.

Another simple example is using a torque wrench with a 24" handle VS using a torque wrench with a 30" handle to do the same work let say re-torquing your F150 wheel lug-nuts at 150 pound feet. You will most often look down and verify the setting on the 30" wrench vs struggling with the 24" wrench.

Agreed. Except the difference between these two is like the difference between a torque wrench that is 30.0" long and one that's 30.1" long.

 

[roadPilot]

I have a 2024 XLT 5.0 FX4 with ~400 miles on it now. On my 87 mile round trip, that is mostly freeway driving 65-75 mph with no traffic, I was averaging 19.9 to 20.5 MPG with 3.73 gears. My previous 2013 F150 with 3.5 Max tow and 3.73 would get 19-22 mpg...(that was a 2wd and 6speed)

Also I am stuck with Commie e10 gas. When I was able to get e0. I would get 1.5 MPG more in my previous truck.

Unless EVERY single factor was identical except for one (the ring & pinion), you cannot accurately compare your results.

 

[Samson16]

? If I wasn't concerned I would be driving a Super duty ?
My 19' with 3:31 definitely got better Gas mileage than my 24' with 3:73. Different generations, but pretty sure the 19' was a 10 speed...don't remember.

Your 2019 got better than 20mpg? ?

 

[Samson16]

Agreed. Except the difference between these two is like the difference between a torque wrench that is 30.0" long and one that's 30.1" long.

Not 0-60

 

[adyaro77]

For me to get 20 miles per gallon even when I drove it off the lot with my 24' I had to drive like a grandpa. My 19' had 60 lb BF Goodrich on each wheel an averaged 18 and a half.

I just threw on new wheels and tires on my 24' and added 4.6 lb per wheel/ tire. I now average 18. So yeah unfortunately I thought the 3:73 rear end would help with that.

Edit: when I first made the swap I didn't think it made that big of a difference but apparently it did. I was always under the impression that the bigger rear end would help with the additional weight. I even went back and made sure that my 19' had a 3:31. Either way 18 miles per gallon for a lifted truck with bigger tires I think is good. Now this is my normal daily commute stop and go traffic. Now if we're talking freeway my 24' is much better.

 

[HoustonScrew]

My 2018 5.0 with 3.31 gears would easily get 27+ mpg on the highway at 65. If i pushed to 75 it went down to low 20's. City driving is what killed it for me, maybe 14mpg. my 2024 with 3.73 powerboost easily gets 24+ on the surface streets, but drops into the low 20s at highway speeds.

 

[Saturnfire]

My 2018 5.0 with 3.31 gears would easily get 27+ mpg on the highway at 65. If i pushed to 75 it went down to low 20's. City driving is what killed it for me, maybe 14mpg. my 2024 with 3.73 powerboost easily gets 24+ on the surface streets, but drops into the low 20s at highway speeds.

Wow. I will need to check once the truck is broken in a bit more.


my 2024 is an XLT 5.0, FX4 with 3.73.

I should have more real world driving to log as this will be my daily. Again. too many variables, but I am good with 20mpg. None of my other vehicles get that. My TRX, if I drive it auto model, it would get ~14- 14.4 mpg on that loop. However, in my custom setup which is essentially in baja mode with some traction control safety enabled I am getting ~12.8 MPG on my R/T's of 87 trip. No biggie I did not buy any of my vehicles to compete on hyper mile-ing bragging rights with Prius owners.

 

[TacoJon]

The estimated MPGs are identical. That means there's very little difference between the two. Again, not rocket science.

Automakers mostly self-report MPG to the EPA. Not many people are getting the EPA 24 MPG unless they are driving 65 mph or below in favorable conditions. Physics says there will be some difference. I've gathered it will be quite small, 5-10%.

 

[HOTRODTIM]

2023 RCSB 4X4 2.7 , 3;31 diffs. 10 speed auto. 20,000 mi. "Running average " total since bought is 20.8 Mpg. No matter what. Very Happy.

 

[Samson16]

For me to get 20 miles per gallon even when I drove it off the lot with my 24' I had to drive like a grandpa. My 19' had 60 lb BF Goodrich on each wheel an averaged 18 and a half.

I just threw on new wheels and tires on my 24' and added 4.6 lb per wheel/ tire. I now average 18. So yeah unfortunately I thought the 3:73 rear end would help with that.

Edit: when I first made the swap I didn't think it made that big of a difference but apparently it did. I was always under the impression that the bigger rear end would help with the additional weight. I even went back and made sure that my 19' had a 3:31. Either way 18 miles per gallon for a lifted truck with bigger tires I think is good. Now this is my normal daily commute stop and go traffic. Now if we're talking freeway my 24' is much better.

I think the lower gearing improves your effective torque. More crankshaft rotations per wheel rotation. I think the mpg would improve slightly at lower speeds under load but be offset by higher revs on the highway once cruising.

 

[Calson]

Most of the horsepower (and fuel) is used to overcome air drag for a vehicle with the drag coefficient of a brick. I care most about acceleration as in the west the onramps are usually very short and I need the truck to go from 15-20 mph to 65 mph in less than 100 feet or I can get hit by a tractor trailer rig as the drivers seldom move into the next lane to allow motor vehicles to safely merge.

There is also the time needed with a heavy payload or trailer in tow to safely pass the tractors pulling doubles on steep grades where they are traveling at 20 mph or less. Even with a diesel engine I wanted 3.73 gears to minimize my exposure time when passing.

The additional speeds on current transmisions provide more overdrive gears to improve fuel economy but do nothing to compensate for gas engines that produce maximum torque at 3400 rpms. This is when the read end gear ratio is important for trucks carrying a heavy load or cab full of passengers.

Fuel economy is trivial by comparison to these safety concerns as well as the mental stress when merging with trucks and traillers to get onto the freeway. I purposely bought a F-150 with the 36-gallon gas tank even though it meant having to buy a supercrew cab model as that was all that was available in California.

 

[Samson16]

According to Ford my PB produces 570 lb-ft at 3,000 RPM. I feast on short on ramps ?

ps 3.73
pps Just noticed this is a 5.0 thread so allow me to rephrase. The wonderful 5.0 would benefit in short on ramp situations utilizing the 3.73 rear end especially with heavier aftermarket wheels and tires. ?

 

[mwallace]

I have a 2023 crew cab XLT 4x4 5.0 w/3.73 gears. It has about 29k miles on it. I typically get 20-21 mpg on interstate road trips in eco mode (not sure that the mode matters...). That's interstate driving at around 75-80 mph. Stock wheels and tires right now.

Otherwise its around 19 mpg.

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