Maximizing MPGs at high speed

[nitrobass24]

So I recognize that driving fast is counter to getting the best MPG, so I’m just trying to figure out what settings I can change to maximize for my situation.

I drive mostly highway miles where the speed limit is 75-85mph so I’m usually driving 90 just to not get run over.

I usually run 87octane in Eco mode and get about 18MPG.

Thoughts on locking out gears, ECU tune or changing rear diff gearing?

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[HammaMan]

Honestly just try and stay out of boost. Think of the trucks being tuned for NVH over economy. It does good-nuff in the EPA stuff because it's 55mph. The truck doesn't want to shift, and it doesn't need to bringing in the boost. You'd really have to look at your roads and see what works. Maybe a gear lockout is better, maybe it's not. At those speeds aerodynamics quickly becomes the limiting factor. EVs start taking hits at those speeds as well because of aero -- at those speeds you gotta pay to play -- in fuel that is.

 

[powerboatr]

good one
i drove from east of big d to mobile al to south fl, back to mobile al then to east of big d last december in 4 days
best i got was 23 in eco on the very flat parts of florida highway 10, but i switched back to normal real fast as truck was slow to regain speed and i was in a HURRY
speeds were 70 to 80 almost the entire time until the 2hrs of stop and go near gainsville fl
most was in 4A and some in 2H..it didnt make much difference
75 was the cruise set speed
you will notice boost was low most of time

what got me, was the year before i made same trip in my 2021 f250 6.7 . with same gear sets 4x4 8600lb brick and got 23 easy as falling off a log

 

[Samson16]

Stay out of cruise control. It hurts mileage even on gradual downhills. Normal mode. 2wd. No cruise. Let the vehicle speed increase and decrease as you follow terrain. Take any opportunity to ease the accelerator pressure while maintaining speed.

The PB is a serious power machine. Getting 19mpg or so doing 90 in a gas engine truck with that much power is pretty amazing. The mileage only gets better from there. It's a worse case scenario.

 

[Gros Ventre]

Recognize that the big square front of the truck has the aerodynamics of a brick... Then the aerodynamic resistance is a cubic function. So do the math, 18-19 mpg isn't so bad... Then if you add in a headwind...

 

[oneinch]

90? MPG? That's an oxymoron.

 

[amschind]

The simple answer is slow down or consider the economics of a second, more fuel efficient vehicle. I'm in the same boat, and for me the answer is no second car and slow down. The issue with speed is that on longer journeys it rapidly loses importance. Think about 90 MPH vs 60 MPH: it's a 1.5x speedup if you get in your truck in one place and drive for x hours until you reach your destination. HOWEVER, when you average in time spent at 0 MPH or very low MPH (stops and slowdowns), the effects of that speed begins to outweigh the advantage of a higher maximum speed. That realization led me to 62-68 behind big trucks and doing my best to minimize stops.

For shorter journeys where you don't stop, you may just conclude that your time is more valuable than any fuel savings. I think that's useful: calculate the fuel cost at 60 vs 90 MPH, caclulate the time cost and convert it to "pay" in terms of $/hr saved. It helps me to consider "is this cost worth the time that I get back?"

 

[bgalakazam]

Where are these US highways... I remember 60-65, some 70 out of town.... Anyway, anything over 65 starts to kill MPG. 80+ really start to notice it. I drive on the autobahn, limit is 130km/h (~80mph) and some sections unlimited. I've driven the limiter ~106mph. MPG approaches single digits. This is with 5.0 and 3.31. Sweet spot is 60mph which is not feasible in Europe. US, I routineley drove 65-70 max.

 

[Ziggy73]

So I recognize that driving fast is counter to getting the best MPG, so I’m just trying to figure out what settings I can change to maximize for my situation.

I drive mostly highway miles where the speed limit is 75-85mph so I’m usually driving 90 just to not get run over.

I usually run 87octane in Eco mode and get about 18MPG.

Thoughts on locking out gears, ECU tune or changing rear diff gearing?

Get rid of any unnecessary weight. Install Tonneau cover, fold in side mirrors on straight sections (if you are ok with that). Air tires to 40. AEM air filter(dry flow).

 

[nitrobass24]

Where are these US highways... I remember 60-65, some 70 out of town.... Anyway, anything over 65 starts to kill MPG. 80+ really start to notice it. I drive on the autobahn, limit is 130km/h (~80mph) and some sections unlimited. I've driven the limiter ~106mph. MPG approaches single digits. This is with 5.0 and 3.31. Sweet spot is 60mph which is not feasible in Europe. US, I routineley drove 65-70 max.

Texas 130 toll road. I have a construction services business in Austin so I am driving all the time between job sites and this is the fastest way to go north/south. Quite remarkable to be going 90 and get your doors blown off.

 

[nitrobass24]

Stay out of cruise control. It hurts mileage even on gradual downhills. Normal mode. 2wd. No cruise. Let the vehicle speed increase and decrease as you follow terrain. Take any opportunity to ease the accelerator pressure while maintaining speed.

The PB is a serious power machine. Getting 19mpg or so doing 90 in a gas engine truck with that much power is pretty amazing. The mileage only gets better from there. It's a worse case scenario.

Interesting comment about cruise control as I am almost always in cruise because I thought it would do better than I could at managing it.

 

[Samson16]

Interesting comment about cruise control as I am almost always in cruise because I thought it would do better than I could at managing it.

I have found with my PB that regen breaking is used to maintain the assigned speed and momentum is lost as a result. Just letting it freewheel on the downhills is better for maximizing mpg.

 

[DcnPat]

Interesting comment about cruise control as I am almost always in cruise because I thought it would do better than I could at managing it.

CruIse control is trying to maintain a set speed. It has zero knowledge of terrain or other factors. Throw in the tendency of the programming to favor boost over down shifting and you can definitely do better manually if you have the skill and will to do so.

 

[Samson16]

CruIse control is trying to maintain a set speed. It has zero knowledge of terrain or other factors. Throw in the tendency of the programming to favor boost over down shifting and you can definitely do better manually if you have the skill and will to do so.

Good point there. Cruise will try to maintain RPM for your ride comfort and use nearly imperceptible boost increases instead.

 

[RuggedGoods]

If you want mpg at speed, you need to focus on aerodynamics. I took a 2003 Jetta TDI from 46 mpg to 56 mpg hwy (70-75 mph cruising) with some extensive aero-modding.

Do you have a tall antenna flapping in the wind? Go shorty.

Is your bed covered with a tonneau cover?

Tow mirrow or regular mirrors? Do you reeeeallly need that passenger side mirror?

Adding more front air dam, wheel spats, seal front gaps, partial radiator block off, full underbody panels, rear diffuser under the spare tire, partial wheel-well covers. And more.

LRR (low rolling resistance) tires at 50 psi. Michelin defenders are excellent.
Lightweight wheels can help around town, less effect on the highway.

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