Speed and MPG

[Spiritof76]

Interesting data point (at least to me).

2021 Silverado 2500HD Crew, 4x4, longbed with the factory MTs (loud as hell). Pulling a 28' travel trailer weighing in at about 8500#.

Ordinarily, drive with traffic at around 75mph, and get around 8mpg. Took the family from home (Sacramento area) down to Palm Springs, down the 5 and up the Grapevine. Had a few slowdowns and stops with accidents and such, and with traffic around LA. Set the cruise at 65 and let her roll (except when slowing for traffic). The factory lie-o-meter said 11.4mpg, and hand calc'd said 11.3.

Before this, I pulled it with a 2015 Tundra Crewmax SR5 with the 5.7L. At 65, it was not happy, and was getting maybe 8-9mpg. At 75, it was hating life and getting 6.5mpg.

 

[tracyb]

imagine that...


i run cheap trucks and gear to suit. haven't decided if the 5.4l f350 is getting 4.88 or 5.13's.

speed costs and gearing matters.


epa dont give a fuck about loaded mpg

 

[Tiha]

I am convinced it takes a certain amount of fuel to do a certain amount of work.

My 1995 F350 PSD will run 18mpg at 70
My 2014 SD 6.7 will run 18mpg at 70
My kids 2004 6.0 excursion. 18mpg at 70.

Sure you can make improvemtns up or down, but the most part they average the same over time.

 

[Derp]

I am convinced it takes a certain amount of fuel to do a certain amount of work.

My 1995 F350 PSD will run 18mpg at 70
My 2014 SD 6.7 will run 18mpg at 70
My kids 2004 6.0 excursion. 18mpg at 70.

Sure you can make improvemtns up or down, but the most part they average the same over time.

Is that empty or towing a significant load? From what I've heard, those seem to get near gasser mpg when loaded. Still, being able to get that mileage empty would sure be nice.

Agreed on needing a certain amount of fuel to do a certain amount of work, but I've heard fuel delivery method plays a part too, and high pressure diesel injectors do a much better job of atomizing the fuel over gas injectors, and there are future improvements to be had in direct injected gas engines?


Still, hearing that new gas trucks are getting about the same mpg as my '05 make it a lot easier to swallow driving an older truck

 

[Tiha]

Is that empty or towing a significant load? From what I've heard, those seem to get near gasser mpg when loaded. Still, being able to get that mileage empty would sure be nice.

Agreed on needing a certain amount of fuel to do a certain amount of work, but I've heard fuel delivery method plays a part too, and high pressure diesel injectors do a much better job of atomizing the fuel over gas injectors, and there are future improvements to be had in direct injected gas engines?


Still, hearing that new gas trucks are getting about the same mpg as my '05 make it a lot easier to swallow driving an older truck

Just running empty down the road.

My 1995 is about 8000 pounds
My 2014 6.7 I think is about 6900 pounds.
Not sure what my son's excursion is.

Yeah some days it might be 18.5, and some days it might be 17.5 but over all there really isn't a change between 1995 and 2014.

geez, even my old 6.2 two wheel drive suburban never did much better than 18.

 

[Derp]

Just running empty down the road.

My 1995 is about 8000 pounds
My 2014 6.7 I think is about 6900 pounds.
Not sure what my son's excursion is.

Yeah some days it might be 18.5, and some days it might be 17.5 but over all there really isn't a change between 1995 and 2014.

geez, even my old 6.2 two wheel drive suburban never did much better than 18.

My 1990 F350 with a 460 was about 6000lbs
My 2005 F250 with a V10 is about 7200lbs

Both got/get 9-10 empty. No matter the load the 460 truck would get the same mileage, it would just slow down. No matter the load the v10 truck goes as fast as I want, the mpg just goes way down.

I'm just throwing this out there for the sake of information and comparison. I agree that mpg for a given amount of work being done doesn't seem to have changed much over the years,

 

[ironman_gq]

I found it pretty similar with my 08 silverado diesel, triple axle car hauler with 2 rigs on it. I figured it was close to 14k, at 65mph I managed 12mpg at 75mph it was pretty close to 9mpg. Empty that truck would do 21-23mpg at 65 and 18-20 at 75.

 

[Provience]

2500 suburban 7.4l would get 11 MOG unloaded, 12mpg if I could cruise 80 all day and about 10 hooked up to a trailer.

Too much motor for no load and good fuel economy, so increasing speed came with little downside. Same with adding load, takes a bit to get it loaded enough to be maxed out and fully utilized.

 

[[486]]

my service truck is a shitty 5.8/e4od f350 with 4.10s and 30s
it'll get 5 if I do 85
it'll also do nearly 15 if I keep it to 55

 

[DMG]

MPG/speed is as much about frontal area and aerodynamics as weight.

 

[[486]]

MPG/speed is as much about frontal area and aerodynamics as weight.

esp where you don't got any hills

 

[tracyb]

btu/ gal vs resistance.

complicated and simple. a little bit of gearing does wonders.

 

[Projectjunkie]

I’ve always figured it’s the right rpm/boost to get those numbers with those motors. 1900-2100rpm and 10-15psi iirc were my sweet spots

There's some cummins paperwork somewhere that says their engines get the best mileage at 1700rpms, my next build will likely have super duty 3.23 axles and whatever size tires gets me the best mileage at 75mph, then I'll upgrade the engine as needed to keep it in 6th as much as possible to use some boost to roll over the smaller hills

 

[Clb]

Imagine that....
Now put some aero to the equation.

 

[[486]]

There's some cummins paperwork somewhere that says their engines get the best mileage at 1700rpms, my next build will likely have super duty 3.23 axles and whatever size tires gets me the best mileage at 75mph, then I'll upgrade the engine as needed to keep it in 6th as much as possible to use some boost to roll over the smaller hills

those BSFC charts are at maximum governed output
meaning it gets best fuel economy in terms of hp per gallon at that range when you've got the governor lever pinned

on an automotive application you'll generally not be asking maximum output from it unless you're accelerating
meaning don't worry too much about the RPM range, just run it in as low an RPM as it'll hold speed at

or don't worry about it as without a throttle plate running along at partial load doesn't come with as much of a penalty
also, you're gonna turn the pump up and make lugging the engine like that require a lot more attention than just matting the pedal so you may find yourself doing a belben roadside rebuild or two

 

[Projectjunkie]

those BSFC charts are at maximum governed output
meaning it gets best fuel economy in terms of hp per gallon at that range when you've got the governor lever pinned

on an automotive application you'll generally not be asking maximum output from it unless you're accelerating
meaning don't worry too much about the RPM range, just run it in as low an RPM as it'll hold speed at

or don't worry about it as without a throttle plate running along at partial load doesn't come with as much of a penalty
also, you're gonna turn the pump up and make lugging the engine like that require a lot more attention than just matting the pedal so you may find yourself doing a belben roadside rebuild or two

After posting, I looked, it's more like 1500-1600 for the 6bt
My typical load is a crew cab 4wd, a 10k lb trailer, or empty, well under the 26k a 6bt was designed for, so I figure I could get a reliable 300k out of a setup like that...


The ford diffs are 3.31 the factory 20s are 34" tall and the zf6 has a .72 od so I'd be
1765rpm at 75mph

But speed kills mileage, so as fuel prices rise, ill just leave earlier if needed

 

[[486]]

After posting, I looked, it's more like 1500-1600 for the 6bt
My typical load is a crew cab 4wd, a 10k lb trailer, or empty, well under the 26k a 6bt was designed for, so I figure I could get a reliable 300k out of a setup like that...


The ford diffs are 3.31 the factory 20s are 34" tall and the zf6 has a .72 od so I'd be
1765rpm at 75mph

But speed kills mileage, so as fuel prices rise, ill just leave earlier if needed

you ain't getting what I'm saying

that BSFC chart is at max fuel delivery, like if you were loading down a water pump or DC generator
your road going fuel economy will be best at the minimum RPM that will produce enough power to maintain road speed

 

[DMG]

There's some cummins paperwork somewhere that says their engines get the best mileage at 1700rpms, my next build will likely have super duty 3.23 axles and whatever size tires gets me the best mileage at 75mph, then I'll upgrade the engine as needed to keep it in 6th as much as possible to use some boost to roll over the smaller hills

I have thought about putting together something simlar with a 7.3 psd/6 speed but it seems a lot of expense and effort when i can use the prius or tundra for lighter loads and average out pretty cheap, only using the V10 truck for really heavy stuff.

 

[WTF?]

over 60-65mph its all about aerodynamics/ wind drag; 55-60MPH on the hwy is gonna deliver the best fuel economy but may take a while to get where your going or get your doors blown off by traffic.

a camper has a huge flatish profile on the front extending above the trucks roofline. a kite doesn't weigh much but it sure pulls on you when flying in bigger winds.

my 5.7 tundra got the same Hwy MPG towing a 3k boat with t-top as it did towing a 7.5k boat with t-top. again; you're basically towing a kite at speed.

It's less noticeable when you get below the roofline, but towing my steel flat deck utility trailer empty with the 4' sides up ( think fence) vs the side panels pulled and laid flat on the deck would make a ~2mpg difference behind my half tom

 

[arse_sidewards]

MPG/speed is as much about frontal area and aerodynamics as weight.

To a point.

The Aerostar and the Ranger both get 20mpg using the same engine and trans and a slight tire size difference.

I get 20-24mpg in my ~2900lb Lesbarus doing ~3000rpm at ~70

My buddy, who has the biggest fattest car (2015 Ouback) they ever stuffed the same engine (with more displacement, twice as many cams and whatnot) into gets 30+mpg doing what's basically the same commute on a different highway. Having a CVT counts for a lot.

There's defintly some crossover territory where something that punches a big clean hole in the air beats out something that punches a small inefficient hole in the air.

 

[DMG]

To a point.

The Aerostar and the Ranger both get 20mpg using the same engine and trans and a slight tire size difference.

I get 20-24mpg in my ~2900lb Lesbarus doing ~3000rpm at ~70

My buddy, who has the biggest fattest car (2015 Ouback) they ever stuffed the same engine (with more displacement, twice as many cams and whatnot) into gets 30+mpg doing what's basically the same commute on a different highway. Having a CVT counts for a lot.

There's defintly some crossover territory where something that punches a big clean hole in the air beats out something that punches a small inefficient hole in the air.

My old subarus got close to 30 mpg. even my LGT wagon got 30 if I wasnt running over 70. how old are yourbold subarus? a lotnof older japanese stuff was geared for the 55 mph speed limit, even years after that went away.

 

[TrailTamer]

I’ve always figured it’s the right rpm/boost to get those numbers with those motors. 1900-2100rpm and 10-15psi iirc were my sweet spots

10-15 psi sound high to me if unloaded. Is that normal for the PSD?

My 12v Cummins run at about 5 psi unloaded at 70mph on flat, or ~10-15 psi pulling two jeeps (~20k gcw)

 

[arse_sidewards]

My old subarus got close to 30 mpg. even my LGT wagon got 30 if I wasnt running over 70. how old are yourbold subarus? a lotnof older japanese stuff was geared for the 55 mph speed limit, even years after that went away.

3.73s in the FWD wagons. 3.90s in one of the AWD wagons and 4.10s in the other. 20-25mpg across the board driving like an asshole. 25mpg is the "all highway" number. All are 1992-1994 non-turbo EJ22 and most have automatic transmission. I haven't driven the manual one enough to get a good feel for the kind of fuel economy it puts down.

 

[Stingray]

MPG/speed is as much about frontal area and aerodynamics as weight.

yup.

2006 Dmax crew cab long bed 2wd. same trip every time, out to AZ and back. Cruise set at 80 no radical driving 14-14.5 MPG every time.

Same truck same trip towing around 11k, Two 4runners on a 30' trailer Cruise set around 60-65, no more. 16-16-5 mpg.

Pushing all that wind cost more than the weight does.

It's the same drive with the same setup since 2006, and its the same every time unless we hit a heavy head wind then all bets are off, the truck will hold the speed but you pay in MPG.

you push 25lbs of boost for an hr, you pay

 

[DT75FLH]

Iirc aero comes in to play at 45 mph plus.....

Wind resistance comes up 4x times at 65 vs 45....


With My large toyhauler ...it was worth 2 mpg....from 6 to 8 mpg towing at 70 to 75 vs 80

 

[DT75FLH]

The power wagon at 83 mph is 11 to 12 mpg..we have hills here ...

Slow down to 65 to 70..I got 14 mpg out of it in a recent trip to cali in heavy traffic

 

[Weasel]

MPG/speed is as much about frontal area and aerodynamics as weight.

esp where you don't got any hills

Except reality doesn't show that.
D-max 2007.5, 38's 4.88's -> 10-11 empty and towing at 75-80mph. Just did a round trip to Billings pulling a 20 foot enclosed trailer. Mostly empty. Thrown on the jeep behind a gooseneck and it's the same. Drop the speed to 65mph and can hit 14mpg. Hand calc, but that's the averages.

Dad's 2010 Duramax all stock, he gets 12-13 pulling same trailer. He runs around 70mph and never in a hurry.

 

[[486]]

Except reality doesn't show that.
D-max 2007.5, 38's 4.88's -> 10-11 empty and towing at 75-80mph. Just did a round trip to Billings pulling a 20 foot enclosed trailer. Mostly empty. Thrown on the jeep behind a gooseneck and it's the same. Drop the speed to 65mph and can hit 14mpg. Hand calc, but that's the averages.

Dad's 2010 Duramax all stock, he gets 12-13 pulling same trailer. He runs around 70mph and never in a hurry.

I meant that as "weight doesn't matter a lick when you don't got hills"

 

[Tiha]

10-15 psi sound high to me if unloaded. Is that normal for the PSD?

My 1995 ran 3-5 PSI at 70. I also ran about 950 EGT. Totally stock.

After adding gear Vendor and lowering RPMS I would run 7-10 psi at 70 mph and get the same mileage.

After 4" lift and bigger tires I run 10-12 psi at 70mph and lost a couple mpg. But also now I have bigger injectors, intercooler and turbo. so a lot has happened.
As I am getting older I want to lower it back down most of the way and put OEM tires on it. See if I get those 2 mpg back. Of it the other mods have ruined that permanently.

Got to thinking about this thread, I had a 99 24 valve dodge and I don't think it had ever got 18 mpg in it's life.

 

[Elvis-n-texas]

I'll play.

03 GMC Sierra 2500HD.
LB7. New nozzle style injectors, EFILive 5 tune. No lift, 265 75's

Unload, at peak torque . 20 mpg

Towing the Yota-toter ( 25 foot, 14k gooseneck with a Lance and my rig on the back, total Cat scales at 19.9k ) to Hammers on the flats, zero headwind = 12 mpg. Peak torque equates between 65 to 68.
Headwind drops me to 10.
If I jump up to 70, lose one mpg.
But. With a tailwind its on.
I've seen 14 at 70 mph.
Those are good hauls :)
I really like the early GMC for the rounded nose.


Very aerodynamic.