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Odd thought about a diesel pickup

O

oldtanker

Well-known Member
A semi, tipping the scales at 80,000 pounds can get 6 to 8 miles to a gallon. A diesel pickup with a curb weight of [b:1f1f110719]less than [/b:1f1f110719] 10,000 pounds gets under 20 empty. Doesn't it seem odd that they can't get that pickup up into the 20's empty? Heck even at the magic 26K range they are less than 1/3rd the weight, they should therefore loaded get more than 3 times the mileage shouldn't they? Like 3X6+18 LOADED? Or 3X8+14 LOADED?

Just thinking about it. Not knocking anyone of diesels. If anything it's the engineers and management.

Rick
 
If the pickup's maximum acceleration was equal to that of the loaded semi, it
probably would get plus 20 mpg. But then nobody would buy it. Ben.
 
I don't know if it makes much difference here or not, but I believe the larger diesels like large marine diesels are also more efficient. Maybe this accounts for some of it.

Dan
 

I think you will find the new pickup diesels have/can have similar HP to that of the diesels used in the large trucks. It would be interesting to know what mileage the diesel engine from a large truck would would return in an unloaded pickup. Conversely, would would the 400 HP diesel from a pickup return in a large truck at 80,000 Lbs.

I am thinking the mileage would be similar. The problem is for an unloaded pickup, the engine is operating well below its best efficiency range.
 
(quoted from post at 07:16:32 06/13/16)
I think you will find the new pickup diesels have/can have similar HP to that of the diesels used in the large trucks. It would be interesting to know what mileage the diesel engine from a large truck would would return in an unloaded pickup. Conversely, would would the 400 HP diesel from a pickup return in a large truck at 80,000 Lbs.

I am thinking the mileage would be similar. The problem is for an unloaded pickup, the engine is operating well below its best efficiency range.
Click to expand...

Then if that's true why does mileage drop when loaded?

If you look at torque there is a huge difference between a semi and a pickup. Semi's are running about 3 FT pounds of torque for every HP. Diesel pickups are close to 1-1. So take your Ram Cummins engine and stick it in a semi, most likely get about 1 MPG. Yea modern semi engines are running 1400 or more FPT.

AS far as the other comment about acceleration: It's a truck, not a drag car! And back not that many years ago it took forever for a semi to get up to speed. The time it takes one today to get up there isn't bad at all.

Rick
 
"If anything it's the engineers and management."

If anything, it's the government.

Dean
 
Its straight forward. Pickup a book and read up if you'd like.

On level ground:
-Coefficient of drag x Frontal Area is the largest factor as its a squared relation to speed
-Weight is just a straight ratio to rolling resistance and semi trucks have very high pressure low rolling resistance tires so they do better lb for lb
-Pickups have a lot of excess displacement in their engines they have to power (bearing areas, rings sliding on walls, etc), where a semi doesn't have nearly
the excess.

Result is an aerodynamic semi and a modern truck at the same speed get fuel economy roughly proportional to their frontal areas.

Throw in some hills and things get much different as the fuel to drag the weight up the hills will put the lighter vehicle far ahead in a relative comparison.
 

I've been thinking about the fuel mileage difference for years!Haven't got it figured out yet and probably never will.As to the get up and go of the heavy semi trucks,get behind one as it pulls away from a toll booth or comes on to a super high way and gets through the first 4 or 5 gears,shifts to the high side and puts the hammer down he will pick up speed in a hurry.I'm amazed every time I see it.
 
Two major differences at play, semi has 15
or maybe 18 gears, pick up has maybe 6, so
semi engine can be kept at peak torque far
more easily.
And engine size, pick up has 400hp out of 6
or 7 litre engine, semi has 500hp out of an
engine twice the displacement with far more
torque.
Torque is what makes everything move.
Modern pickup trucks are made to be toys
not work machines it seems, not saying they
can't work, but the designers seem more
inclined to make them car like.
 
I've always wondered about that too, but on cars, not trucks.

I spent a lot of time on the road with several Olds 88's, years '92 to '99. All would get 30 mpg+ on the open road and average 25-26 all around. And they were full sized comfortable cars. Then you read road tests of dinky econobox 4 bangers that wouldn't do any better. Something didn't add up.

I admit, GM's 3.8 V6 was an extremely efficient engine, but something just didn't seem right.
 
Pre-2003 diesel pick up engines easly got 20+ mpg. Hmmmm wonder why new one's get less???????? Oh yea,EPA regulations. Thanks EPA.
 
Jonathan Winters on Torque vs Horsepower.....
a229376.jpg
 
Exactly, cant count the number of car magazine testers who think anything that takes more than 6 seconds 0-60 can not be driven as it's just too slow.
 
Would be interesting if a trucker would comment, how much does that semi tractors mpg improve when run down the highway without a trailer attached, does it improve in proportion to the weight reduction, or not so much ? 80,000 pound loaded tractor trailer making 6 mpg VS a 12,000 ? pound tractor running the same road at the same speed ?
 
I can't speak to your exact scenario, but I can offer up one of my own.

My service truck weights in around 33,000 lbs, all the time. When I figure my quarterly fuel tax the usual fuel mileage comes around 5 to 5 1/2 MPG. That's about a 80+/20- mix of around town/highway miles.

A few years back I had to make a lot of trips one quarter down to Greenville, NC. That meant a lot of highway driving where I could get into 6th gear (double overdrive) and cruise. At the end of the quarter my mileage, best I can remember, was just shy of 10 MPG.

So, same truck, same basic weight, and the only difference being highway/cruising miles -vs- stop and go around town, meant just shy of double the usual MPG.
 
My old 96 F250 with a powerstroke will get
22-23 if I don't hammer on it in summer. In
winter if I drive it easy it'll do about 18.
It has an automatic (unfortunately) it is
4wd, seems like last time I crossed the
scales empty with it, it was around 6,250
pounds. It has zero emissions on it to choke
it. And is touching 300,000 miles with not a
bit of engine work.

Ross
 
You ain't kidding. Can't get a manual, can't get one with no frills, can't get a body that won't dent if you bump it, or bumpers that will "bump" stuff without causing $4000 in damage
 
They will really roll unloaded. The log
truck I run has a C15 Cat tweaked up to
about 650 HP. I'll get someone that will try
to speed around me as I turn a corner, and
usually they have a hard time passing if I'm
on the throttle very hard. With 60' of
doubles behind me...
 
(quoted from post at 10:32:33 06/13/16)................................
Torque is what makes everything move.
Modern pickup trucks are made to be toys
not work machines it seems, not saying they
can't work, but the designers seem more
inclined to make them car like.
Click to expand...

Yeah and add in the short shift automatics working against a 3:55 or higher rear axle along with all that the EPA requires to meet emissions requirements and the torque capabilities of the engine are negated. My '05 Dodge 4x4 with a 5.9, 3.73 finals, strayed cat, low restriction muffler, and manual 6-speed would get 20+ running empty, easy. (weighed 6,640# across the scale). 8)
 
My 90 2wd Cummins has got an estimated 23-
24 highway. I say estimated because the
Speedo is about 5% off and odometer about
9. That's at 70 with the original or oe-
replacement slipmaster converter and 3
speed auto. I'm sure it could hit 25 with a
good converter. It's not a rocket sled but
fast enough for me. The bossman's 6.7
Cummins is scary fast.

These big-horse isx at work will get 5-5.5,
if you drive it right. If you don't they
will still get the 2-2.5 they did before
deletes.

The old freightshaker won't get out and run
with them but the 400 hp N14, 10 direct
2.64 gears will get 5.5-6. I expect that to
improve when the boss gets back with my new
shocks, so my drive tires stay on the road
better.

I'm going to try to talk him into an rtlo
13 speed and 3.36s when we do an in-frame.
Give me the same final ratio in top gear,
which is a compromise for the work we do,
but it would drop my crawl ratio from the
current 39:1 to 45:1 and let me keep my rpm
down on the high side.

That's the big thing with these big
engines. Lug them. The C15 and isx are just
blowing fuel out the stack over 1500 rpm.
They still pull good but you can just about
hear the fuel tank vents sucking in the
air.

And I wouldn't run one of these 13 speed
isx powered Pete's without a governor. They
can get you in trouble in a hurry.
 
Never paid much attention fuel usage as I wasn't the one footing the bill, but last fall I actually did pay attention on the runs for one job I was doing. It was 155 miles unloaded to the logging location, then, after loaded, it was 260 loaded miles to the sawmill. Then after unloading, it was 95 unloaded miles back home. A big circle each day, and almost 50/50 loaded and unloaded of the 510 miles, weighing between 140 and 165,000 pounds. It was between 175 and 190 gallons of fuel each day, so pretty close to 3 MPG, which I didn't figure was too bad, there are ALOT of long steep hills that had to be traveled along the east side of the state.

Ross
 
Had an 01 lesabre with that engine. If I could buy that exact car brand new still I would. Sold it with 270k on it. Was burning 1qt oil every 3k. Transmission was doing some funny things, but still drove good. Never touched the internals of the engine or transmission. Wheel bearings and rear air shocks were changed about once per year, but could live with that (both were aftermarket lifetime warranty) got the 30mpg in it's hay day, still got 26 wore out.
 
Reading these replys I haven't seen
anyone mention this. A pickup diesel
will turn upwards of 3000 rpm : a semi
in the 1800-2100 range. That being
said: no 400 hp pickup diesel can come
close to pulling what a 400hp big
diesel will
Pickup around 600 lb /ft torque:
semi 1500. Both are diesel engines but
have different standards for rating
evidently.
 
(quoted from post at 11:14:57 06/13/16) Would be interesting if a trucker would comment, how much does that semi tractors mpg improve when run down the highway without a trailer attached, does it improve in proportion to the weight reduction, or not so much ? 80,000 pound loaded tractor trailer making 6 mpg VS a 12,000 ? pound tractor running the same road at the same speed ?
Click to expand...

For the two years that I had my triaxle I got 6.5 MPG with 475 Cat. A few years back I bob-tailed two road tractors from Omaha to southern NH. I was expecting to get up around 10MPG each time, but it was nothing like that. More like 7to 7.5. It looks like the full cab wind resistance was holding them back quite a lot.
 
If you're turning an isx, c15, or dd15 2100 rpm you're gonna keep opec real happy. They work best in the 1100-1500 rpm range.

Even the n14 celect and 60 series work best from 1200-1800.

The old big-cams, a and b models, and 2-strokes, however......

The 12 valve Cummins torque peaks about 1800 and was governed at 2500, but believe it or not, installing the 3200 rpm governor springs will net you better fuel mileage. All 3 heavy-pick-up diesels work best about 1800 rpm and up. The 24 valve will get better fuel mileage at 65 in 4th than 70 in fifth loaded.
 
IT has a lot to do with the wind resistance. Another words if I am hauling high and wide like a gravel screen in WY, and try to run even close to speed limit. I can watch the fuel gage drop almost. It will be about 4.7 MPG If I run closer to 60 it will be more like 5.5 or so. This is with a 379 Pete 550 cat at 1800 torque 18 spd with 3.55 gears on 11r-24.5 tires. If I come back empty run speed limit I can get about 6.5 -7.0. Now if I load brick and stay about 60 it will get about 6.0-6.5. Most of the time my quarterly fuel tax is paid on about 5-6 mpg. Depends on where I go and what I haul. The gross weight will be about the same for both of these examples.
 
(quoted from post at 12:29:16 06/13/16) Reminds me, railroad freight trains are -by what I've heard- 10x more fuel efficient than trucks.
Click to expand...

freight trains are big generators all drive motors are electric

small diesel engines can get great mileage not a truck but a VW rabit 1980 or 81 and it got 52 mpg
 
(quoted from post at 20:17:07 06/13/16) IT has a lot to do with the wind resistance. Another words if I am hauling high and wide like a gravel screen in WY, and try to run even close to speed limit. I can watch the fuel gage drop almost. It will be about 4.7 MPG If I run closer to 60 it will be more like 5.5 or so. This is with a 379 Pete 550 cat at 1800 torque 18 spd with 3.55 gears on 11r-24.5 tires. If I come back empty run speed limit I can get about 6.5 -7.0. Now if I load brick and stay about 60 it will get about 6.0-6.5. Most of the time my quarterly fuel tax is paid on about 5-6 mpg. Depends on where I go and what I haul. The gross weight will be about the same for both of these examples.
Click to expand...

I went to pick up a hay rack about 70 miles away with a trailer behind my 7.3 diesel. This is one with the tubular steel sides. I wasn't checking mileage, but the way those sides caught air really was a huge noticeable additional load to the truck.
 
(quoted from post at 07:37:47 06/13/16)
(quoted from post at 07:16:32 06/13/16)
I think you will find the new pickup diesels have/can have similar HP to that of the diesels used in the large trucks. It would be interesting to know what mileage the diesel engine from a large truck would would return in an unloaded pickup. Conversely, would would the 400 HP diesel from a pickup return in a large truck at 80,000 Lbs.

I am thinking the mileage would be similar. The problem is for an unloaded pickup, the engine is operating well below its best efficiency range.
Click to expand...

Then if that's true why does mileage drop when loaded?

If you look at torque there is a huge difference between a semi and a pickup. Semi's are running about 3 FT pounds of torque for every HP. Diesel pickups are close to 1-1. So take your Ram Cummins engine and stick it in a semi, most likely get about 1 MPG. Yea modern semi engines are running 1400 or more FPT.

AS far as the other comment about acceleration: It's a truck, not a drag car! And back not that many years ago it took forever for a semi to get up to speed. The time it takes one today to get up there isn't bad at all.

Rick
Click to expand...

Efficiency is a measure of the engine's useful work output per the amount of fuel consumed. As the load is increased the efficiency typically increases and becomes a maximum at about 80% of the engine's maximum output. As load is increased to greater than 80% the efficiency tends to decrease. At idle the engine is consuming fuel however, the engine is not connected to an external load. Since there is no useful work output, the efficency is zero. The only engine load is to over come friction, drive the cooling pump/fan, alternator, etc. Once an external load is applied we have useful work output and efficiency, however fuel consumption will increase to produce the additional power. In a similar manner as a truck's weight is increased the engine may be operating in a more efficient load range, however the fuel mileage will decrease due to the additional power production required.

Heavy truck engines are typically designed for lower RPM operation, which in part provides for increased life. HP = (torque x rpm) / 5252; if power is to remain the same then the engine's torque output must increase. The higher torque can be obtained by increasing the engine's displacement and/or cylinder pressure. Given equal HP of a high speed or low speed engine the driving force (torque at the rear alxe) can remain the same by selection of the proper gear ratios in the transmission/rear end.
 
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