Most fuel-efficient speed / best fuel economy?

[RomaniGypsy]

I'm looking to make some calculations in my head, if possible. My 2004 CE300 w/DT466E and AD2000 gets about 11 mpg at wide-open throttle on the highway (governed to 65 mph).

I know that certain speeds are the most fuel-efficient for various vehicles and just wondered if any of y'all have an idea of the most fuel-efficient speed for this powertrain combination (or any similar powertrain combination), and about what that fuel economy number would be.

(I can estimate that it'd be maybe 20% higher at 55 mph compared to 65, but that's using what I've found to be true for older carbureted passenger vehicles using 3-speed automatic transmissions.)

 

[david.dgeorge07]

Pretty sure that if you can hit ideal rpm (1700 to 1900 for most of us that have inline 6 motors) at 45mph that is going to be your best mpg.

That’s pretty slow for the interstate, but if you are just looking for best possible, I think that would be it.

 

[rensch26]

I believe peak torque begins at 1,400 rpms on the 466E. Trying to cruise at RPMs slightly above that (not lugging the engine) is probably going to yield your best fuel economy.

 

[Tango-SKO]

What he said. Peak efficiency on any diesel is going to be at just above (maybe 10%) peak torque RPM's. Also known as the "Sweet Spot".

 

[cadillackid]

ive found from experience that unless you are running an unlocked driveline.. (ie AT-545)... you will lug about any diesel engine at just a couple hundred above its peak torque..



a tale of my 2 diesels.. one V8 (T444E) and one inline 6 (DT-360).. both list their peak torque at 1300-1400 RPM respectively.. driving. the V-8 at 65 MPH at 1550 RPM results in when I need to climb a bit of an inline I watch my EGT (exhaust gas temperature) Sky-rocket. and I really dont gain much speed... high EGTs can be damaging to an engine.. as is Lugging. Lugging has many definitions.. but in general you'll know when you lug a diesel.. you wont gain much speed if any. and the engine will sound like its struggling or laboring.. you dig into the throttle more only to hear it labor more..


that bus (T444E) is admittedly over-geared. even with the engine pumped up to 250 HP / 700 Tq. it isnt happy beinbg pushed at 1500-1600 RPM.. now I drop the trans out of 6th and into 5th and im in that high 1700s low 1800s range.. and i can watch my EGT gauge drop.. (as well as the computer's engine load factor).. it almost always gains speed and doesnt sound labored when im up in that range.. go into 4th gear and run it up to 2400-2500 RPM and now im beyond even the rated Horsepower which is 2300 rpm .. the enmgine just doesnt pull hard.. oh its great to run 65 up the mountain hills. with EGTs in check and turbo push 21 lbs of boost.. but im making a lot of noise and a lot of heat..



now lets get back to MPGs.. RPM is only part of the story.. yep at 65 if im in 6th at 1550 RPM and I crowd it just a bit to maintain 65.. my MPG's go through the floor.. dropping down to 7-8 or so.. now go to 5th gear runs up to 1800 or a little above and wow im back up to 12 MPG.. crowding a little gains speed so any drop on the MPG is momentary as I am easier able to maintain my speed without making a lot of heat (as shown by the lower EGTs).. go to 4th gear. and my MPG's at the same speed same incline may drop down to 8 or 9.. so actually higher than when you run at too low RPM and lug it..



so the idea here is that you need to build your driveline for the speed you want to drive and not too low and not too high RPM..



speed affects MPG greatly.. you are after all pushing a square box with about 0 aerodynamics down the road.. even the slightest head or tail wind makes MPG's changed pretty significantly.. that said.. with my gearing in my red bus.. at 60 im in its absolute sweet spot getting upwards of 12-15 maybe even 16 at times..



at 65 it drops to an average of 10-13. or most usually 11-13 or 14...



at 55 in 5th gear it tends to lug.. since my 5th OD is 0.71 and my 4th is 1.00 ratio a downshift to 4th even at 55 results in too high RPM to be efficient.. so my MPGs at 55 tend to be similar or just above my 65 MPG;s..





now the other bus the DTA360.. I recently tosserd the AT545 for MT643. and the driving ive done on that one suggest that 55-60 is its sweet spot.. even though at those speeds my engine is faster than what id like.. its running about 2100-2200.. at 45 MPH im in the 1700-1900 range and it gets fantastic mileage..(13-16) but is a real chore to drive and no longer fun.. so I wont run it 45.. I hear a lot of horns and see a lot of middle fingers at that speed.. regearing to the common 4.44 from its current 4.78 i have a feeling will result in 12-14 at 55-58. which is where ive almost always driven that bus..



so again, build for the speed you want to drive.. that seems in my expeirence more important than just arbitrary one-size fits all MPG's.. becuase its a mixture of engine speed, wind resistance, and terrain..

-Christopher

 

[o1marc]

ive found from experience that unless you are running an unlocked driveline.. (ie AT-545)... you will lug about any diesel engine at just a couple hundred above its peak torque..



a tale of my 2 diesels.. one V8 (T444E) and one inline 6 (DT-360).. both list their peak torque at 1300-1400 RPM respectively.. driving. the V-8 at 65 MPH at 1550 RPM results in when I need to climb a bit of an inline I watch my EGT (exhaust gas temperature) Sky-rocket. and I really dont gain much speed... high EGTs can be damaging to an engine.. as is Lugging. Lugging has many definitions.. but in general you'll know when you lug a diesel.. you wont gain much speed if any. and the engine will sound like its struggling or laboring.. you dig into the throttle more only to hear it labor more..


that bus (T444E) is admittedly over-geared. even with the engine pumped up to 250 HP / 700 Tq. it isnt happy beinbg pushed at 1500-1600 RPM.. now I drop the trans out of 6th and into 5th and im in that high 1700s low 1800s range.. and i can watch my EGT gauge drop.. (as well as the computer's engine load factor).. it almost always gains speed and doesnt sound labored when im up in that range.. go into 4th gear and run it up to 2400-2500 RPM and now im beyond even the rated Horsepower which is 2300 rpm .. the enmgine just doesnt pull hard.. oh its great to run 65 up the mountain hills. with EGTs in check and turbo push 21 lbs of boost.. but im making a lot of noise and a lot of heat..



now lets get back to MPGs.. RPM is only part of the story.. yep at 65 if im in 6th at 1550 RPM and I crowd it just a bit to maintain 65.. my MPG's go through the floor.. dropping down to 7-8 or so.. now go to 5th gear runs up to 1800 or a little above and wow im back up to 12 MPG.. crowding a little gains speed so any drop on the MPG is momentary as I am easier able to maintain my speed without making a lot of heat (as shown by the lower EGTs).. go to 4th gear. and my MPG's at the same speed same incline may drop down to 8 or 9.. so actually higher than when you run at too low RPM and lug it..



so the idea here is that you need to build your driveline for the speed you want to drive and not too low and not too high RPM..



speed affects MPG greatly.. you are after all pushing a square box with about 0 aerodynamics down the road.. even the slightest head or tail wind makes MPG's changed pretty significantly.. that said.. with my gearing in my red bus.. at 60 im in its absolute sweet spot getting upwards of 12-15 maybe even 16 at times..



at 65 it drops to an average of 10-13. or most usually 11-13 or 14...



at 55 in 5th gear it tends to lug.. since my 5th OD is 0.71 and my 4th is 1.00 ratio a downshift to 4th even at 55 results in too high RPM to be efficient.. so my MPGs at 55 tend to be similar or just above my 65 MPG;s..





now the other bus the DTA360.. I recently tosserd the AT545 for MT643. and the driving ive done on that one suggest that 55-60 is its sweet spot.. even though at those speeds my engine is faster than what id like.. its running about 2100-2200.. at 45 MPH im in the 1700-1900 range and it gets fantastic mileage..(13-16) but is a real chore to drive and no longer fun.. so I wont run it 45.. I hear a lot of horns and see a lot of middle fingers at that speed.. regearing to the common 4.44 from its current 4.78 i have a feeling will result in 12-14 at 55-58. which is where ive almost always driven that bus..



so again, build for the speed you want to drive.. that seems in my expeirence more important than just arbitrary one-size fits all MPG's.. becuase its a mixture of engine speed, wind resistance, and terrain.

-Christopher

I bet my stint behind the wheel of Red didn't result in good fuel economy.:whistling:

 

[RomaniGypsy]

a whole bunch of info

I haven't had my bus out in a while, because it's been too freaking cold and snowy... but if I remember correctly from the trips I have taken in it, it spins about 2,100 rpm in top gear (which I believe is 5th) at 65 mph.

Once it gets into 5th gear, even if I have the pedal to the floor, it has to drop to 55 mph before it'll downshift into 4th. (I don't remember what its lowest upshift point into 5th is.) Since 5th is a locked gear, it would seem that the engine speed at 55 in 5th ought to be (55/65)*2100 = around 1,800 rpm.

10% above 1,400 would be 1,540, and logically that ought to be about 45 mph if I could be in 5th by then.

But if it's really all about getting into this "sweet spot", then maybe I should try that and stick to minor highways. I could probably get away with 55 on the interstate but when the speed limit is 70, even though they say "minimum 45", you get people zipping around you with the smallest of clearances even at 65. I mean, not like they're going to do much damage to a bus if they "miss", but still...

I guess I have to look up the torque curve for my engine, and then get a feel for the shift points under various loads.

You talked about "build to your driving habits" or something like that - I ain't building to anything. I'm not that mechanically inclined, at least not right now. Plus, I imagine it would take money to build like that... and I'm not convinced that this bus is worth putting a lot of money into. It's an Ohio bus, it has rust, who knows how long the engine will last before developing timing cover problems, so I'm going to drive it as is and try for the best economy I can get in its stock form.

 

[cadillackid]

I bet my stint behind the wheel of Red didn't result in good fuel economy.:whistling:

You know what’s funny is since I turned the power up from 190 to 250 my MOg at higher speeds like that went up... I did have to break 80 once since I did that just because.. we tried so hard that night and only hit 79...

 

[cadillackid]

I haven't had my bus out in a while, because it's been too freaking cold and snowy... but if I remember correctly from the trips I have taken in it, it spins about 2,100 rpm in top gear (which I believe is 5th) at 65 mph.

Once it gets into 5th gear, even if I have the pedal to the floor, it has to drop to 55 mph before it'll downshift into 4th. (I don't remember what its lowest upshift point into 5th is.) Since 5th is a locked gear, it would seem that the engine speed at 55 in 5th ought to be (55/65)*2100 = around 1,800 rpm.

10% above 1,400 would be 1,540, and logically that ought to be about 45 mph if I could be in 5th by then.

But if it's really all about getting into this "sweet spot", then maybe I should try that and stick to minor highways. I could probably get away with 55 on the interstate but when the speed limit is 70, even though they say "minimum 45", you get people zipping around you with the smallest of clearances even at 65. I mean, not like they're going to do much damage to a bus if they "miss", but still...

I guess I have to look up the torque curve for my engine, and then get a feel for the shift points under various loads.

You talked about "build to your driving habits" or something like that - I ain't building to anything. I'm not that mechanically inclined, at least not right now. Plus, I imagine it would take money to build like that... and I'm not convinced that this bus is worth putting a lot of money into. It's an Ohio bus, it has rust, who knows how long the engine will last before developing timing cover problems, so I'm going to drive it as is and try for the best economy I can get in its stock form.

Hey no one says anyone has to build their bus any certain way.. 2100 rpm isn’t going to blow up your dt466e.. it’s a decent rpm. Your overdrive if you have Allison 2000 is likely .74 or .75, as for downshift it seems I’m not the only one that’s had downshift issues with Allison and navistar DLC computers... until I started tuning my own trans my down shift schedule seems off like it had to go really slow before it would go down even foot to the floor..
Christopher

 

[Napabavarian]

are you able to track your economy in real time? Are these smaller busses? Those numbers sound pretty good, of course I'm looking at 40' beasts.

ive found

REDACTED

a tale of my 2 diesels.. one V8

REDACTED

so again, build for the speed you want to drive.. that seems in my expeirence more important than just arbitrary one-size fits all MPG's.. becuase its a mixture of engine speed, wind resistance, and terrain..

-Christopher

 

[Cjgoertzen]

I get 13 to 14.5 mpg at 57 mph with my 03 fs 65. Now when i drop the speed to 54-53 mph usually get around 15.5 mpg.

 

[Napabavarian]

I get 13 to 14.5 mpg at 57 mph with my 03 fs 65. Now when i drop the speed to 54-53 mph usually get around 15.5 mpg.

Do you know what engine transmission and gear ratio you have?

 

[cadillackid]

in my case they are smaller busses.. my redbyrd is a full chassis 3800. but sits lower because of smaller wheels.. and GVWR is 17500.. my DEV bus is heavier, sits higher (has the DT-360)). and is GVWR 27500.


so im definitely working with equipment that is lighter and in the case of my best MPG runner it sits lower to the ground with a slight rake toward the front. so that probably helps..

-Christopher

 

[cadillackid]

my MPG is tracked real time and its tracked in my ECM as a lifetime average of the Bus.. which of course the lifetime includes all the idling, city traffic, its life as a school bus, etc..



while i can 0 out my odometer and gallons burned in the ECM I havent ever done that since i got the bus..

 

[Cjgoertzen]

Do you know what engine transmission and gear ratio you have?

Yes I have a 185hp, 5.9l cummin with a vp44 coupled to a fs-5205a Eaton Fuller. Currently running 9r 22.5s on a metiror 4.10 axle. I am intalling some 11r22.5s on alcoa's and some 100hp injectors this weekend. I will post later if and how much the mileage had changed.

 

[o1marc]

I get 13 to 14.5 mpg at 57 mph with my 03 fs 65. Now when i drop the speed to 54-53 mph usually get around 15.5 mpg.

I'd give my left nut to get that kind of mileage in mine. I have yet to see over 9mpg in mine. Not too many here get that kind of mileage.

 

[Cjgoertzen]

I'd give my left nut to get that kind of mileage in mine. I have yet to see over 9mpg in mine. Not too many here get that kind of mileage.

Keep in mind that is canadian mpg so in U.S mpg its about 12. My bus is only a 36 passeger aswell so not near as much weight to lug around.

 

[Seriousracer]

The other elephants in the room on mileages is wind ( aerodynamic) resistance , clutch or air fans ,and auxiliaries being driven by the engines.
Sometimes the sweet spot falls in the range of aero efficiency. Sometimes it is above or below the aero efficiency “sweet spot “. Think about it this way. As you go faster the harder it is to move the frontal area of the bus through the air. Air is gas but behaves like a liquid. So in the same respects closing the large negative area behind the bus will help efficiency.

Parasitic draws on the engine are also killers to fuel mileage. Is your air compressor running to compensate for leaks ?
Is the alternator working overtime to charge house and bus batteries?
Power steering an efficient pump or jus an old keep it over boosted pump.

Clutch fans. They are the best and extremely reliable. But hey SUCK fuel mileage down are hard on water pump /mounting bearings due to weight and load the generate. Also need to be kept in balance
Is your cooling system in good working order or is your clutch fan working over time.
Air engagement fans air much more efficient when not engaged because they free wheel truer. But the suck fuel when engaged.
Look at your frontal area close there is a lot to gain in moving air efficiently in and out of the engine compartment to keep the clutch fan from engaging.

 

[capnron-SKO]

School buses by nature of their purpose as stop and go low speed vehicles, are geared for lower speeds. Check the gear ratio of the rear end and perhaps investigate the possibility of a slightly taller ratio. It could mean maintaining a decent highway speed at a comfortable rpm while increasing fuel mileage.

 

[Dirtdoctor]

I’d be glad toPull 11 mpg- 190 hp DT466 with a 545 or 645 Allison and I pull 5-6mpg.