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Old 07-06-2008, 06:14 PM   #1
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Impressive MPG -- why?


When I drove Millicent home 600 miles from the school district, I recorded 9,5 MPG. This last trip, close to the same distance in similar terrain, we recorded 13 MPG. Is that possible, or are we filling the tank unevenly?

That glorious day in December 2006, Millicent had...
Stale fuel.
Cracked fuel hoses, drawing air into the fuel.
Old motor oil.
Old "regular" tires with unknown pressures (although they looked inflated).
And she had just plain sat parked a couple of years, except for one recent short drive.
And she may not have been up to 55 MPH... ever.
It was December; cool weather.

Last weekend, we had...
Fresh fuel.
Good fuel hoses.
Synthetic motor oil.
New "low-profile" tires.
And she has seen more regular use, specially long trips at 55-57 MPH.
It was June; hot weather.

On the downside, she now carries 16 square feet more frontal area, and pulls a trailer.

Analysis:

The old fuel would have been gradually replaced with fresh fuel on the original drive home, and I remember being pleased that they sold her with more than half a tank. Maybe she got just a couple of MPG those first couple hundred miles on the two year old fuel?

I know the fuel hoses drew air, because I had a starting problem that went away when I replaced the hoses. Air in the fuel may have led to less power, but I don't quite see how that would affect milage.

Old motor oil is a known problem. When I change the oil in my Dodge Dakota, it immedidately feels like I have more power. So fresh, and full synthetic, oil is bound to help some. But 30%?

New tires generally yield worse milage, because of more squirming in all that rubber. But the modern "lo-pros" like we use on the 18-wheelers at work, may have an advantage in casing design.

"Blowing out the cobwebs" will probably help a little -- re-breaking-in parts that have sat and oxydized in that old oil in a humid climate for a couple of years.

What might the change in ambient, and fuel, temperature have done to fuel milage? Less power, yes. But milage?

Aerodynamics: The 20% increase in frontal area is bound to hurt. The trailer may not have had any negative effect -- heck, it may even have helped, aerodynamically.

Thoughts? Facts? Superstitions?
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Old 07-06-2008, 08:26 PM   #2
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Re: Impressive MPG -- why?

I have found that it is incredibly difficult to fill my tank consistently. You know as well as I do that diesel fuel REALLY foams. My tank is tucked way back under the body and the filler neck comes out almost from the side of the tank which only compounds the problem. I've found that it takes about 15 minutes and a flashlight to get my tank consistently filled to the same spot.

That said...if you're actually getting 13 mpg with that two story brick I think I'm trading my 6.6 in on a 6BT!
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Old 07-06-2008, 08:39 PM   #3
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Re: Impressive MPG -- why?


Right. Peter did the filling both times, and he did pay attention to getting it consistent, but I do know it is difficult to do. So there will be further fillings and calculations. Meanwhile, my money is on the fresh synthetic oil as the most important factor.
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Old 07-06-2008, 09:17 PM   #4
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Re: Impressive MPG -- why?

My thoughts are winter fuel gives less mpg. But I don't know the area you bought your fuel from. My other thought is maybe since you got the bus you have re taught you right foot to take it easier on start up from 0 to 55.

I know over the last 6 months my right foot is very light now. On a transuttle bus I have been able to go from 7 up to 10.6 mpg. This is cause of winter fuel and my foot. Most of the time I can make it do 9.5 mpg. This bus doesn't have the big tires since it has the flat floor either. 265r70 19.5 tires, 3.55 rear end. About 2750rpm at 70mph, but as of late I don't push it anymore than 45 to 50. I get a lot of horns from the cars because of this but what do I care. I even got one horn from a pepsi truck.
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Old 07-06-2008, 09:23 PM   #5
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Re: Impressive MPG -- why?

About 2 years ago I took my bus on a 3,000 mile trip and kept track of my fuel mileage along the way. My best tank was cruising at a steady 55mph and I got 12.89mpg, my worst tank we were going into a headwind with the pedal to the floor the entire tank, and only got 6.17mpg.

I'm not sure how accurate those numbers are since I was not being as careful as you guys when filling up. I never really thought much about making sure there was the same amount of fuel in the tank - I just filled it till the nozzle shut off, maybe round up to the next dollar after that.
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Old 07-06-2008, 09:46 PM   #6
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Re: Impressive MPG -- why?


We don't have very cold weather on this coast, so no #1 fuel blend in winter for us.

Peter and I are both long time experts at easing miles out of a gallon of fuel; no change there.

But good thinking, guys.
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Old 07-06-2008, 09:47 PM   #7
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Re: Impressive MPG -- why?

Quote:
Originally Posted by phillbus914
my worst tank we were going into a headwind with the pedal to the floor the entire tank, and only got 6.17mpg.

don't forget, that phill's bus is no mere mortal bus....his pedal to the floor speed is well over 70 mph!

not like us average skoolie people who drive pedal to the metal to go 60 mph.
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Old 07-07-2008, 09:39 AM   #8
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Re: Impressive MPG -- why?

Elliot, did the new tire profile change your final drive ratio any?
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Old 07-07-2008, 10:15 AM   #9
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Re: Impressive MPG -- why?

what rear end gear ratio do you have?
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Old 07-07-2008, 01:15 PM   #10
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Re: Impressive MPG -- why?

Elliot,
I wonder if those low profile tires aren't fooling your speedometer into believing it's going further that it really is?
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Old 07-07-2008, 01:25 PM   #11
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Re: Impressive MPG -- why?


Millicent's rear end ratio is 4:78.

The new tires are roughly one inch smaller diameter. That would affect the odometer a little bit, yes. You guys who went to school can figure out how much. Certainly a valid point!

Edit: Forgot to say: Old tires are 41" tall. New ones are 40". (Old ones were 11R22.5. New ones are 295/75R22.5.)
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Old 07-07-2008, 04:06 PM   #12
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Re: Impressive MPG -- why?

Hey Elliot,
You haven't switched to that fancy new "downhill" mileage I've been hearing about have you.
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Old 07-07-2008, 04:38 PM   #13
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Re: Impressive MPG -- why?


Downhill? That could refer to so much, like my bank account.

Rolling off Donner Summit with a heavy load last year, I found that there was no point in keeping the speed down with the brake pedal and a low gear. All that did was overheat the brakes. So I let the old gal roll, and she topped out around 60 MPH -- butting firmly against the brick wall of air.
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Old 07-16-2008, 09:42 AM   #14
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Re: Impressive MPG -- why?

Quote:
Originally Posted by Elliot Naess
Rolling off Donner Summit with a heavy load last year, I found that there was no point in keeping the speed down with the brake pedal and a low gear. All that did was overheat the brakes. So I let the old gal roll, and she topped out around 60 MPH -- butting firmly against the brick wall of air.
Sounds scary, and I'm afraid my bus being a lot more aerodynamic would get to 80 or so.
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Old 07-16-2008, 10:19 AM   #15
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Re: Impressive MPG -- why?

Your change in tire size only affects the odometer and speedometer to the tune of 2.5%. That is hardly noticeable and falls well within the range that I think tire pressure and loading affects overall tire height. Basically, 65 mph indicated is actually 63.375 mph. I don't think that matters.


This last weekend I was actually able to get 10 mpg for a period out of my bus. The trick was traveling 45-50 mph. I don't think it was the aerodynamics so much as the engine speed. At those speeds I had all kinds of power on tap yet the bus was happily cruising around. I think I found my peak torque and it's right around 2000-2250 RPM. Unfortunately I don't like driving around at that speed. To achieve those engine speeds while cruising at a normal highway speed I would need to get my overall gearing 14% higher (assuming no need to compensate for wind resistance). That would mean instead of 4.56's I would have 3.92's. Of course I WOULD need to compensate for the wind resistance so I think 4.10's would be the magic number there.

Not surprisingly that is the gear ratio Phill has I believe. I would tend to believe that many of us have lower gears because of the terrain and the stop and go nature of school buses. Phill's certainly seems to fit into that activity bus category used for transporting sports teams long distances on the highway. I think with a similar gear ratio my bus might even come close to his mileage and it all makes sense looking at the engine speeds.
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Old 07-16-2008, 10:34 AM   #16
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Re: Impressive MPG -- why?

I've got 4.33 gears in my bus, and I've double checked it's also got the At545 - so no lockup converter.

I happened to go to the school district that my bus came from, we had 4 TC2000's I tried to get the one I rode in middle school and part of high school but it had already been used as a parts bus due to a bad motor I think. Anyway aside from it's regular route, they did use it for field trips and ski club (I used to ride it to the local ski slope about 45 miles each way)

When I was bus shopping I never even thought to check the rear end ratio but if I were to ever buy another bus that's one of the first things I'd check.
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Old 07-16-2008, 01:17 PM   #17
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Re: Impressive MPG -- why?


Quote:
Sounds scary...
Yes, safety must always be the first concern. Downhills are serious business. And a First Student driver tipped a school bus on its side on the flat level highway near here just a couple of days ago! Skidded 200 feet on its side! (Minor injuries only. A windshield popped out, and the nine students and the driver walked out thru it. Bus looked 100% structurally intact.) But after 26 years of rolling 18-wheelers off hills, I hope I have a better feel for this sort of thing than the average beginner. Of course, 24 or 25 years ago, I set the brakes on fire on a Kenworth. Put it out by throwing sand on it. Learned a lot that day!

Yes, engine speed is important to fuel consumption. Also the load (current power output), and the efficiency of the engine at various revs, like you mention, but more bangs per mile generally means more fuel per mile.
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Old 07-16-2008, 07:57 PM   #18
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Re: Impressive MPG -- why?

What's the rated horsepower of your engines just out of curiosity? Mine is still at the stock 185hp/474 ft lbs setting.
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Old 07-16-2008, 10:21 PM   #19
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Re: Impressive MPG -- why?


I have not touched mine yet, so I expect it is the stock rating at 190 HP. No torque listed.

And now that I think about it... I'm not getting all 190, because the foot feed linkage does not open all the way. But that has not changed since I bought it, so that does not account for the sudden 13 MPG.

Leaving on a 1200 mile round trip tomorrow -- Kinetic Sculpture Race in Corvallis, Oregon, this weekend. Will try to keep track again.
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Old 07-24-2008, 11:58 AM   #20
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Re: Impressive MPG -- why?

Synthetic motor oil. ?

Hey Elliot which one did you use? I have using using Mobil 1 in my tacoma(273K miles) and harley with great results for years, am afraid to add up what it would cost to switch the bus. And afraid it will leak more oil than it presently does, have you had any issues since changing?

I am planning on changing the oil & transfluid next weekend.

Thx

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