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Old 07-07-2008, 02:25 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, 05:06 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, 05:38 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, 10:42 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, 11:19 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, 11:34 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, 02:17 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, 08:57 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, 11:21 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, 12:58 PM   #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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