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Old 10-02-2014, 03:11 PM   #11
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Join Date: Apr 2013
Location: Colorado
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Re: 75mph cruising? is it worth it?

cool beans bansil.... thanks....

i looked, its right there in black and white. p18. downhill instructions

NOTE: Transmissions that do not have a lockup clutch (AT 540,
AT 542, AT 545) provide less engine braking than units which have a
lockup clutch. Downhill speed control for non-lockup units without a
retarder must rely on the service brakes.

found that out the hard way


ok, ok, i can have patience on the highway..... 65 is ok by me.

re gearing the rear end is too much money.....
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Old 10-03-2014, 12:02 AM   #12
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Re: 75mph cruising? is it worth it?

Our 36' Crown has a Detroit Diesel 6-71 with a turbo and an Allison Auto. It can do about 75 mph maxed out on the flats. Too fast for me in a large bus. My 40' tandem Crown with its Cummins 855 turbo (equivalent to a 14L engine) can do 70 mph going up hills. That is saying something for a vehicle with a GVWR of almost 50,000 lbs. That is plenty fast for me. My recent trip from Southern California to Northern California's Bay Area on Interstate 5 I kept the tandem at 65 mph. I was still passing all the trucks on the freeway and at 65 was getting optimal fuel mileage. These big guys don't stop in the same distance as a car. We all have to keep this in mind when interacting with all those small cars on the road that always seem to not see us.
Greg and Donna Marvel

Danville, California
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Old 10-03-2014, 08:04 AM   #13
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Re: 75mph cruising? is it worth it?

Originally Posted by bansil
here is pdf,d.aWw

I use factory engine temp gage to judge the overall tranny temp (rely on coolant heat exchange as bases)
thanks bansil. good stuff!
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Old 10-03-2014, 12:20 PM   #14
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Re: 75mph cruising? is it worth it?

Originally Posted by claydbal
Originally Posted by bansil
here is pdf,d.aWw

I use factory engine temp gage to judge the overall tranny temp (rely on coolant heat exchange as bases)
thanks bansil. good stuff!
your welcome
go to it's my battle when starting out...might help
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Old 10-27-2014, 10:45 PM   #15
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Re: 75mph cruising? is it worth it?

You save a lot by keeping it at the minimum speed on the interstates. Wind resistance goes up by the square of your speed, which means that to double your speed you need to quadruple your power and since we're basically driving a small building down the road, high speed really sucks the fuel. I'm retired now (and cheap) so I cruise with the 18-wheelers these days. I'd much rather spend my money on toys than fuel.
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Old 10-28-2014, 11:53 AM   #16
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Re: 75mph cruising? is it worth it?

Hey guys, would it be a good Idea to hook up one of the heaters that I removed from inside and set it up inline outside for the purpose of extra cooling for the motor?
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Old 10-29-2014, 03:51 PM   #17
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Re: 75mph cruising? is it worth it?

with the exception of pulling grades in the mountains in the summer, i've never had a diesel inline 6 bus engine that had any trouble keeping itself cool.

In the event that your engine is making more heat than the system is able to dissipate, turning on the heaters inside the bus will aid in removing heat from the engine.
Quis custodiet ipsos custodes (who will watch the watchmen?)
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Old 11-03-2014, 11:57 AM   #18
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Re: 75mph cruising? is it worth it?

I'll join the chorus here. When Peter drives Millicent he gets 11 MPG -- going 53-54 mph. I generally get around 10 or 9.5, going 57 or so. When I feel pressed for time and go top speed of 63-65, fuel mileage drops like a stampeding herd of rocks.

Engines generally get better fuel mileage at lower RPMs, when all other factors are the same. This is a simple matter of fewer combustion cycles per mile covered. So this suggests that changing to longer-legged gearing would improve fuel mileage at all speeds. But note that I said "generally", because...

...Engines run most efficiently at a certain RPM, determined by factors from bore/stroke ratio to cam-lobe profiles and timing to turbo-charger specs. So it is possible to get worse MPG at lower RPM. Some truckers learned this when the national double-nickle speed limit was imposed back in the day. The smarter ones corrected this by driving in 12th gear instead of 13th, bringing the engine back into the efficient operating range.
It is also possible that your engine will not have enough power at the lower RPM to keep the bus moving at the intended speed. Then you would be driving back to the shop in 3rd gear to have the old ratio re-installed, since you don't have a 12th gear.

Regardless of the details, more speed will always burn much more fuel, because of the "square of speed" that was mentioned above. Ask any Bonneville racer who makes run after run at, say, 240 in a vain pursuit of a 241 record. That last click might require an extra 50 HP.

And I have personal experience with Millicent: Every September I drive down I-80 from Donner Summit to the Sacramento valley -- a stretch notorious for fiery truck crashes in the old days. There are two or three stretches where it is vital to manage braking-capacity and speed, to avoid overheating the brakes. To maintain a legal 55 I must use the brakes quite a bit, and they overheat. But I can stay off the brakes almost entirely by letting her roll up to 65. At 65 I can practically feel the brick wall of air holding her back.
This matches what I learned from 27 years of trucking cross-country.

But wow, yeah.... (Caution. Trucking anecdote follows.) There were times when I was in a royal hurry to get from California to Colorado for an important delivery appointment, and I would set the truck on 76 MPH pretty much from the moment I entered Nevada, and just steer and grin until I turned off at Laramie, Wy. The computer tracks of those trips were automatically red-flagged in the office, but nobody ever said a word. Sometime fuel is cheap. I doubt that happens much with skoolies, though, or we would be driving Prevosts instead.
Millicent The Bus - roof raised two feet, toy-hauler tailgate.
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Old 12-03-2014, 10:20 AM   #19
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Re: 75mph cruising? is it worth it?

As a newbie with a 12 window buebird and dt360 at545, I can say only a little so far.
I picked mine up last weekend near Pensacola and drove 10 east to 75 south and then the turnpike to Orlando. With my foot to the floor, I had the windows open and did a top speed of 62 with luck and no headwind. Uphill, it was more like 55 and downhill was 65. I got passed by everything but didn't care. I stayed in the right lane. The previous owner recently installed 11r22.5 all around. All seats are still in at this time.
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