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Old 09-10-2009, 10:36 AM   #1
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Purchase descision - need advice

Hey folks,
I'm getting ready to join the ranks of the skoolies, and I'm looking at a particular bus right now, but have one concern so far: The seller says is is running 3600RMP at 60mph. The only relevant information I have so far is that it has a 7.3 IDI non-turbo International engine (the bus is a 1991 Carpenter/International), and an Allison AT500 transmission. I don't know what the gear ratio is, but the wheels/tires appear to be a typical size for a bus. So is it the transmission that is causing the high RPMs? Or can we assume it is the differential gear ratio?

The RPMs sound too high to me. I would not want to be screaming down the highway like that all time. I have very specific requirements for a bus, and this bus meets most of them, so I'm willing to make reasonable modifications if necessary.

Thanks all!
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Old 09-10-2009, 10:49 AM   #2
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Re: Purchase descision - need advice

The RPM's sound awful high in my opinion but I am no expert... Mine runs around 2400-2600 if I am not mistaken & tops out at 62MPH...
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Old 09-10-2009, 12:27 PM   #3
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Re: Purchase descision - need advice

sounds like it may have been geared for transit. a lot of skoolies are set up for the country and go fast, but some are set up for city transit. from what i have seen most are set up just like the country ones and have a gear ratio that let it go fast, but there are a few that have a transit gearing ratio. that way the bus will run more efficiently at a slow pace. will usually still give you the high speed, but your gonna be pushing those rpms up and losing what little gas milage you had. i will say that if you can handle 55 mph....it would probably drop that rmp quite a bit. the gearing ratio works on a curve, so when you get towards the top end of it, just going an extra 5 mph takes a lot more turns of the gears. just my two cents.
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Old 09-10-2009, 01:17 PM   #4
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Re: Purchase descision - need advice

I wonder how much it would cost to get a better gear ratio installed, and how hard would it be to find parts? This is not a job I would attempt myself, so I'd have to pay a mechanic.
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Old 09-10-2009, 01:22 PM   #5
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Re: Purchase descision - need advice

you can do it, or have it done....but if you haven't purchased yet it may me easier and cheaper to find one that you dont have to do that to. there are a ton of buses out there right now, and they are inexpensive right now too. try and find the best foundation to build your home away from home on. thats my feeling.
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Old 09-10-2009, 01:42 PM   #6
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Re: Purchase descision - need advice

Yeah, but there aren't many that meet my criteria: viewtopic.php?f=12&t=4790

Problem is, I'm too damned picky!
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Old 09-10-2009, 04:00 PM   #7
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Re: Purchase descision - need advice

Actually, gear ratios do not work on a curve, they are linear.

I have a 6.17:1 ratio on my differential, although I would like to find something lower. With that ratio, 1st gear is unusable unless you want to pull a house of the foundation. At the other end, 4th gear output is direct, or 1:1

60 mph is at about 3000rpm. Engine is turning 3000rpm, and so is the driveshaft. Driveshaft drives the differential, generating 1 wheel revolution for about every 6 driveshaft {or engine} revolutions.

Sure would like to find a GM 14 bolt 10.5 inch diff with a ratio in the 4's or 5's! get my first gear back, maybe a little better cruise speed at a few less rpm!
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Old 09-10-2009, 07:13 PM   #8
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Re: Purchase descision - need advice

Quote:
Originally Posted by Smitty
Quote:
Originally Posted by Ob1

I have a 6.17:1 ratio on my differential, although I would like to find something lower.
You mean higher (numerically lower)...what you have would definately be a stump-puller

Smitty
Lower, as in a lower number of driveshaft revolutions per wheel revolution!
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Old 09-10-2009, 11:22 PM   #9
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Re: Purchase descision - need advice

Okay, the seller just said he was mistaken, it's actually 2,900RPM at 60mph. Still a little high, but hot as bad. He also said the bus has 232K miles, but the engine and transmission were rebuilt at 150K "according to the school district" - no documentation of this. New paint and new tires as well. Hmmmm....
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Old 09-11-2009, 09:54 AM   #10
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Re: Purchase descision - need advice

I recommend reading this link and then figure out the actual age of the tires prior to paying for the bus.

People will tell you a vehicle has "new" tires simply because it has 98% tread left, even though the tire might be 10 years old. It is good to educate yourself about this.

hope this helps,
jim
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