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Old 08-31-2010, 02:05 PM   #1
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Location: Twin Falls, Idaho
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Changing rear gears

To begin the gear changing process, gather all the information possible. Decide the rear gears presently in your application. Make some measurements about the present complete unit. Go to a truck salvage yard or gear shop for big trucks.These places can give information about what other trucks use the same model or size rear axle assembly. New gears are expensive, so changing a complete rear axle is more cost effective. I got a used rear assembly from a Freightliner with a 3:55 for my 1968 Crown bus. The used unit was $1500, drum to drum. The 3:55 has all the upgrades from 1969-1996, with good brakes, drums and no leaking seals. The u-joint was $25 and near new. All the air brake stuff was good, and only some new u-bolts were bought. My bus had a 4:10
when purchased and could go 57 mph, as now the bus will travel about 79mph. Frank
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Old 09-02-2010, 10:45 AM   #2
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Re: Rear end gear ratios

I think the gear ratio in our bus is 5:10 (don't remember, don't care). Folks can just pass us. Bring the bus out west, some semi's would pass, others ended up dropping either just behind or just in front. With a fully loaded bus, the stopping distance is ... long, even when going slow. Since we like to run the back roads/scenic highways we think the original gearing will end up working just fine. As close as we can tell, we were getting 10 to 15 mpg bringing the bus out, going slow. David recently got the governor readjusted so that will probably change... hopefully not drastically.

The Class C does 80 mph and is heavy (my brakes are good... the RV is just very heavy... it's maxed out weight wise on the chassis). It also is a PITA to get it stopped (those little cars either are hiding at my rear bumper -- if I'm lucky I can see their antenna in the rear window -- or trying to fit into my front grill), I'm always worried that I can't the thing stopped in time. I worry that I will run over some little speck of a car, and I do mean run over it.

You may want to rethink doing higher speeds. It's not a question of can you go faster, it's a question of can you stop it.
This post is my opinion. It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
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Old 02-23-2011, 10:47 PM   #3
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Location: Columbus, Indiana
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Engine: DTA 360, Fuller 6-speed
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Re: Rear end gear ratios

Mostbig box trucks have the same axle gearing--I've been looking. Just like when you buy a pickup--engine power has everything to do with what gears are installed. An old bus with a gas engine, or a non-turbo diesel probably had something near 6:00-1 gears. I owned a straight truck with an FT361 with a 2-speed axle and the gears were 6.50/9.00. If you see a medium- duty box truck going 70mph you should know 2 things: the driver is an employee and doesn't own the truck; his foot is to the mat. Won't hurt anything, but I don't drive my bus that way.

A bus is delivered with a straight-truck drivetrain. A typical bus has a maybe 26,000# GVWR and weighs 10,000# less. The bus my kids rides is about half full (2-per seat, not 3) with kids from K-6. It's an 11-row, a 6th-grader is 100# tops maybe (guessing) and a K is maybe 40#. You've got maybe 1,600# in that bus. Besides that, your chassis is probably rated at 50,000# GCWR (my old straight-truck mentioned before was) which means you have to have enough power/gearing to get a small dumptruck pulling a backhoe going, but you are dealing with only 18-20,000#. Can you safely gear it up? Hell yeah.

I have a Fuller 6-speed but I don't use first. It's not a barnyard gear either--it's a smooth shift from first to second. I just don't need first because I don't have enough weight. If I start in first, the engine slams into the governer before I can get the clutch up. With 50,000#, yeah, I could use it. I plan to go down to 3.70s, when I can find them. It's not easy though, because people don't order medium-duty trucks with 300hp.

As far as brakes go, you should have excellent brakes or something is wrong. My bus is hydraulic disks and they are great--very surprised at the stopping ability, especially since they are only 15" disks. Disk brakes are THAT much better. I'm actually frightened by private owners with old air brakes and manual adjusters. I hate laws, think everyone should own a gun, etc., but air brakes do need to be adjusted to function properly. If you have them, go get a class on operating and maintaining them so you don't kill someone. Hydraulic disks are the same as what's on your Hyundia--just bigger.
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Old 03-03-2011, 11:52 AM   #4
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Re: Rear end gear ratios

I agree about the air brakes. If you have them know how to adjust them. I drove truck for 10 years and even when drivers have to go through scales and get safety inspections it always amazed me how often I would get a tractor/trailer with out of adjusted brakes. Major problem when most of your driving is through the Sierras! When set right air brakes are very effective.

Ounce you get a feel for it, it is not difficult to do and takes 5-10 min. (Note: do not trust "self adjusting" air brakes)
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Old 03-03-2011, 01:32 PM   #5
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Re: Rear end gear ratios

Originally Posted by Cotton
Even though it sounds like a good amount of work, it seems like a rewarding project to be able to go 79 in a bus! Thanks for the advice!!!
And you feel 79mph in a bus is a safe thing to do?
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