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Old 10-25-2016, 01:44 PM   #1
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Rear end ratio change.

I'm really wanting to change my ratio of my rear end from 4.78 to 3.73 if I keep my bus to make highway speeds more attainable. I found a remanufactured rear end for 1500 with core deposit. The local shop said it would only be about 3 hours of labor plus fluids. So out the door at 1900. Does this sound reasonable? It seems a little silly as I probably wouldn't see a return in the investment for years. But would make driving much more comfortable.

What does everyone else think?

Bus is 1993 Amtran Genesis DT466 MT643
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Old 10-25-2016, 02:21 PM   #2
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keep us posted on the swap.

i have the same 4.7 differential and spent plenty of time thinking about doing the same. your DT motor should make enough power to make use of the taller gear and get more speed, or cruise at the same speed your at now at a lower rpm.
if you're going to see a lot of interstate time, it'll be a nice upgrade.
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Old 10-25-2016, 02:30 PM   #3
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I've got the same bus, with 4.44.
Have you looked into running a bigger rear tire? I doubt it would be as dramatic a difference, but could be a better investment. Especially if your tires are worn.
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Old 10-25-2016, 02:44 PM   #4
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I actually may have 4.44 as well. I'm doing 60 mph at around 2250 RPM.

Wouldn't that only give about 3 mph difference? I do need two rear tires. But the other two are pretty good
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Old 10-25-2016, 03:36 PM   #5
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i may be wrong, but i believe the speedometers are run on their own gear off the transmission. either rear end or tire swap will get you faster but your speedo will still read the same based on rpms.
fwiw
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Old 10-25-2016, 04:41 PM   #6
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I'm really wanting to change my ratio of my rear end from 4.78 to 3.73 if I keep my bus to make highway speeds more attainable. I found a remanufactured rear end for 1500 with core deposit. The local shop said it would only be about 3 hours of labor plus fluids. So out the door at 1900. Does this sound reasonable? It seems a little silly as I probably wouldn't see a return in the investment for years. But would make driving much more comfortable.

What does everyone else think?

Bus is 1993 Amtran Genesis DT466 MT643
Cost seems reasonable. However, you probably don't need to replace the entire rear axle - assuming the desired gear set is available for your current one (and given you're finding a reman, I'm assuming there is). I located a shop that would redo my axle module with my desired gearing for ~$850 or so (exchange), and with a transmission jack, this can become a DIY job (the module is somewhere around 300-350 pounds).
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Old 10-25-2016, 04:42 PM   #7
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i may be wrong, but i believe the speedometers are run on their own gear off the transmission. either rear end or tire swap will get you faster but your speedo will still read the same based on rpms.
fwiw
CadillacKid can explain better than me, but the gauges are easy to recalibrate.
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Old 10-25-2016, 04:43 PM   #8
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I actually may have 4.44 as well. I'm doing 60 mph at around 2250 RPM.

Wouldn't that only give about 3 mph difference? I do need two rear tires. But the other two are pretty good
Check out some online gear ratio/speed calculators. You can plug in different tire sizes and stuff and see what you'd gain from what.
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Old 10-25-2016, 04:45 PM   #9
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I think you're right.... a rear end swap would require dropping a new speed gear or sensor in that's calibrated to the new gearing to keep the speedometer and odometer accurate.

YMMV, they may not all be the same way. Check it out to see what, if anything, needs to be done on that side of things.
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Old 10-25-2016, 04:46 PM   #10
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I think you're right.... a rear end swap would require dropping a new speed gear or sensor in that's calibrated to the new gearing to keep the speedometer and odometer accurate.

YMMV, they may not all be the same way. Check it out to see what, if anything, needs to be done on that side of things.
Internationals' are adjustable. no need for any new parts.
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Old 10-25-2016, 04:56 PM   #11
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i may be wrong, but i believe the speedometers are run on their own gear off the transmission. either rear end or tire swap will get you faster but your speedo will still read the same based on rpms.
fwiw
They sell rear end gear sets but not speedo gears?

My genesis is dt466 and 643 @22250 or 2300 at 60mph. I would be afraid of getting the gears to tall and not being able to stay in your tall gear? Seems like the slightest grade would kick you out of the gear and then your not able to use it and its not worth a dam. I cant est this myself as i don't have a load on the bus while I'm tearing it down. Also I don't know the HP ratings and what was most common but my sticker on the new engine says 250hp and the old engine was suppose to be 195hp I think. that's got to help hold a gear at high speed. Burn more fuel to trying to get more MPG? its balancing act for sure.

For $2k dollars how many years will it take you to recover that cost. I must be getting old but id rather run 5mph under every one else. You never have brake lights in front of you and the bumps that rip the counters from the walls are so much more timid.

Time wise there is a huge amount to be had at 50 to 70mph. Its to bad we don't have those big 14 liter engines turning 1200 rpm. Manuals are a pain in the ass when your out on vacation trying to hold your bloody marry and match RPMs. But they sure keep that engine right were it wants to be.
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Old 10-25-2016, 05:15 PM   #12
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For $2k dollars how many years will it take you to recover that cost. I must be getting old but id rather run 5mph under every one else. You never have brake lights in front of you and the bumps that rip the counters from the walls are so much more timid.
100% this. I've learned to love 55mph. Just take it at a medium pace...
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Old 10-25-2016, 05:24 PM   #13
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Internationals' are adjustable. no need for any new parts.
Good to know.... will have to keep that in mind for when the day comes that I score another International.
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Old 10-25-2016, 05:32 PM   #14
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Considering I've seen my exact bus with a 3.73 gear, I'm pretty sure it can handle it. Yes it may downshift more on hills, but i drive primarily in the Midwest. I don't think it will have too much trouble. With 3.73 gearing I would be turning 1977 RPM at 65, vs the current 2534 which I fear will shorten the life of my block. Even traveling at 55 with my current gearing I'm doing 2144 RPM which is simply too high IMO. I don't mind going 55, but my gearing is set for optimum cruising speed of 50.

So to extend the life of my engine, and drastically shorten trips, AND possibly improve my MPG, I feel like it may be worth my time. ESPECIALLY if changing the rear end module is a option. If I could get the same quote for $800, it would be in the shop now
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Old 10-25-2016, 08:39 PM   #15
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For $2k dollars how many years will it take you to recover that cost. I must be getting old but id rather run 5mph under every one else. You never have brake lights in front of you and the bumps that rip the counters from the walls are so much more timid.
I make two trips a year that average around 3200 miles each. At $2.39/gallon on diesel and estimating a 9 to 12 MPG increase for the diff change (that's what I've heard to expect from mine), that saves me about $425 a year. But that's not the real benefit. Right now my engine is screaming to hold at 55-60 MPH. With a diff change, I can knock off 500 RPMs to do the same speed. Those 500 RPMs are the difference between having to scream to my passengers or having to speak up a bit. Totally worth it.
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Old 10-25-2016, 09:53 PM   #16
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Gear/speed/RPM Calc...run some numbers. The bottom section does it best.

Engine RPM Calculator
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Old 10-25-2016, 10:09 PM   #17
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Those 500 RPMs are the difference between having to scream to my passengers or having to speak up a bit. Totally worth it.

Now I know why my bus driver was always yelling at us kids

He wasn't pizzed off, it was the gears!
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Old 10-25-2016, 10:43 PM   #18
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Bread, I loves ya like a brother, but I want you to think about this proposed swap. The DT466 is *perfectly* happy spinning along at 2500-2600 rpm. I think you are going way too far numerically with this rear-end gearing. 1900-ish is too slow. I think you will regret this swap.

Diesels are not like gassers. You run a gasser at its "service RPM" and you'll drive over the crankshaft after 25k miles. Gassers are made to loaf along at 30% load for 95% of their lives with occasional bursts to full power near redline. Diesels are different. A DT466's "service rating" is 2600 rpm. Redline is 2800 rpm. But it will run 2600 rpm at 80% load basically forever. Its a hoss. It just takes it. Feed it oil, water and clean fuel and its gonna bark along happy as a clam for quite a few hundred thousand miles before you need to get into the oily bits.

For what it is worth, I was of the exact same mindset as you. I was convinced 2600 rpm was just cruel and unusual punishment. I wanted to swap the rear enf badly and get it down near the torque peak. And then I drove the thing halfway to hell and back. 5000 miles taught me the guys who designed these things kinda knew what they were doing. The gearing worked. The radiator and cooling worked. The trans worked. It was a SYSTEM.

Me? I roll along about 2500 rpm. Thats about 57 mph (from memory), and its fast enough to get there without getting into any trouble. Yup, its a pretty good roar in the cockpit, but its a happy roar. Power is there when you need it. 18 psi of boost is about 1 second away anytime/anywhere you mash the big pedal.why? Because its already wound-up and happy to breathe even on short notice.

Take my thoughts for what they are worth. You might be really happy with the swap. Dunno. But DO let us know how it goes for you and your impressions before/after. Lots of people contemplate this swap, butfew do it, so it'll be a wonderful learning experience for all of us.
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Old 10-25-2016, 11:19 PM   #19
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I'm really wanting to change my ratio of my rear end from 4.78 to 3.73 if I keep my bus to make highway speeds more attainable. I found a remanufactured rear end for 1500 with core deposit. The local shop said it would only be about 3 hours of labor plus fluids. So out the door at 1900. Does this sound reasonable? It seems a little silly as I probably wouldn't see a return in the investment for years. But would make driving much more comfortable.

What does everyone else think?

Bus is 1993 Amtran Genesis DT466 MT643
If you can turn a wrench you can change a gear set.
No need to pay someone to do it.
All you need is a floor jack and some wrenches.
I am sure that there is a video and instructions online somewhere so I won't spell it out here.
But if not ask.
You can do this one.
Save your money.
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Old 10-25-2016, 11:20 PM   #20
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Bread, I loves ya like a brother, but I want you to think about this proposed swap. The DT466 is *perfectly* happy spinning along at 2500-2600 rpm. I think you are going way too far numerically with this rear-end gearing. 1900-ish is too slow. I think you will regret this swap.

Diesels are not like gassers. You run a gasser at its "service RPM" and you'll drive over the crankshaft after 25k miles. Gassers are made to loaf along at 30% load for 95% of their lives with occasional bursts to full power near redline. Diesels are different. A DT466's "service rating" is 2600 rpm. Redline is 2800 rpm. But it will run 2600 rpm at 80% load basically forever. Its a hoss. It just takes it. Feed it oil, water and clean fuel and its gonna bark along happy as a clam for quite a few hundred thousand miles before you need to get into the oily bits.

For what it is worth, I was of the exact same mindset as you. I was convinced 2600 rpm was just cruel and unusual punishment. I wanted to swap the rear enf badly and get it down near the torque peak. And then I drove the thing halfway to hell and back. 5000 miles taught me the guys who designed these things kinda knew what they were doing. The gearing worked. The radiator and cooling worked. The trans worked. It was a SYSTEM.

Me? I roll along about 2500 rpm. Thats about 57 mph (from memory), and its fast enough to get there without getting into any trouble. Yup, its a pretty good roar in the cockpit, but its a happy roar. Power is there when you need it. 18 psi of boost is about 1 second away anytime/anywhere you mash the big pedal.why? Because its already wound-up and happy to breathe even on short notice.

Take my thoughts for what they are worth. You might be really happy with the swap. Dunno. But DO let us know how it goes for you and your impressions before/after. Lots of people contemplate this swap, butfew do it, so it'll be a wonderful learning experience for all of us.


Thanks for the words. Definitely is making me think. I am reading on some of the truck forums that they run 2550 all day and the truck has 500k etc etc. but my truck mechanic was blown away that I had been driving it at 2339 rpm at 60 mph. What do you get for fuel mileage at that speed? And what trans do you have
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