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Buskid
Top Member

USA
3368 Posts

Posted - 06/04/2003 :  07:49:15 AM  Show Profile  Reply with Quote
My knowledge on the subject is limited, but if I understand correctly, the rear end can be swapped out in order to add speed to the vehicle.

We had a bus in the shop about a month ago having the rear end swapped out for another one so that it could maintain highway speeds — this was a privately owned church bus, not a school bus. They wanted it to be able to go 70 m.p.h. instead of only 60 m.p.h.

Now, I have one that will only go 52-55 m.p.h. It has a 250 horsepower 671 Detroit Diesel Engine coupled with a Spicer (model 6852, I think?) 5-speed transmission. Would changing the rear end be a futile attempt to add some speed to my top end, or could it actually help?

I was told that the gear ratio is very low because it was in service up in the hills. Its maximum speed is somewhere around 52 m.p.h. @ 2500 RPM. Is there any way to change that?

I realize that I may lose some of my low end power if I change out the rear end, but I do mostly highway travel, so I really need the top end speed.

PS: I really have no idea what rear end is in it right now. I'm not sure how to tell.

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PHW
Top Member

USA
1345 Posts

Posted - 06/04/2003 :  08:23:35 AM  Show Profile  Reply with Quote
There are several factors that determine top end speed. Transmission gearing, differential gearing, engine RPM and tire size.

I would first try to determine exactly what ratios you have compaired to a standard set up. If you can't get the part or serial numbers off the rear end I would think you could count the number of turns the drive line makes to one turn of the rear wheel. If the driveline turns 4.5 times to one turn of the wheel thats your gear ratio. 4.5 to 1.

BTW 2500 rpm for a 6-71 seems to be set a little high. If I recall right the specs are 2200 to 2250 rpm.

PHW

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wagonmaster
Top Member

USA
2298 Posts

Posted - 06/04/2003 :  08:49:23 AM  Show Profile  Reply with Quote
Most 6-71 engine are rated at 2350 rpm. 2500 is indeed high, but if limited to that rpm and no more engine life wouldn't be drastically affected. Usually, I would suggest increasing rpm a little higher, but that's not really an option in this case. Changing out the ring and pinion should not be too hard, and it will work. The trick will be not to go too far in the other direction with the ratio. This could give you a problem pulling hills, etc. The ring and pinion won't be inexpensive, and you may even find a used unit out there that you could use. I would suggest a 3.90 or 4.11 ratio if your transmission is a "direct" drive and not an overdrive type unit. The 4.11 with a direct (1-1) 5th gear and 10.00 Rx20 tires should give you about 65-68 mph with 2500 rpm. The 3.90 would be a little higher, maybe 70-72. Hope this helps. Sounds like you have about a 4.56 or a 4.88 ratio, but you can figure it pretty close, like PHW said, by turning the driveshaft (with the rear jacked up) and dividing the number of turns the driveshaft makes into the number that the wheel makes. Usually for every 1 rotation of the wheel the driveshaft would make a "tad" more than 4 1/2for a 4.56 ratio, or similar.
Joe



Edited by - wagonmaster on 06/04/2003 08:50:49 AM
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Aint Skeered Racing
Senior Member

United States
68 Posts

Posted - 02/08/2009 :  6:58:04 PM  Show Profile  Visit Aint Skeered Racing's Homepage  Send Aint Skeered Racing an AOL message  Send Aint Skeered Racing a Yahoo! Message  Reply with Quote
Same question but what's the best gear ratio for racing school bus for example 1990 chevy 8.2 diesel automatic tranny...... would a rear gear out of a 1986 c60 chevy manual tranny make the 1990 bus go faster?

Jimmy Volk #76
Aint Skeered Racing
School bus racing asphalt Figure 8s and oval dirt racing

Owner of 26 racing buses
14 buses destroyed
12 buses running
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wagonmaster
Top Member

USA
2298 Posts

Posted - 02/09/2009 :  05:38:10 AM  Show Profile  Reply with Quote
Jimmy, Check the ratio from each and see. If that Chevy had a gas engine with more usable RPM it may be higher (numerically lower) and thus, will provide a higer road speed.

Joe
Land of the Free, because of the Brave!
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professorwrench
Active Member

United States
16 Posts

Posted - 02/19/2009 :  11:22:01 AM  Show Profile  Reply with Quote
I had the same problem in an international bus with a 3208 Cat. Top crusing speed was limited to 53mph. I can't remember exactly what ratio was in the bus but it was around a 4.7 and I bought a new ring and pinion at around a 4.1 ratio and installed them. Made all the difference in the world for going down the highway. I also changed the gear for the speedometer by having one calibrated at the local shop. The ring and pinion were around $400 if I remember right and the few hours necessary to pull the rear end and set up the new ring and pinion. This was cheaper than an overdrive unit. I was warned by a friend to not change the ratio too much because I could lose all hill climbing ability. The 3208 was no horsepower king. So I hope this encourages you to go for it. It has be 10 years since I did the swap and it has served the bus well.

Do it right the first time or do it over when you don't have time.
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Jared
Top Member

USA
1865 Posts

Posted - 02/19/2009 :  1:45:56 PM  Show Profile  Visit Jared's Homepage  Reply with Quote
quote:
Originally posted by professorwrench

I had the same problem in an international bus with a 3208 Cat. Top crusing speed was limited to 53mph. I can't remember exactly what ratio was in the bus but it was around a 4.7 and I bought a new ring and pinion at around a 4.1 ratio and installed them. Made all the difference in the world for going down the highway. I also changed the gear for the speedometer by having one calibrated at the local shop. The ring and pinion were around $400 if I remember right and the few hours necessary to pull the rear end and set up the new ring and pinion. This was cheaper than an overdrive unit. I was warned by a friend to not change the ratio too much because I could lose all hill climbing ability. The 3208 was no horsepower king. So I hope this encourages you to go for it. It has be 10 years since I did the swap and it has served the bus well.



I am contemplating doing this as well. I have a 1987 Chevy 366 / AT545 that tops out at 53 mph. How do I go about searching for a ring and pinion?

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Aint Skeered Racing
Senior Member

United States
68 Posts

Posted - 02/20/2009 :  4:44:56 PM  Show Profile  Visit Aint Skeered Racing's Homepage  Send Aint Skeered Racing an AOL message  Send Aint Skeered Racing a Yahoo! Message  Reply with Quote
Does this applies to the axles because I've seen different number of splines on the axles does that mean the more splines the slower the bus is?

Jimmy Volk #76
Aint Skeered Racing
School bus racing asphalt Figure 8s and oval dirt racing

Owner of 26 racing buses
14 buses destroyed
12 buses running
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International-9.0
Advanced Member

USA
459 Posts

Posted - 02/20/2009 :  4:56:00 PM  Show Profile  Reply with Quote
The number of splines on the axle will not affect the gear ratio. The splines slide into a gear and are directly driven by the gear body. Even if the splines were driven by gear teeth, a different number of splines would not change the ratio. Only a shaft larger or smaller in diameter would affect the ratio. Usually as a gear size changes, the number of teeth will change as well.
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