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Old 04-08-2019, 05:10 PM   #1
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Gearing for 80 MPH

After searching the forums for a while, on multiple occasions, I couldn't find any posts discussing general information about gearing skoolies for high freeway speeds. I can find threads discussing fine details but no threads discussing how to make an informed decision on gearing modifications.

Has anybody geared a skoolie to sustain 80 MPH under 2000 RPM? Or Under 2500 RPM? Did you modify or change the transmission? What did you put downstream of the transmission to lower the gear ratio? Did you swap the differential? Did you install splitter gears or a “hi-lo differential”? Other than higher diameter drive tires, What is the simplest mod to minimize the gear ratio?

Does sustained bogging usually damage the engine? What parts will wear faster in this configuration? Should I upgrade driveshafts & drive lines to avoid snapping bolts due to higher than rated for torque? What else do I need to know to make an informed decision?

I’m doing this to have the choice between high speed or high fuel economy. So I can haul ass if I need to at 80 MPH where I legally can (like most of Interstate 80 in Northern Nevada).
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Old 04-08-2019, 05:17 PM   #2
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My bus will do a little over 80.
Its got 3.42 gearing in the rear and 1:1 4th gear in the Allison 545.

Its happiest at 1800 rpm cruising at 65mph.

I'm getting good fuel economy. Better than any bus I've owned.

The school set the bus up for high speeds, so I haven't done anything to it.
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Old 04-08-2019, 05:33 PM   #3
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The first question that comes to mind for me....

Why would you want to go 80 mph in a school bus?

For me, safety and fuel economy keep me at 60-65 mph. I must be getting old
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Old 04-08-2019, 05:36 PM   #4
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The first question that comes to mind for me....

Why would you want to go 80 mph in a school bus?

For me, safety and fuel economy keep me at 60-65 mph. I must be getting old
Some schools have 80 mph buses, man.
Mine is from Atlanta where going 55 can be more dangerous than going with the flow.
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Old 04-08-2019, 05:41 PM   #5
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Some schools have 80 mph buses, man.
Mine is from Atlanta where going 55 can be more dangerous than going with the flow.
When you have 200 miles of straight 80mph freeway, there's nothing unsafe about going 80mph if all is in good condition. I ran a full tank of fuel out on CC at 80mph. The only negative I found was 7mpg at that speed. Mine has a 4.38? gear and is turning 2350 at 85mph if I recall correctly. All gauges stayed in the safe zone. And that was with only 5th gear, opening 6th I think would substantially improve fuel mileage.
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Old 04-08-2019, 05:41 PM   #6
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Some schools have 80 mph buses, man.
Mine is from Atlanta where going 55 can be more dangerous than going with the flow.
I agree that going 55 when the flow of traffic is going 80 creates its own hazards. I have never had trouble other slow traffic in the slow lane.

Driving 80 in a 30k lb vehicle on tires rated for 70 just is not my thing. Nor is 55 in an 80. On good road, I feel good at 65.
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Old 04-08-2019, 05:43 PM   #7
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I agree that going 55 when the flow of traffic is going 80 creates its own hazards. I have never had trouble other slow traffic in the slow lane.

Driving 80 in a 30k lb vehicle on tires rated for 70 just is not my thing. Nor is 55 in an 80. On good road, I feel good at 65.
The tire rating thing is way over-blown on this forum. Most modern 10R and 11R tires are rated for 81 mph. Even the chinese ones.
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Old 04-08-2019, 06:06 PM   #8
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I agree that going 55 when the flow of traffic is going 80 creates its own hazards. I have never had trouble other slow traffic in the slow lane.

Driving 80 in a 30k lb vehicle on tires rated for 70 just is not my thing. Nor is 55 in an 80. On good road, I feel good at 65.
Driving 80 on a tire rated at 75mph is not unsafe at 80mph. That rating is a compromise due to liability of the tire company. They make a tire that is safe to 100mph and then rate it at 75mph. If your tires are in good condition, going 10% or even 20% over is not going to cause the tire to fly apart.
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Old 04-08-2019, 06:55 PM   #9
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the tire rating takes into account how hot tires get on very hot roads. and even if they are somewhat underinflated by accident.. youd be surprised at how hot that rubber gets at high speed on hot summer roads...



other part is centrifigal force on the tire.. the rubber and the steel belts...



agreed that like most things they rate them for the safest and easiest to get out of liability-wise.. if you blow a tire thats got a 75 MPH stamp on it driving 80 the tire manufacturer says "not our fault"....



the big thing is properly inflating the tires for your load..



as far as centrifigal force it is real... ever watch the top fuel dragsters do a burnout? and watch those tires stretch out? granted those wrinkle wall mickeys are designed to do that... but its a concept that does apply in a lesser degree to all tires..



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Old 04-08-2019, 07:46 PM   #10
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I did mention that you need to keep things in good condition. If you want to be a fool and push the limits on unsafe conditions, then deal with circumstances. That's the same fool that would tell the tire company he was exceeding the rated speed when it let loose.
Dealer just got back to me. I have a 4.78, not a 4.35 gear.
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Old 04-08-2019, 07:50 PM   #11
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I did mention that you need to keep things in good condition. If you want to be a fool and push the limits on unsafe conditions, then deal with circumstances. That's the same fool that would tell the tire company he was exceeding the rated speed when it let loose.
Dealer just got back to me. I have a 4.78, not a 4.35 gear.
With OD 4.78 is a REAL nice ratio.
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Old 04-08-2019, 07:57 PM   #12
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With OD 4.78 is a REAL nice ratio.
With the 2nd overdrive it would be much sweeter.
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Old 04-08-2019, 10:10 PM   #13
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Quote:
Originally Posted by debit.servus View Post
After searching the forums for a while, on multiple occasions, I couldn't find any posts discussing general information about gearing skoolies for high freeway speeds. I can find threads discussing fine details but no threads discussing how to make an informed decision on gearing modifications.

Has anybody geared a skoolie to sustain 80 MPH under 2000 RPM? Or Under 2500 RPM? Did you modify or change the transmission? What did you put downstream of the transmission to lower the gear ratio? Did you swap the differential? Did you install splitter gears or a “hi-lo differential”? Other than higher diameter drive tires, What is the simplest mod to minimize the gear ratio?

Does sustained bogging usually damage the engine? What parts will wear faster in this configuration? Should I upgrade driveshafts & drive lines to avoid snapping bolts due to higher than rated for torque? What else do I need to know to make an informed decision?

I’m doing this to have the choice between high speed or high fuel economy. So I can haul ass if I need to at 80 MPH where I legally can (like most of Interstate 80 in Northern Nevada).


Tell us more about your bus.
Engine, transmission, rear end gearing, tire size?
I have a 250+ horsepower engine, .78 overdrive, 5.29 rear end and 11r24.5 tires.
My bus will go 80 MPH.
However 80 is too fast in a 40+ year old school bus, at least for me and I generally drive to get there.
For me the biggest benefit of changing from a 5 speed direct to the 10 speed overdrive transmission is comfortably cruising at 65 instead of being maxed out at 67. I also went from 8 MPG to 9 MPG.
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Old 04-08-2019, 11:46 PM   #14
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Quote:
Originally Posted by debit.servus View Post
After searching the forums for a while, on multiple occasions, I couldn't find any posts discussing general information about gearing skoolies for high freeway speeds. I can find threads discussing fine details but no threads discussing how to make an informed decision on gearing modifications.

Has anybody geared a skoolie to sustain 80 MPH under 2000 RPM? Or Under 2500 RPM? Did you modify or change the transmission? What did you put downstream of the transmission to lower the gear ratio? Did you swap the differential? Did you install splitter gears or a “hi-lo differential”? Other than higher diameter drive tires, What is the simplest mod to minimize the gear ratio?

Does sustained bogging usually damage the engine? What parts will wear faster in this configuration? Should I upgrade driveshafts & drive lines to avoid snapping bolts due to higher than rated for torque? What else do I need to know to make an informed decision?

I’m doing this to have the choice between high speed or high fuel economy. So I can haul ass if I need to at 80 MPH where I legally can (like most of Interstate 80 in Northern Nevada).
If your gearing for a top speed of 80 mph your rpm will need to be close to the governed rpm for max rated HP. It most likely will need all of the rated engine HP to push a bus through the air at that speed and be able handle a slight grade.

Keep in mind going to a lower gear ratio you lose gradeabilty which is how long the tranny can hold a gear before downshifting on a hill and startability which is how well the bus can start from a stop.

The easiest way to change gearing is to swap the differential carrier for one with a lower ratio.
If you want to play with gear ratios check out cummins power spec. It is a free download and has the best gear calculator I've found.

Ted
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Old 04-09-2019, 12:38 AM   #15
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All of this talk of gearing to go faster and tire safety and, no one has even mentioned brakes and stopping distances.

I'm sure all of you who want to go faster are safe and alert drivers. I have over 2 million mile accident free in a semi. I have seen very few trucks, buses or cars that leave enough room in front of them. I couldn't find much real data for truck but there is lots of data for cars.

A car traveling 55 mph has a thinking distance of 54 feet and a braking distance of 151 ft for a total stopping distance of 205 feet.
At 80 mpg the thinking distance is 79 feet with a braking distance of 320 ft with a total of 398. That's over a football field for a car, trucks while not necessarily heavier than a bus, have much more breaking surface in both the brakes and the tires and can probably stop in less distance than a school bus.

In my opinion anything over 70 in a skoolie is just too fast I have driven a semi over 80, out west when there were NO other vehicles in sight. Probably shouldn't have.

On the tire issue all of those specs quoted are for new pristine tires. NONE of us have those. Have you never hit a large pot hole hard, bumped a curb with a side wall, ran over anything you couldn't identify as being soft?

There is also the expense my truck would get 1/4 mpg worse fuel mileage for ever 5 MPH over 55. So at 80 I was getting 1 1/4 LESS MPG. Don't sound like much do the math for 150,000 miles a year with a fuel price north of $4.00 at 7 MPG. When fuel got up around $5.00 I drove 55 and didn't feel unsafe at all

Dick
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Old 04-09-2019, 12:58 AM   #16
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All of this talk of gearing to go faster and tire safety and, no one has even mentioned brakes and stopping distances.

I'm sure all of you who want to go faster are safe and alert drivers. I have over 2 million mile accident free in a semi. I have seen very few trucks, buses or cars that leave enough room in front of them. I couldn't find much real data for truck but there is lots of data for cars.

A car traveling 55 mph has a thinking distance of 54 feet and a braking distance of 151 ft for a total stopping distance of 205 feet.
At 80 mpg the thinking distance is 79 feet with a braking distance of 320 ft with a total of 398. That's over a football field for a car, trucks while not necessarily heavier than a bus, have much more breaking surface in both the brakes and the tires and can probably stop in less distance than a school bus.

In my opinion anything over 70 in a skoolie is just too fast I have driven a semi over 80, out west when there were NO other vehicles in sight. Probably shouldn't have.

On the tire issue all of those specs quoted are for new pristine tires. NONE of us have those. Have you never hit a large pot hole hard, bumped a curb with a side wall, ran over anything you couldn't identify as being soft?

There is also the expense my truck would get 1/4 mpg worse fuel mileage for ever 5 MPH over 55. So at 80 I was getting 1 1/4 LESS MPG. Don't sound like much do the math for 150,000 miles a year with a fuel price north of $4.00 at 7 MPG. When fuel got up around $5.00 I drove 55 and didn't feel unsafe at all

Dick
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Old 04-09-2019, 06:59 AM   #17
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I guess I have a different opinion than the general publc.. one of the things getting a bus did was teach me to re-learn hpw to road-trip... I venture to say that very few of us are getting paid by the mile to drive our busses across the country..



I built the red bus to cruise at 65... its actually over-geared for that purpose.. 6th gear lugs.. ill gear it down once I sort out exactly which tires im going to run. and then can gear accordingly...



65 to me seems to be a great speed to travel the main freeways of 90% of the country..



I wanted the Over-drive to drop my RPMs and have a lockup converter.. I wanted the horsepower upgrade so I didnt pull hills at 35... although there are times I WANT to pull hills at 35 knowing I gotta slow it down on the other side..



I rode in the back of my bus when marc drove it 79 MPH... the MPG meter on the dashboard was at someplace like 6 MPG.. and the windows of the bus were buffeting some in the wind...



I have a 400 Horsepower HEMI pickup anytime I feel the need to get there quick.. and a friend who tosses me the red key to his HellCat anytime I feel the true need to just fly.. beyond that I pull up my Delta Airlines App on my phone...



remember you are driving your HOME.. or your RECREATIONAL vehicle.. often with your FAMILY...



sorry Ill get off my rant now and go back to enjoying my busses for what they are..
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Old 04-09-2019, 07:12 AM   #18
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80mph buses aren't really that rare.

I'm happiest at 65 and so is the bus, but its good to know it'll do more if asked to.
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Old 04-09-2019, 09:25 AM   #19
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I'd like my bus to travel comfortably at 100 KPH ( 62 MPH ), the speed limit on most of the highways I'll be driving, and NOT be the vehicle at the front of a long line of vehicles going up the mountain passes - by doing some skirting and air dams, I hope to lessen the air drag and maybe gain a couple of easy MPH - I hope I can find a bus with the right gearing to do that and a transmission that allows me to lower the bus down a mountain pass without burning up the brakes
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Old 04-09-2019, 11:37 AM   #20
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80mph buses aren't really that rare.

I'm happiest at 65 and so is the bus, but its good to know it'll do more if asked to.
The lower video I noticed the cars RPM's, I bet the bus was turning the same RPM. The second video shows a dumbass behind the wheel, concerned about a bus going 80 he almost crashes dropping his cell phone at 80mph. He should get a ticket and the bus is probably legal.
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