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Old 04-08-2019, 04:10 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, 04:17 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, 04:33 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, 04:36 PM   #4
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Originally Posted by PNW_Steve View Post
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, 04:41 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, 04:41 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, 04:43 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, 05:06 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, 05:55 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..



-Christopher
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Old 04-08-2019, 06:46 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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