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Old 06-21-2013, 09:40 PM   #11
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Re: SKOOLIE - Governed or Geared?

Quote:
Originally Posted by Motobus
Jack one rear wheel up, put it in neutral. Turn the wheel one revolution and count how many times the drive shaft spins. For example, if the drive shaft turns about 5 1/4 turns for every spin of the wheel you have 5.29's.
I think she meant how do we figure what ratio we want, not what we already have.
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Old 06-21-2013, 09:49 PM   #12
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Re: SKOOLIE - Governed or Geared?

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Originally Posted by JakeC
... If my engine is spinning at 2000 rpm's, my gear ratio is 4.78:1, and my tires are 42", the math would be as follows. 2000/4.78 = 418.42 tire rpms. 418.42*131.88 (tire circumference) = 55181.23 in/min which converts to 52.25 mph. To be in my powerband with my current gearing, I need to drive 52 mph. Not gonna happen. Now if I plug in 3.55 for a gear ratio, the numbers become 2000/3.55 = 563.38 tire rpms. 563.38*131.88 (tire circumference) = 74298.59 in/min which converts to 70.36 mph. That's more like it
We do not know what the RPMs are either. I only know the ratio because it's stamped on a plate inside the bus.
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Old 06-21-2013, 09:56 PM   #13
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Re: SKOOLIE - Governed or Geared?

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Originally Posted by lornaschinske
I only know the ratio because it's stamped on a plate inside the bus.
And even then it could still be wrong.
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Old 06-21-2013, 10:07 PM   #14
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Re: SKOOLIE - Governed or Geared?

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Originally Posted by Das Mel
Quote:
Originally Posted by lornaschinske
I only know the ratio because it's stamped on a plate inside the bus.
And even then it could still be wrong.
Hi Mom, how about a poke in the eye. LOL.
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Old 06-21-2013, 10:16 PM   #15
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Re: SKOOLIE - Governed or Geared?

Hey, I only poked me eye once!

Okay, twice.

...fine, I admit I did it more than twice I never said I wasn't accident prone. At least I haven't managed to punch myself this week! ...yet. Oh, shut up. :P
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Old 06-21-2013, 10:17 PM   #16
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Re: SKOOLIE - Governed or Geared?

Quote:
Originally Posted by lornaschinske
Quote:
Originally Posted by JakeC
... If my engine is spinning at 2000 rpm's, my gear ratio is 4.78:1, and my tires are 42", the math would be as follows. 2000/4.78 = 418.42 tire rpms. 418.42*131.88 (tire circumference) = 55181.23 in/min which converts to 52.25 mph. To be in my powerband with my current gearing, I need to drive 52 mph. Not gonna happen. Now if I plug in 3.55 for a gear ratio, the numbers become 2000/3.55 = 563.38 tire rpms. 563.38*131.88 (tire circumference) = 74298.59 in/min which converts to 70.36 mph. That's more like it
We do not know what the RPMs are either. I only know the ratio because it's stamped on a plate inside the bus.
You actually don't need to know what actual rpms you are turning currently. The calculations are all hypothetical I'm surprised you don't have a tachometer, though! Do you know what transmission you have? It can all be calculated from there. If you tell me what speed you like to spend the majority of your time at, what transmission you have, and what size your tires are, I can give you an estimate of an ideal rear gear ratio. I could do a google search of your engine and see where your powerband is. I realize you don't actually want/need to do this, but sometimes it's fun to pretend, lol
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Old 06-21-2013, 10:37 PM   #17
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Re: SKOOLIE - Governed or Geared?

Quote:
Originally Posted by lornaschinske
Quote:
axle ratio versus tire size for top speed information
Is there a way to figure out just what is a good axle ratio? Ours is 5.29. It's not going to be changed. I'm just curious. We've not got a problem with meandering down the primary highways. MPG's are more important to us than MPH.

Documentation on schoolbuses suck! I don't even know what make our rear is (plate inside says R125NX5). "R" = Rockwell?
You might know this already but the lower the number, the more it is highway gearing. Ideal in a school bus would be in the 4-4.5 range. If you're under 4, you will need a lot more HP to push you down the road or have a tranny with more than 4 gears to find the right sweet spot for your engine so you are not lugging your bus. Also, if you have an AT545, changing anything before you change that tranny is a waste of time and money. These are just my opinions, nothing else.
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Old 06-21-2013, 11:04 PM   #18
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Re: SKOOLIE - Governed or Geared?

Quote:
Originally Posted by crazycal
Quote:
Originally Posted by lornaschinske
Quote:
axle ratio versus tire size for top speed information
Is there a way to figure out just what is a good axle ratio? Ours is 5.29. It's not going to be changed. I'm just curious. We've not got a problem with meandering down the primary highways. MPG's are more important to us than MPH.

Documentation on schoolbuses suck! I don't even know what make our rear is (plate inside says R125NX5). "R" = Rockwell?
You might know this already but the lower the number, the more it is highway gearing. Ideal in a school bus would be in the 4-4.5 range. If you're under 4, you will need a lot more HP to push you down the road or have a tranny with more than 4 gears to find the right sweet spot for your engine so you are not lugging your bus. Also, if you have an AT545, changing anything before you change that tranny is a waste of time and money. These are just my opinions, nothing else.
I realize I may be a little optimistic thinking I can spin 3.55, but keep in mind most of these buses are geared for 55 mph or less. Many geared for rural gravel roads, or cities. When we take them out on the highway, we are spinning them well past the powerband. I do understand what you are saying with needing more gears to find the sweet spot, though. That is a problem for sure if you go with a numerically lower gear ratio. Just need more fuel and boost to bridge the gap
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Old 06-21-2013, 11:30 PM   #19
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Re: SKOOLIE - Governed or Geared?

Put in 3.55's and throw in a 10 spd Road Ranger as well. Problem solved.
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Old 06-21-2013, 11:37 PM   #20
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Re: SKOOLIE - Governed or Geared?

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Put in 3.55's and throw in a 10 spd Road Ranger as well. Problem solved.
I like your way of thinking Seriously, I would be in heaven.
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