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Old 05-31-2024, 07:53 AM   #1
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Tire size and Engine RPM

If I go from a 10R to a 11R tire, how much would that drop my engine RPM?

As a percentage maybe?

I guess I could calculate this--- but I thought maybe someone else had already invented this wheel. (Pun intended...)

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Old 05-31-2024, 11:58 AM   #2
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Quote:
Originally Posted by PorchDog View Post
If I go from a 10R to a 11R tire, how much would that drop my engine RPM?

As a percentage maybe?

I guess I could calculate this--- but I thought maybe someone else had already invented this wheel. (Pun intended...)

Youll want to just use the calculator. Its going to change depending on your rear end and trans overdrive settings.


https://spicerparts.com/calculators/...rpm-calculator


11r is roughly 41.75" and 10r is about 40"
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Old 05-31-2024, 04:36 PM   #3
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Thank you for pointing me to that link.

My bus has a 6.14 rear axle and a 0.75 overdrive-- and 10R tires.

Unlocking the second overdrive will cost about $1500 for programming and shipping, and because it is an MD3060 with WTec II, a little bit iffy.

I drove my bus 2100 miles coming home at 63 MPH, and it did fine. But when cruising at highway speed if you feather the throttle to unload the driveshaft, there is quite a bit of noise. If you are accelerating slightly or decelerating, there is no noise. You have to force it into the no load condition for the noise to appear. I have looked at the U-Joints and don't see anything wrong, so I wonder if I have excessive ring gear and pinion clearance or a loose bearing back there. Bus chassis has over 300,000 miles...

My thinking is maybe I should spend the $1500 to get a reman 5.38 or 5.63 rear differential assembly, and be happy with 5 gears.

It does need tires, (Due to age) and I will be going to 11R-- mainly because they are commonly available, versus the 10Rs. I had a leaky front tire on my way home, and the tire shop had one used 10R-- and they did not want to patch a front tire, but I finally talked them into it. They had plenty of 11Rs in stock.
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Old 05-31-2024, 05:30 PM   #4
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Quote:
Originally Posted by PorchDog View Post
Thank you for pointing me to that link.

My bus has a 6.14 rear axle and a 0.75 overdrive-- and 10R tires.

Unlocking the second overdrive will cost about $1500 for programming and shipping, and because it is an MD3060 with WTec II, a little bit iffy.

I drove my bus 2100 miles coming home at 63 MPH, and it did fine. But when cruising at highway speed if you feather the throttle to unload the driveshaft, there is quite a bit of noise. If you are accelerating slightly or decelerating, there is no noise. You have to force it into the no load condition for the noise to appear. I have looked at the U-Joints and don't see anything wrong, so I wonder if I have excessive ring gear and pinion clearance or a loose bearing back there. Bus chassis has over 300,000 miles...

My thinking is maybe I should spend the $1500 to get a reman 5.38 or 5.63 rear differential assembly, and be happy with 5 gears.

It does need tires, (Due to age) and I will be going to 11R-- mainly because they are commonly available, versus the 10Rs. I had a leaky front tire on my way home, and the tire shop had one used 10R-- and they did not want to patch a front tire, but I finally talked them into it. They had plenty of 11Rs in stock.

you'll get noise with the high driveshaft speed you are turning.. with OD and maxing out the engine, your driveshaft spin speed is pretty high so when its unloaded you'll definitely hear it as there is always a little slack in everything, esp some lash at the pinion gear maybe the shaft can vibrate around a little at the carrier bearing where it goes into the rubber.. the set might be a little out of balance
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Old 05-31-2024, 06:08 PM   #5
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"Unlocking the second overdrive will cost about $1500 for programming and shipping, and because it is an MD3060 with WTec II, a little bit iffy"



Maybe it is the WTecII (I don't know what that is) that jumps the price, but I though unlocking 6th on an Allison MD3060 was more like $400 ?
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Old 05-31-2024, 07:59 PM   #6
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The later models with WTEC III are easier to unlock. I contacted a couple of the cheaper fellows, but they could not do it. ECM Kings in Texas said he thought he could do it, but he would not guarantee it. $1480 and the he would hold my good core (Or maybe my original programming) until I could see if what he did worked.

If high driveshaft speed is making my noise, then that's another reason to go the re gearing route to lower the shaft speed some.

This is a 210 HP DT466 on a 7 window bus, so not a huge bus. 5.38 or 5.63?

5.38 with 11R tires shows an engine RPM of 2100 at 64.67 MPH.

5.63 with 11R tires shows an engine RPM of 2100 at 61.8 MPH. I lean toward the 5.63.

Peak Horse Power is at 2300 RPM. That would be 67.68 MPH with 5.63 gears.

With 5.38 gears-- 2300 RPM will be 70.83.

Comments?
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Old 06-01-2024, 07:02 AM   #7
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Quote:
Originally Posted by PorchDog View Post
The later models with WTEC III are easier to unlock. I contacted a couple of the cheaper fellows, but they could not do it. ECM Kings in Texas said he thought he could do it, but he would not guarantee it. $1480 and the he would hold my good core (Or maybe my original programming) until I could see if what he did worked.

If high driveshaft speed is making my noise, then that's another reason to go the re gearing route to lower the shaft speed some.

This is a 210 HP DT466 on a 7 window bus, so not a huge bus. 5.38 or 5.63?

5.38 with 11R tires shows an engine RPM of 2100 at 64.67 MPH.

5.63 with 11R tires shows an engine RPM of 2100 at 61.8 MPH. I lean toward the 5.63.

Peak Horse Power is at 2300 RPM. That would be 67.68 MPH with 5.63 gears.

With 5.38 gears-- 2300 RPM will be 70.83.

Comments?

im assuming you are talking with 6th gear turned on?


since 5th is 0.75 and 6th is 0.65, it gives you a bit of play room.. you can gear to where your 6th gear RPM at cruise might be a little lower than you might otherwise want to cruise but then a little throttle andit drops back to 5th which is going to bump you up a couple hundred RPM.. but its not like going from 5th to 4th which is a big change...
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Old 06-01-2024, 09:22 AM   #8
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No, my calculations were based on just 5 gears, with the 0.75 5th gear.

Spending the money to swap the differential rather than unlocking 6th.
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Old 06-01-2024, 01:31 PM   #9
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Originally Posted by PorchDog View Post
No, my calculations were based on just 5 gears, with the 0.75 5th gear.

Spending the money to swap the differential rather than unlocking 6th.
Run the 5.38 if staying with 5 gears. You be able to drop to 4th and run up that hill a little faster. The 5.38 was crazy expensive for me because I would have had to import it out of the states. I stuck with 5.63 because I found one in Canada. My diff would growl bad under load. Carrier bearing was bad, someone was in it in the past and set it up wrong. Ring and Pinion was trashed.

I have that 5.63 you mentioned. Seems fine behind me T444e. Really hear the turbo constantly spooled at 100kph.

Currently running 10r. Switching to 11r. It's about 2-3% difference depending on manufacture
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Old 06-12-2024, 05:27 PM   #10
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If I recall correctly, the 11R tire is 1 inch wider than the 10R. Changing to wider tires wont change your speed. The link provided to determine gearing/tires/engine rpms looks like the one I used to determine what my gearing needed to be. Larger diameter tires will lower your engine RPMs on the highway. The engine in my "new Crown" is good for 425 HP with over 1250 ft/lbs of torque so I run 3.42 gear ratio. That gives me about 78 mph at redline but I don't think I want to go almost 80 mph in a school bus even though the speed limit is 80 mph in some states. With my gearing I can cruise 70 all day and be comfortable.
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Old 06-12-2024, 05:51 PM   #11
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If I recall correctly, the 11R tire is 1 inch wider than the 10R. Changing to wider tires wont change your speed. The link provided to determine gearing/tires/engine rpms looks like the one I used to determine what my gearing needed to be. Larger diameter tires will lower your engine RPMs on the highway. The engine in my "new Crown" is good for 425 HP with over 1250 ft/lbs of torque so I run 3.42 gear ratio. That gives me about 78 mph at redline but I don't think I want to go almost 80 mph in a school bus even though the speed limit is 80 mph in some states. With my gearing I can cruise 70 all day and be comfortable.

Width x aspect ratio equals height. I believe aspect ratio is in the high 80 percentile.

Easy math here. A 10 inch tire with aspect ratio of 90% gives you a 9 inch sidewall. A 11 inch tire with 90% aspect ratio gives you a 9.9 inch sidewall.

I'm pretty sure that's what it is.

So yes, an 11r 22.5 tire is taller. I just laid marks on my garage floor. 11 r22.5 tire rolled 4 inches further than my 10r22.5. I crunched the numbers and it was a 3.33% improvement between the tires. My 10r22.5 only had 7/32nds lift for tread. Likely reason why it pushed past the 3% range I suggested earlier
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