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Old 12-22-2019, 12:22 PM   #1
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Gear ratio help

I need imput on figuring out a better gear ratio then i have currently. I just added a Md3060 with 6th gear. The engine is a 262 Cummins and i am running 1200 22.5 tires. Take off is great however top speed is about 55 mp.
Thatís not going to cut it. Any recommendations on what ratio to go to?
Do you think maybe the trans ecm needs new programming due to my lower rpm on the Cummins 1800 vs 2400 rpm the computer says its for ?
Thanks
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Old 12-22-2019, 12:55 PM   #2
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I found this helpful when I was thinking about tweaking my gearing of 4:44.

https://www.strangeengineering.net/g...io-calculator/
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Old 12-22-2019, 02:01 PM   #3
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Having a little trouble understanding your post. Are you saying that you're doing 55mph at 1200rpm but can't go any faster?

MD3060's 6tth gear ratio is 0.65:1, plug that into your ratio calculator to figure out how fast you should be able to go.
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Old 12-22-2019, 05:58 PM   #4
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What we need from you - your engine RPM, max and desired cruising speed. Tire size and transmission top gear (both easily obtained from the info you've given). Current gear ratio.


I'll point out that this is approaching Big Rig territory as far as gearing goes and due to the lower engine speeds, many big trucks have gear ratios in the mid 3's, such as 3.54 or 3.23 or somewhere close to that (and many don't have overdrive gears).


*Edit* Did the math with the above linked calculator - with tire diameter 42.5", 1800 RPM and 4.44 gears. you'd get ~52 in 4th gear (1:1) and ~79 in 6th (.65:1)
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Old 12-22-2019, 06:37 PM   #5
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Quote:
Originally Posted by brokedown View Post
Having a little trouble understanding your post. Are you saying that you're doing 55mph at 1200rpm but can't go any faster?

MD3060's 6tth gear ratio is 0.65:1, plug that into your ratio calculator to figure out how fast you should be able to go.
Ya I hit about 55 it shifts to 6th and if I push the gas barley it goes back to 5th .
Found the ratio calculator and its looking like low 4.00 will be a good spot for me.
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Old 12-22-2019, 06:41 PM   #6
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Quote:
Originally Posted by Brad_SwiftFur View Post
What we need from you - your engine RPM, max and desired cruising speed. Tire size and transmission top gear (both easily obtained from the info you've given). Current gear ratio.


I'll point out that this is approaching Big Rig territory as far as gearing goes and due to the lower engine speeds, many big trucks have gear ratios in the mid 3's, such as 3.54 or 3.23 or somewhere close to that (and many don't have overdrive gears).


*Edit* Did the math with the above linked calculator - with tire diameter 42.5", 1800 RPM and 4.44 gears. you'd get ~52 in 4th gear (1:1) and ~79 in 6th (.65:1)
Ya 1800 is right, tire size is correct and 79 in 6th thatís plenty.. so I think something is out of wack as im not able to get the rpm past say 14-1500 before it down shifts to fifth and then Iím stuck doing around 55.
Possibly programming on the ecm or tps out of adjustment ?
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Old 12-22-2019, 10:05 PM   #7
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Not to hijack your thread, I have a T444E, a 5-speed manual (1:1 top gear), 10R22.5 tires (~40" diameter) and 5.38 rear ratio. Right now I'm getting around 55 MPH at 2600 RPM (give or take). I mathed it out, with a drive tonight, and I'd like to gear for a 2200-2300 cruising speed at 70. Calculating as above, a 4.10 rear axle would hit that very closely; at 2600 RPM that would give me about 75 MPH, or at 55 it would be around 1900 RPM.
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Old 12-22-2019, 10:33 PM   #8
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sounds like your kickdown is too tight, adjust it so you can stay out of 5th and run in 6th. Just installed a 4.44 behind my 3060, 385-75-24.5 tires which I believe are the same as a 10.00 20 tire or 42". Running 1650 RPM at 70 MPH or 1700 at 78 MPH, just where I wanted to be, sips fuel. Drops into 5th when climbing hills then back into 6th when topping them or kicks down into 5th when floored pedal, perfect in my book.
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Old 12-23-2019, 07:26 AM   #9
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if your engine max RPM is 1800 and the TCM is built for 2400 then yeah its nit going to operate correctly.. you'll never stay in 6th.. 1800 is main cruise RPM for say an 8.3 with 2400 RPM redline.. so anything above about 50% thriottle and it will want to drop down and then hang against your 1800 RPM governor..



you definitely want to get a shift program that better fits your RPM and torque profiile.. the throttle to shift-point maps in these TCMs are generally tailored to the engine its connecgted to..



building a custom rig means you either need t opay an allison dealer like stewart and stevenson (they will do custom work) to talk to allison and have a shift CAL made.. knowing the RPM / TQ / HP curves of your engine will help.. or if you have a 4th gen trans / TCM.. buy the necessary software and build your own shift Cal.



I have spent MANY hours dialing in my custom trans to my liking..



-Christopher
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Old 12-23-2019, 11:16 AM   #10
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Ok this makes since, the trans came from behind a dd and I know the power curve and rpms are different. Looks like i will be getting some power added then off to the dealer for some computer work.. then see if gears are in the future.
Thanks
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Old 12-23-2019, 11:18 AM   #11
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Quote:
Originally Posted by cadillackid View Post
if your engine max RPM is 1800 and the TCM is built for 2400 then yeah its nit going to operate correctly.. you'll never stay in 6th.. 1800 is main cruise RPM for say an 8.3 with 2400 RPM redline.. so anything above about 50% thriottle and it will want to drop down and then hang against your 1800 RPM governor..



you definitely want to get a shift program that better fits your RPM and torque profiile.. the throttle to shift-point maps in these TCMs are generally tailored to the engine its connecgted to..


building a custom rig means you either need t opay an allison dealer like stewart and stevenson (they will do custom work) to talk to allison and have a shift CAL made.. knowing the RPM / TQ / HP curves of your engine will help.. or if you have a 4th gen trans / TCM.. buy the necessary software and build your own shift Cal.



I have spent MANY hours dialing in my custom trans to my liking..



-Christopher
Ok this makes since, the trans came from behind a dd and I know the power curve and rpms are different. Looks like i will be getting some power added then off to the dealer for some computer work.. then see if gears are in the future.
Thanks
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Old 12-23-2019, 12:42 PM   #12
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You’ll want to find someone willing to work on custom stuff. I have no idea who to look for in California .. I know Stewart and Stevenson did some custom work for a forum member here. They are a national company.. many Allison dealers only want to do warranty or repair work. Might ask around in some of the crown and gillig enthusiast groups as repowers and drivetrain upgrades are pretty common on those rigs vs commercial trucks or school busses.
Christopher
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Old 01-01-2020, 08:08 PM   #13
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Ok got this all done !! The tps that my mechanic set up was wrong.. multimeter, welder and 40 trips from under the bus to start it and back under to check the voltages it needs to work correctly and bam.. shifts super smooth and was approximately 70 mph at 1800-1900 rpms before it shifted to 6th !! No gears needed !! I will post exact rpm when i calibrate the speedo.
Thanks for all the help.
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Old 01-07-2020, 03:23 PM   #14
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Glad you came out on the winning end of that one, you should be grinning from ear to ear, and it matches mine to a tee and I love mine.
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