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Discussion Starter · #1 ·
Got her in and she's running but will run only 55 mph wide open. My truck has 4.10 gears and I am using the original 727 out of a 90 model dodge D-250. Does anyone know what the gear ratio was in those trucks. I know it should run faster that this.
 

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1990 Cummins Dodges with the 727 transmissions came with 3.07 rears and normally 31.5 inch tires.
 

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Got her in and she's running but will run only 55 mph wide open. My truck has 4.10 gears and I am using the original 727 out of a 90 model dodge D-250. Does anyone know what the gear ratio was in those trucks. I know it should run faster that this.
Will help if you give more info !
 

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Discussion Starter · #6 ·
I figured it with a calculator and at 2700 rpms and 30 inch tires about 55 is wide open. I was wondering what the gear ratios were in 1990 trucks with this engine. I took this engine out of an aircraft tug with 7.17 gears and it ran 35 mph. I am using the 727 3 speed trans. I think I just need to try and find some taller gears. The trouble I am having now is I have a 9.5 semi floating axle GM differential and the highest gears I can find are 3.42:1 and that is not high enough. I need 3.07 or 2.73 gears in order to cruise at 1800 rpm where the torque peaks. It is alot cheaper to change gears than to change the transmission. Do any of you know where I can purchase these higher gear ratio ring and pinions for a GM 9.5 semi floating diff.?
 

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May I suggest you keep a close eye on local craigslist and free shopper ads? MANY guys want to dump their high geared rear ends for more bottom end grunt on their gasser trucks, and the 3.23 to 3.07 rears are not highly sought after, That normally translates into cheap. To me it's a lot easier and quicker, and probably cheaper, to swap the whole rear.

The other option is to swap in an A518 O'drive trans and chop the driveshaft. Sometimes these show up fairly cheap, I paid $400 for one with lo miles at a wreckers. Same-same 727 with an OD unit on the rear. The Cummins version that came in CTD Dodges is an A618, stronger, same trans. either one should hold up fine behind a fairly stock 4BT with a decent diesel torque converter.
 

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what application? I wouldn't run 4.10's with anything less than a 39" tire... and that's with overdrive! couldn't imagine 4.10's w/ no OD and a stock governor :(
 

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The 727 came with 3.07 rear end do to not having a OD. The A518 that came in 91.5-93 have a electric OD, kinda like a 700R4. It is nearly impossible to get it to go any faster. So your choices are 1) Find a 3.07 D71U rear end 2) Get much bigger tires or 3) Get a new tranny.

If you put the spring in you do 2 things. One is just being able to go about 65mph WOT and you bring it out of peak torque a long time before that. Your engine will hate you.
 
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