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Discussion Starter #1
i have an 01 cummins 5 speed going 65 on the highway im run about 2000 rpm so im pretty sure i have 4 10 rear end i thought about puttin a set of 3 73 in it but i dont want to lose the low gearing ive heard something about some kind of over drive kit you can get and one no any thing about that ??????
 

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gear vendors has a kit and they used it on project x in diesel power mag, and it seems to be taking their punishment, probably gonna need to look into it
 

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actually 4.10's are the stronges, it balances stress on individual teeth with size and strength of the teeth, in the 3.54's the teeth are small and weaker and can crack individually, but any higher than a 4.10 and you have so few teeth that the stress in each tooth is great, so 4.10 is good, just not for mileage
 

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i supose u have 5:54 gears, although my speedometer says i am going 75 at 2000 rpm but that is the tire size throughing it off.
 

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7.3stroker said:
i have an 01 cummins 5 speed going 65 on the highway im run about 2000 rpm so im pretty sure i have 4 10 rear end i thought about puttin a set of 3 73 in it but i dont want to lose the low gearing ive heard something about some kind of over drive kit you can get and one no any thing about that ??????
I think you got 3.55:1 ratio... I'm running stock tire size 265/75 R16 and I'm twisting 1,900 RPM's at 65 MPH...

At 55 MPH I'm about 1,600 RPM's...

At 75 MPH I'm about 2,200 PRM's...

All these are roughly...
 

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Discussion Starter #8
i have stock tires to might put 33s on it i jus need somein to get the rpm down cause im goin to be driving over 500 miles to school when i go
 

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Discussion Starter #9
i dont really need all the stuff that the gear vendors does i jus need to drop the rpm in 5th gear would it be better to jus throw a higher set of gears in it
 

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7.3stroker said:
i have stock tires to might put 33s on it i jus need somein to get the rpm down cause im goin to be driving over 500 miles to school when i go
You want to have the RPM's about 1,800 - 2,100 range at your cruise speed... If you drop below that you lose MPG...

Larger tires would do it the quickest but still your going to lose MPG over it... Most people with larger tires lose MPG quickly to rolling resistance...

Cummins was designed to run between 1,600 (55 MPH) to 2,100 RPM (75 MPH) normally with 3.55 gears...
 

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If he's hauling he'll want his RPM's there. Not if he's running empty like alot of us. Cummins designed the engine to run at 1600RPM to 2000RPM for MPG while hauling. If he wants MPG unloaded he can drop his RPMS if his speed is low enough. But putting larger tires isn't the answer. In 6th gear my mileage doesn't drop going from 1800RPM to 1400RPM's, it almost doubles.
 

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Discussion Starter #12
mine is a 5 speed i dont want to do bigger tires i guess swaping out the gears is the best way to go
 

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just some info for you.i have a 6speed and 4:10. at 2000 i'm at 60 with stock bald tires, both needles are exatly at 12:00 position. i put 285/75/16 on it i did the calculations and with that tire i am now doing about 65 at 2000. i do wish my mpgs were better, but i do alot of hauling and towing so i like the low gears.
 

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I wouldn't change your ratio. I have 285s and got around 18mpg on a trip i took, and i was usualy running around 80mph with alot of hills, and got back home had 1/4 tank left and used the rest around the farm. So if i was going down the interstate 500 miles and kept it going 70 i would probably average around 19 or 20.
 

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with my 01 3.54's and an auto on the highway rolling 80-90 the whole trip and my 33x12.50 dunlop mudders I go 19.5. I like my 3.54's and now I am going to see how my 35's help. each time I increased tire size I have gained mpgs.. you have to calculate in your loss for bigger tires if you don't recalibrate the speedo.. i would say go tires they are cheaper than gear swaps but thats just me. also add 2stroke oil and it will help too. even hauling with my 33's rolling 65-75 with a 30' gooseneck horsetrailer I was getting 15.25. running the country roads out here with my 33's I get 16.5 on average and have some good climbs. but everyone sees different mileages in different areas. if your doing a lot of flat highway driving buy a taller but kinda narrow tire it will help too
 
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