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Discussion Starter #1
i have 4.10s right now and with my zf5 i top out at 120 km(75miles). if i swapped to 3.55s how much more top end would i be able to get? also how much would it affect towing? i dont tow alot once or twice a month.
 

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Discussion Starter #4

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Discussion Starter #5
may be 3.73s would be the best of both worlds
 

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OBS junkie
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I think a set of 3.55s would help your truck out. When I had 3.55s I could cruise at 75mph only running 2000 rpm. With the power of your 12v & your ZF5 you won't have a problem towing.
 

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Flossin'
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My brother has 3.73s, the cruise difference between our trucks is ridiculous. I do somewhere around 62-64 at 2k (E4OD)(my speedo is off), and he can do around 72-73 or more at 2K.(ZF5)

His truck also gets a couple MPG's better than mine, even with AB injectors.
 

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Discussion Starter #10
its hard to choose now whether i should get 3.55 or 3.73
 

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ive hit 94mph with my obs(that was using the speedometer on a GPS) with 4.10's but that was screamin over 3000RPM
 

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Powerstroked
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I think 3.73's are your best bet. I have 4.10's and its certainly not a highway truck.

The heaviest load I tow is a 23ft boat, so 3.73's wouldn't make much difference there. I have a GN hitch but have not had to tow any large loads with the truck yet.

I would prefer 3.73's, and I may actually swap whenever I swap in a D60
 

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Winners never quit!
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I would choose 3.73's over 3.55's any day.. Currently have 4.10's and as Freddie said and everyone else who has them knows theyr'e not highway approved!
 

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Going to larger tires will have the same effect as changing your gear ratio. I'm running 285's and at 70 mph I'm turning a tick over 2300 rpm with the 4.10 ratio. 75 mph equates to around 2500 rpm; 60 mph is 2000 rpm....
 

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OBS junkie
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If you want 3.73s you will need to do ring & pinion on the front & rear axle. If you wanted to go to 3.55 you can just swap axles & save the R&P labor.
 

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most old dodges came out with 3.55's also. Some had the 4.10's.

I would go 3.55's with that 12v unless you are trying to tow with 35inch tires or taller.
 

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Flossin'
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Did he regear his OBS to 3.73?
MAybe they are 3.55, I coulda swore we looked and it was stamped with 3.73, but thats been years and I'm probably wrong.
 

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OBS junkie
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MAybe they are 3.55, I coulda swore we looked and it was stamped with 3.73, but thats been years and I'm probably wrong.
They never made an OBS with 3.73 gears, only 3.55 or 4.10 gears. You will have to look at the rear axle tag & (assuming he has a F250 with a Dana 50 TTB) look at tag on the pass side I beam. Seen alot of people claim they had 3.73s but discovered they really had 3.55 gears. The speed & RPM your brother's truck is running makes me believe it has 3.55 gears.
 
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