I'm thinking of putting 3.07's in one of my trucks, it has a 14 cm wastegated housing, stock injectors, governor spring, pump turned up a little, BHAF, 5 speed, 235/85-16(stock size) tires, when I tow, its around 6-7K. Am I still going to be able to tow easily w/ 3.07's, or would they be better suited for an empty truck? I usually go around 75 on the highway, and I'd think they'd really benefit me there. Thanks

 

I like them!! I have the 3.07's in my truck w/5spd, I junked the slushbox and swapped in the G360. I tow with my truck enough be concerned about the gearing, but after several trips grossing 13K to 15K I decided it is a good match. Just a little slip of the clutch on the steep hills to start out. I have the fuel turned up a little-approx 1 turn on the full fuel screw, and backed the smoke screw out, BHAF, and Stan's. I run 285's. As for future mods to improve the marriage, I need a gov. spring, a little timing bump, and possibly a smaller turbo housing to really waken it up. Until then I'll just load the wagon!!!

 

My Ram 1500 with a 4.7 and 3.54 gears can tow 6-7k at 75mph (although not in overdrive), so I would think a Cummins would have no problem at all.

 

Thanks for the post, what do you get for mileage?

 

I love my 3.07's and 5sp set up. No problems with power and i also run a 35 in tire. As close as i can tell it get's about 23mpg.It is fun to drive on the inerstate 70mph in 5th with the 35's is still lugging the motor.So when i pull a trailer i just leave it in 4th and at 70 the rpm's are still very low. Dont have a tach just going by ear.

 

If you run on the freeways at 70-75 all the time the 307 gears are the greatest in my
opinion . I swapped my 410's across with 307's and I love em . Your stock 235/85/16 are
also the choice height for rpm I think . I had 285/75/16 that were a little tall for the foothills where I drive mostly lots of curves and not letting you wind out enough or bring the rpm down enough to find that perfect rpm . I now have 225/70/19.5 that are closer to 235/85's and just 2" in tire hight makes the
ratio change enough for the rpm"s I like . For looks the 245 / 19.5 would look much better but
225's are easier on my tranny I think and work better in the snow ((( 410 likes - towing
anything - dislikes driving 75 on the highway maxxed out and people still passing you .
307's likes . driving 75 on the highway and the rpm just perfect no problems and you
passing people .

 

www.tinytach.com That will let you know where you at.


I think the main thing is that with the Getrag that 4th gear is the best for towing.

 

I've tried 285/75-16 and 315/75-16 tires in the past on my first and second gens and with 3.54's you can feel the difference between them and stock tires, at best mileage seemed to stay the same w/ 3.54 trucks, w/ 4.10s it increased my economy. On my 91 right now I have 235/85's and like them, keeping the truck at stock height makes it easier to work on and to load things into it also. The thing that matters to me is economy, as it is I can get 20-23 with a first gen 4x4 manual 3.54, and have done in the past more often than not. 75 in a 3.54 truck is around 2200 if I recall correctly, if I can get it to around 1800 w/ 3.07's it might make a difference. If 3.07's will give me at least 2 mpg I'll probably pursue it, otherwise it doesn't seem to be worth the hassle of finding 2 diffs in decent shape. I was also thinking of putting 255/85-16's on but can only find them in D range w/ an all terrain tread, they'd likely do nothing to help economy.

 

If your at 20-23mpg with 3.54 and the 5spd I think you will have a hard time beating that with the 3.07's. I'm in that same 20-23mpg range with my truck. I cruise at around 70-72mph which puts me at 1700-1800rpms. If I go 75-80mph my mileage drops to 19-20mpg. One time I got behind a truck and drove 62mph for about 200 miles ended up with 23.8 mpg from what I remember. Best tank ever was 24.4mpg. I have towed about 4-5k on my car trailer and it did fine and I would tow more however I find myself in 4th gear alot which I'm sure is just as common for the 3.54's. I see the 3.54's slightly better for towing and the 3.07's better for cruising empty(no trailer) with a lower rpm but in the end both can tow the same stuff and end up with about the same mpg.

I'm not sure what tire size I plugged in but when I was looking at tires for my alcoas I was getting a corrected ratio of 2.93 or something like that but I'm not sure if its the 265's or 285's I was using. The 235's are the way to go for fuel economy. I just can't seem to get great mpg #'s with these 265's anymore.
Also on mpg I have a 110gal bed tank. When I took my trip down to florida I left with a full bed tank and full truck tank. I logged the mileage for the trip and watched it go from 19.72mpg on the first tank to 22.82mpg on the last tank as I burned the fuel off. It really showed how weight in the truck effects the mpg. I would be curious to see what would happen to the mpg in a truck with 3.54 on the same trip burning down the fuel.

 

I think its not so much the 265vs 235, but the BFG all terrain tread pattern you're using. On my 96 it seems I lost a mpg or two going from 33x12.5-16.5 tires to 285/75-16 all terrains, and the 33x12.5's were wider! As far as the mpg, I've gotten about the same w/ a few trucks, 18-21 going 75-80(once towing a boat!), and 22-23 all the time going to and from the city, which is anything from stop and go to 70 mph. I think no matter what RPM /gears, when you get up in the speed your wind drag becomes so much that if you've got 3.07's, you'll be pressing down more on the pedal to maintain that lower RPM than if you're a few hundred RPM higher w/ the 3.54's. What is your RPM at 75 in 5th BTW? -Nick

 

I'm going to guess about 1850rpm's at 75mph. 82mph puts me at 2000rpms.

70mph= 1700rpms
82mph= 2000rpms

I guess you could do some math to figure that out. Now this is trying to read a 2 inch tach at 2 feet plus away below the radio so plus or minus 100rpms.

On the way into work tonight I'll try and get some hard #'s for this post.

 

My dad lost 2 mpg by putting pro comp 33x12.50's on his 2005 dodge CTD. tread does really make a difference.

 

WOW I wish I had 3.07's those are some sweet numbers. My 4:10's put me at 2000 at 60 mph. I have to baby it with a bunch of highway to get 20mpg. Oh at it tops out at about 75mph

 

With my 518 and 3:07's I come in at 1650 RPM at 60 MPH. 1950 RPM at 80 MPH. Thats with 235's on it

The getrag has a little lower OD ratio than the 518 does. So the RPM will be just a little higher for the same speed.

 

Here are the RPM's VS MPH what i got with my truck last night on the way to work.

1400rpm = 57mph
1500rpm = 61mph
1600rpm = 65mph
1700rpm = 68mph
1800rpm = 72mph
1900rpm = 76mph
2000rpm = 80mph

so about 4mph per 1000rpm