Changing my 3:54 to 3:73 or 4:10

[dream chaser]

I have a 1992 F350 4x4 Crewcab Duallie 8ft bed with a cap on the bed that is flush with the cab but raises up about a foot by the back of the bed. I described all of that for those who factor in wind resistance, to my question. The engine is a 93 5. 9 intercooled, NV4500 trans and 3:54 gears. Not exaclty sure of the turbo but it has a GB Gail Banks part number and is a 16 cm housing. 4" mandrel bent exhaust, magnaflow muffler and 18" resonator, custom downpipe with no sharper bends than a mild 45 degree. I've rotated the fuel diaphram to the richest setting and cranked the starwheel and aneroid adjustment on the pump. Custom K&N air filter also. I've put about 8000 miles on this rig since I finished building it. I can constantly get 19-20. 5 mpg on the highway cruising between 62-68. I have noticed on a couple of tests, I get a little better mileage going faster with the rpm's up around 1900. At 62 the rpm's are 1650. When I tow a camper the mileage drops to an average of 13 mpg and the combined weight of truck and camper is only 12,500#. I've noticed when pulling my loaded horse trailer which tops me out at a meager 14,500# combined weight on 2 lane roads I get around 15mpg. From my research and reading the forums, I think that the size of the truck and the wind resistance is lugging the truck too much with the 3:54 gears. Unlike most people that post, I'd be happy cruising the highway at 62-65 while towing the horses or camper. THe truck is a daily driver that I drive 14 miles one way to work each day on 2 lane country roads never going over 55, and most of the time in the 50 mph range on the back roads. Driving back and forth to work I get a steady 17-18 mpg When on the back roads, it is often hard to find a happy medium between 4th and 5th gear. Is switching gears to 3:73 worth the trouble, or should I go right to 4:10's. Or am I crazy and should be happy with the mileage that I am getting. I know that changing the gears won't improve my empty mileage and may even hurt a little, but would like to maybe help my towing mileage a bit and also increase the pulling power too. These are all hand calculated mileage ratings and verified over many tanks of fuel. I have access to a fairly low mileage (118,000) 4:10 rear end from a reliable source for $200. 00. Is this worth trying? I know I would still have to swap the front gears before the snow flies if I like the change.

 

["Mad Max"]

Your mileage is right on the mark, and I personally don't think it'd increase much if at all by a gear swap. I also think you'd be disapointed by the rpm increase overall. I love my 3. 54's and I have a 35" tire, and I'm sometimes wishing I had even taller gears. I'd totally leave the '54s in there .

- M2

 

[therabbittree]

I almost always tow in direct drive any how. . i have 3. 07s and direct is what i tow in with the 5spd getrag. . I too have found that if I keep the rpm at 1800 or so i get better mpg. 16 towing etc... if i keep rpm around 12 i dont get much over 14mpg towing for me . . what do you gte rpm in 4th gear in your mph range 60-65? if you stay in 4th the trans temps are lower and less stress on your trans, more efficient etc. . plus no 5th gear issues. .

Thanks

Deo

 

[dream chaser]

My rpm's in fourth are around 1900 at 50 mph, I would be screamin at 60-65

 

[therabbittree]

ouch... mumn what size tires do you run?

with the 3. 07s and 285 tires im at 5th gear 1500 rpm at 65mph, 75mph is 1800, 4th gear is 1500rpm 55 mph, 65 is 1800 rpm, 75mph is 1950rpm.

heck for fun tried low range 5th gear 2500rpm is 55mph,65 mph is 2950rpm.



For you the cost of doing a gear change is big... tires are cheaper... maybe go down a size to get the few extra rpm you want. . ?

 

[dream chaser]

6 brand new firestones a couple of months ago, so new tires aren't an option. they are 235/85/16's. For the cost of the rearend I can get ($200. 00} I may give it a shot and see if I like it or not. The other benefit of changing the rearend is this: the rearend in the truck is from a Dodge 3500, and although the Ford rims fit the bolt pattern, they don't center on the hubs, so I had to get creative and center the Ford wheels using metal roll pins. It works, but rotating tires is a problem. It would be nice to have the correct diff housing too so the wheels would fit properly. I could probably sell my Dodge Dana 80 to cover the cost of the gears for the front end. I'd like to be at 1900 at 62-65 like you are, that sounds ideal. Like I said in my original post, if I don't go over 65mph thats fine with me. Now I need to go about 72 mph to get in the right rpm range. It's crazy, going faster to get the rpm's up gets me better mileage. I tow several thousand miles a year too, thats why I was considering the 4:10's. I don't think 3:73's would be much of a difference.

 

[therabbittree]

ah i thought you had a 4x4 so you needed 2 axles. . but if you have 2wd then the cheaper axle makes sense there... try it see if it works for you. what year dodge dana 80 do you have? i may be interested if you do want to part with it as i have aset of 355 axles here too...

Thanks

Deo

 

[dream chaser]

I do have a 4x4 so I would have to change out the front. I think I can just change ring and pinion in it though. To go from 3:55 to 4:10's in the rear I think I have to change the carrier too. The dodge rearend is from a 97 2wd 3500 according to the tag and numbers on it. It also has new drums and brakes too, thats also why I'm hesitant to switch, it's a nice unit. I still have to research and see if I could go to 3:73's with the 3:55 carrier, but then I still have the wheels issue.

 

[R C Cola]

What Next ?

Lots of info on this subject in these forums if you search. Couple of ideas for you, might help. I have 4. 10s in mine and tought often of changing since I tow commercial all the time. With stock 33 inch tire diameter I am in what I feel is the perfect economy range between 55 to 62 mph approx 14-16 mpg when loaded average. I get 20-24 when funning empty, and I do not feel that I can complain with either number. 2 Brands of final drive units available which change your final drive ratio by 20% I plan to use the overdrive unit, but you would need the underdrive unit which will take you to approx. 4. 10 which at the speeds I run should give you better miliage and rpm's while being better for the engine. Added benefit I feel is of equal value when the split is available the engine does not have to rev as high to make a shift and be in the power range of rpm's which I beleive helps miliage and longevity of engine. Cost will be just under $3000

 

[dream chaser]

Yeah I've read quite a few of the posts. Your response is appreciated and seems to back up what I've been finding, that these engines seem to be better off running in the 1850-2000 rpm range. I found a gear ratio and speed calculator while searching the posts: http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls

this link was great, just plug in your info and it will give you the apprx. rpms for speed. My tires are only 31. 75 tall compared to your 33's, so I'd be taching a little higher than you. What do you tow? I know different trailers make a differnce depending on there profile. I'd love to get the extra few mpg's that you are getting compared to my averages. A friend of mine gets the mpg's that you get. I figured his was higher than mine because he is a 2wd no bed, set up for long goosenecks. He is a standard cab and lower to the ground than mine too. He's got 4:10's.

 

[Forrest Nearing]

I say drive faster

 

[MMiller]

I've driven my 93 with 3. 54's, and a slightly tweaked 91 with 4;10's, both getrag trucks. Mine now has 265's and the 91 has 235's. IMHO the 410 truck pulling in OD is almost the same as running my truck in 4th gear. I really think you should put some miles on a truck with 410s before changing. *I* think they are very short geared, and run a bunch of rpms. The milage you are getting is spot on, and probably will not change much with gear changes. I tow upwards of 28-29000# and would rather have my truck with 3. 55's and 265s, then the 91 with 410s. Now I will say the truck with 410s has the advantage of getting trailers started out in the pasture, or in soft fields. Much less abuse to the clutch, but I don't think you are doing that.



I say try towing in 4th gear around 55 mph. The truck won't complain, it will be just fine. With 410s you will find your self in OD by 45 mph anyway, and think the cummins is wound out at 70 mph.



Michael

 

[pepecat]

I prefer to tow in direct with 3. 07s... or 3:54s... never 4:10s...

 

[dpuckett]

Seems like my 92 turned right at 2200rpm at 60 in 4th. Not screaming, but definitely out of the best MPG range. Of course, I had 3800rpm of fuel availbe... ... . Seems like I did better at 65 in 5th with it til I got into the hills, then I'd drop to 4th and let her wind up going down hills.



How many RPMs do you let it turn before shifting? I let mine run to 2500+ (sometimes 3K when accelerating onto interstate) when towing heavy. If you do a lot of stop and go towing, shifting too soon can hurt your MPGs as well. RPMs are your friend, let it rev up.



Daniel