4.10 or 3.55 Gears?

[bike-maker]

Hello all.
I'm currently in the middle of tearing the 460/C6 combo out of my 84 F350 Crew Cab 4x4 for a 7.3IDI/ZF 5 speed. Among the mass of parts I have now acquired, I have the option of either 3.55 or 4.10 gears. I like the 3.55's for fuel mileage since the truck will become my daily driver - 60 miles a day of straight, flat Interstate driving at about 70mph. But I will also be towing a fifth wheel with the truck that weighs about 10k pounds, which makes me lean more towards the 4.10's.
If my math works out, the difference between 3.55 and 4.10 gears @ 70mph is 300 rpm with 33" tires; 1950rpm vs. 2250rpm. Can anyone give an accurate guess of how much this would hurt the fuel mileage? Better fuel mileage is my main reason for doing the swap in the first place.

Thanks, Mike

 

[typ4]

I had the same dilemma, go 355's if you have a turbo, I was dd'ing mine also and the mileage difference was quite different 300 rpms less. Under 2k is the best economy for most diesel pickups.

hell I just noticed your only a little ways away.

Also do you have the diesel frame mounts, that makes life easy for this swap.

 

[Brianedwardss]

I'd lean towards 3.55's if a majority of your driving is on I-5 to Salem. I gained 1.5 mpg when I went to 3.55's, but I've got a C6 so it's a different story. I had an IDI zf5 with 4.10's I used to commute with, and I always longed for one more gear/3.55's...

 

[Ottoparts]

I have an E4OD, 4.10, and 35' tires. I am sitting at 2200 rpms at 65ish.

 

[DragRag]

I would go 3.55's if driving daily.

 

[funnyman06]

If you dont have a turbo i would go 4:10s. Your truck weights in at about 8K lbs, with a 10K trailer your looking at almost 20K loaded. 3.55 will feel like a slug with out a turbo. When i moved to Missouri, i pulled a 3k trailer putting me at around 10K loaded, with 3.54 and a C6 i felt really sluggish. But even so at 55 MPH i was getting about 20MPG.

 

[Diesel JD]

A turbo with 3.55 is about as peppy and maybe a little moreso than a NA with a set of 4.10s. I think as the others have indicated you will need to tell us if you'll be turbo or NA for the best answer.

 

[bike-maker]

So here's my take:
3.55's = a good excuse to go looking for a used turbo. 4.10's = a good excuse to go buy bigger tires. It's got nice 33's on it now. And the 3.55's are already in the truck. 3.55's are sounding pretty good. Increased fuel mileage is my main reason for ditching the gasser. You're not getting anywhere quick with a 32' 5er anyway.
I'm surprised at all the Oregonians lurking around in this forum; good to hear from ya'll. My daily commute is from Albany to Coburg. 32 miles of Interstate. No hills. No turns. BOOOOOORING
I added my info to my signature (assuming I did it correctly) so you guys can see what I'm working with.
Thanks for your input

 

[Ottoparts]

I am N/A if that helps you out at all.

 

[Brianedwardss]

I'm surprised at all the Oregonians lurking around in this forum; good to hear from ya'll. My daily commute is from Albany to Coburg. 32 miles of Interstate. No hills. No turns. BOOOOOORING

Oh, when you said straight and no hills, you meant it!!! Its that very stretch to Eugene that convinced me to obtain a 3.55 rear.....

 

[Hybrid455]

Use the 3.55's. If you don't have a turbo and get in trouble you always have 4wheel low. Just unlock your hubs and gitter dun.

 

[Logroller]

My 93 f-350 turbo came factory with 4:10 and i love having the extra power ,, but the fuel is to be desired ,, so like the others said if your not pulling loads alot 3:55 or convert to 3:73 and be in the middle ,,, if your doing light towing ,,,, figure this heavy loads lower gears lighter loads higher gears and then put into consideration your drive time and fuel your gonna use ... Just a thought