4.10's to 3.55's - Should I do it?

[bikepilot]

I'm considering swaping my 4.10's out for 3.55's or 3.73's to get better highway mileage. Of course a GV would probably be better, but I'm on a tight budget and can do the gears pretty cheeply. I'll probably hang on to my 4.10's just incase a GV comes along later on.

My truck is a 94' F-250 IDI factory turbo, 4.10 gears, E4OD and 100% stock. I primarily use it to pull a <3000 lbs enclosed motorcycle trailer.

I have carefully measured my mileage empty and have found that I consistently get 20mpg @ 55mph, 16mpg @ 65mph and 13mpg @ 75mph. I have thus restricted most of my driving to a max of 60mph, but that gets very old on a long trip. I know there is increased wind resistance at higher speeds, but I suspect that much of the rapid dropp-off in mileage has to do with engine rpm and thus mileage at higher speeds might be greatly improved with taller gears.

I have not done any real scientific measurements with with trailer but am getting around 17mpg between 55 and 60mph pulling the trailer. If I could get these numbers (i.e 20 empty/17 towing) at 65-70mph rather than 55mph I'd be really happy

My concern is that the taller gears may kill it off the line. I don't take off fast, but I don't want it to be a total slug either. Still, as it is now first gear seems pretty short and perhaps it wouldn't be bad with the taller gears.

So any comments, recomendations or whatever would be very much appreciated

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BTW I'm the same bikepilot as on TDS and FTE and like many others seem to be migrating away from TDS....

 

[sle2115]

While not comparing apples, I have a 6.9 NA, I switched the other direction. I went to 4.10's and would not go back, but then again, I pull much more than 3000 lbs. most of the time. Even empty though, I like how my truck performs with the 4.10's, don't care for the RPM's at highway speeds and I don't have an overdrive as you already do, so mine really wines at 60 - 65.

 

[bikepilot]

Thanks for the re. I just talked with a co-worker who has a 92' N/A IDI with 3.55's. He seems to think it takes off ok, but he hasn't driven a similar truck with 4.10's either.

Frankly, I'd be willing to trade quite a bit of acceleration for a more efficent/faster highway cruise as longish trips are about all I use the truck for.

 

[912504x4]

How about running some taller tires?

I have 3.55 gears and wish I had your 4.10's.

I am running taller tires though so my 3.55 gears really struggle to pull the weight of my truck and trailer in 5th gear but if I shift down to 4th it is reving too much for me to get good mileage out of it.
I have a spare rear axle with 4.10 gears so now I just need to track down the front gears or a solid axle with 4.10's

I bet if you stepped up to some 34's or 35's you wouldnt need to swap gears.

Just another option for you.

Eric

 

[bikepilot]

Larger tires are a posibility, but I was concerned that the increased rolling resistance might offset the gain in efficency from reduced crusing rpm. Perhaps some tall skinny tires, but they are hard to find. I was sorta looking for 255/85/18's as they would work with my rims and should be about 33.5" tall, but only 10.5" wide. Availability seems to be very limited though.

Oh, what kind of a load does it take to make 3.55's feel too tall? I assume they are fine empty?


Perhaps 3.73's would be better?

 

[912504x4]

weight

ooopps I forgot to post that part. When I have the Jeep on my homemade piece o'crap trailler and I am headed to LBL Turkeybay offroad park with the dirtbikeson my flatbed I am pushing 15000 total combined gvw.
The truck seems to lug along and smokes a little bit because of it. If I run faster than 70 it clears up but with my junk trailer its hard to do more than 65mph (1400rpm) and stay in one lane.
Last month coming back from LBL I ran with the OD off and my truck liked it much better. No smoke no lug at 65 mph but the rpms were at 2400 which is too high to get any mileage out of.
Empty it runs like a dream at 70 mph.
I usually get 15-16 mpg empty at 70 and only 10-12 at 65 pulling all my junk.
These figures are from the E40d not the 5 speed I just installed.
Hopefully the stick will get me a little better mileage when towing.

I wouldnt suggest the 36's but 33or 34inch tires would benefit you a bunch as far as highway rpms go.

Eric

 

[bikepilot]

Great info, thanks!


It does sound like you are in serious need of lower gears with your 36" tires.

With dirt bikes only my total trailer weight is about 1/5th of yours so I could probably get by with the taller gearing (and smaller tires)

 

[shorthair]

i have 3.55s & a zf5 & tow a 31' 8,000# TT quite a bit no turbo just a bit slow but i always get where i'm going. seems i run empty way more than loaded so if i'm loaded & need to drop a gear or two i just do it early & get on down the road. remember that the difference between 4.10s & 3.55s is about the same as OD & direct.

 

[soulpatchfr]

keep her under 2000 rpm for the best mileage - i get about 20mpg if i can keep her under 70 (about 2050rpm @ 70mph) with 33's on my 2x N/A w/ 3.55s.
personally, i love them.

i'd think you shouldn't even notice 3k lbs back there.

BTW, welcome aboard

 

[DeeseSR]

I run 4.10's with either p285 75r16's when I am towing or 36 12.50 r16.5 when I am just tooling around. I'm running NA and at best get around 15 mpg. But I don't ever get over about 65. There really isn't that much difference with the different tires under her as far as gas goes. Off the line is a big difference though. Hope that helps.

 

[bikepilot]

Great info and thanks for the welcom

I don't really notice the trailer back there very much, its just a tad slower accelerating. The empty weight of the trailer is 1900lbs IIRC, I usually put in a few dirt bikes and some gear. Seriously doubt it would total a 1000lbs worth of load in the trailer. So far I've only had one hill where it downshifted with the cruise set between 55 and 60mph (only way to get decent mileage as is). The trailer is kinda tall and does push some wind though.

I wouldn't mind it at all if I had to go out of OD from time to time to get up hills. I think I'm going to try 3.55's. If its too slow on take off, it'll give me a good reason to go for the ATS turbo upgrade...

I've got a line on a complete TTB D-50 front end (i.e. same as on my truck) with 3.55's that I can get for free. If it turns out to be in decent shape then all I need is to find a ring and pinion set for the 10.25 out back and I'll be

Any pointers on changing out ring and pinion gears would be great too:Sly

 

[MIDNIGHT RIDER]

If I had 4.10's, I would leave them be.

My truck is 3.55 and many times I wish it was 4.10.

I don't think you would gain a drop extra of fuel and could hurt yourself instead.

 

[bikepilot]

wanna trade:Sly

seriously, soulpatch is getting 20mpg near 70mph. I can only get that if I stay around 55mph. The extra 15mph would make a big difference on my drive from NoVA to south FL in a couple weeks (70mph instead of 55mph would cut 12 hours of driving time round trip). Of course this could be down to difference in our trucks, but it sure seems to be rpm related to me:O

 

[The Warden]

seriously, soulpatch is getting 20mpg near 70mph. I can only get that if I stay around 55mph.

FWIW, I'm seeing the same thing, with 4.10's and a ZF5. If I can keep to 55mph, I can break the 20mpg mark...but in "normal" driving, it looks like I'm getting about 17 (can't say that number's set in stone, though; I've only refueled 5 times since the transmission swap). That's on CA fuel and an engine that needs to have the timing set properly

I thought very seriously about switching to 3.54's (in fact, I almost went and did it about a year and a half ago) but in the end decided to do the trans swap and eventualy get an OD unit. The OD would basically make it so that my truck would drive as though it had 3.28 rear end gears in high and still have 4.10 gears in low...with the 0.77 gear ratio in 5th and stock tires, I'd be turning 1900 RPM at 70mph, and could still pull anything I need to Only thing is $$ *sigh*...

{on edit} Where are my manners? Welcome aboard!!

 

[bikepilot]

Well, I just picked up a D-50 front end with 3.55's in it for free from a local fellow who swapped out for a D-60. Now all I need is the 10.25 Ring and pinion and I'll be all set