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do i want 3.55's

3K views 23 replies 7 participants last post by  sixpack 
#1 ·
my truck revs high at 65mph, i have 4.10's would i notice much in pulling with 3.55's so i can get better pulling mpg on the highway.

i found a diff at a salvage for 500 bucks, didnt know if its worth it.
 
#2 ·
if you don't pull heavy loads with your truck you'll be fine. with highway gears the 3.55 you'll get way better mpgs, but you'll have a harder time towing stuff. maybe check and see if they have 3.73 gears thats pretty much in between the 2 gears you are looking at. also you have to have your speedo recalibrated if you do the
gear swap
 
#6 ·
. also you have to have your speedo recalibrated if you do the
gear swap
no you don't

3.55 will run hotter towing heavier, it is harder on the rest of the drivetrain also. we have a 14,000lb horse trailer and we are overthe limit for the 3.73's, towing with the 97 that has 4.10's is much better. if we get a bigger trailer were going to have to get 4.10's to go with it for the superduty
 
#4 ·
i had 4.10 in my dodge diesel, and never really thought it needed them.
also used my buddy 2500 dodge diesel with 3.55 and it pulled the hay trailer just fine.
 
#8 ·
hey man get different gears and im DAMN sure you have to recalibrate the speedometer. reason the tires now spin at a fast rate when the rpms are the same. its the same thing if you put bigger tires on your truck, your speedo is off by a little bit.

jake you right on the part that the truck with 4.10s pulls the load easier
 
#9 ·
Speedometer is calibrated to TIRE size. The VSS is on the OUTPUT of the diff, so it does NOT need to be calibrated unless you change TIRE size. It has NOTHING to do with engine RPM. Think about it; how could it be tied to engine RPM? Your truck still reports the correct speed when you're coasting in neutral, right?
 
#10 ·
I don't want to be an a$$ about it, but it has ALOT to do with the rpms of the motor. think about it, the motor has to spin 3.73 times for one revolution of the tire VERSE 4.10 times it has to do with the lower gears. and to put it into better perception if you put 5.88 gears in the motor now has to spin 5.88 times for every reolution of the tire. so with the lower gears or the higher number it will make the motor run at a higher rpm. and your right if you change the tire size it does change the speedo too. bigger tires make the speedo look as if you are going slower and gears make the speedo look as if your going faster. we do this all the time in rock jeeps.
 
#12 ·
You're not being an a$$, you just have your facts wrong. First, it's the driveshaft that turns 3.55, 4.10, etc. times for each rear axle revolution, not the engine. The ratio between the driveshaft and the engine is the gear ratio of the gear you're in. But even that's not relevant. Because the VSS doesn't give a rat's patoot about driveshaft revolutions. It's reading revolutions of the axle. You could, hypothetically, take all the gears out of the diff, and leave only the tone ring on the diff output, turn the key to RUN, push the truck down the street and it would register on the speedometer.

Maybe Jeeps read for the speedo based on driveshaft revolutions. The Fords don't.
 
#11 ·
When choosing gears for a truck I look at a few things. Like what is the truck 2wd/4x4, stock tire/lifted big tire, daily driver/tow rig & what speed does it cruise at. We went from 4.10 to 3.55 on a buddy's 97 dually & he reported no change on the highway MPG wise. But it cruises nicer unloaded & has more top end. In my 93 I had 4.10s & got 14mpg-16mpg with 17mpg being the best it saw with 4.10s. I swapped to 3.55 & I saw 17mpg-20mpg with stock size tires. However when I installed a 700lb flatbed & a turbo with a large exhaust housing that didn't like to lite till 2100rpms I found myself under powered driving 55mph. I was always under the turbo & caught myself driving 65mph to keep it in the power band & get up hills. My MPG went back down to around 15mpg because it was struggling all the time. Most of my driving was at 55-60mph so I swapped back to 4.10s & the truck was happier again.
For the axle in question, $500 sounds a little on the high side for a 10.25 but you are in a rust free state. If it's sound then it's just a bolt in & drive. Re-sell your current axle & recoup some of the cost.

also you have to have your speedo recalibrated if you do the gear swap
No you don't, I swapped axles from 4.10 to 3.55 & vice versa on many OBS's & 3.73s to 4.10s on SD so many times I've lost count. The speedo stays the same, the only thing that will throw it off is tire size. I checked mine with 2 different 4.10 rears & 1 3.55 rear & the same tire height & it was the same.

k thanks! just didnt know if id save a lot of money on fuel doing the switch for $500
ive only got a 5500lb travel trailer im working with
You may or you may not, but with how open it is out in your area I know for a fact I couldn't stand 4.10s & stock tires. I'd go with 4.10s if you want to run 35"-37" tires or your hooked to a 16k lb trailer all the time.

so with a 3.55 does it take a higher risk of burning up the rearend
No, you'll be fine unless your trying to pull the titanic. But then I believe you'll have other issues. LOL

no you don't

3.55 will run hotter towing heavier, it is harder on the rest of the drivetrain also. we have a 14,000lb horse trailer and we are overthe limit for the 3.73's, towing with the 97 that has 4.10's is much better. if we get a bigger trailer were going to have to get 4.10's to go with it for the superduty
:whs:

so just stick with my 4.10"s? instead of running at 75 everywhere ive learned to just go the speed limit, cause that fuel gauge just moves too fast if im flying.
Switch to 3.55s & cruise at 65-70mph to keep the rpms below 2k & you'll see better mpg. Also won't feel like your hunting for another gear on the highway.
 
#14 ·
yeah sorry my mistake didnt fully read the post about the fords being in the diffs. i got them mixed up thinkin that fords were in the t case too. and yeah cat they are in t case
sorry for all the confusion guys

also when changing gears it does make your rpms run different in high gear- right?
 
#15 ·
also when changing gears it does make your rpms run different in high gear- right?
Yes, mine with 4.10s & 33s run right around 62mph at 2000rpms. If I had 3.55s it would be closer to 68-70mph & 2k. (Sorry it's been awhile since I had 3.55s)

 
#16 ·
lots of great replies guys and a good go around from a couple of you.

so if i switched to the 3.55's id be hurting on the highway with my travel trailer cause the engine would be having a harder time?

and if the gears were lower i would be torquier?

dont know much about diesels, but arent they producing lots of torque in a lower rpm range?
 
#17 ·
so if i switched to the 3.55's id be hurting on the highway with my travel trailer cause the engine would be having a harder time?
Depends what speed you pull it at, if you cruise at 55-60 stick with 4.10s or 3.73s. If your rolling along at 65-75 you'll like the 3.55s.
 
#18 ·
yeah your truck is like my dads, he has 4.10s and goes nowhere fast. but he can pull anything around with ease.
ymy truck has 3.73 with 35" tires and i can go 75 at 2k rpm, but i suck at getting loads started moving

talk bout going slow, our jeep crawls at 0.80 mph with 5.88s
 
#19 ·
when i had my f150 with 3.55's it was easy pulling up to 50mph then id have to rev the engine at 4000rpm just to stay at 65 if i was lucky, the diesel is supposed to fix that so im not sure id have much trouble getting started just need more power on the top end.
 
#22 ·
what speed are you pulling your trailer at? my truck does not like to pull my camper at 55-60, too slow. So if your on the interstate you will like the 3.55's, if it's all 2 lane you may want to think about it.
 
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