better mpg with 3.55 or 3.92? [Archive] - Dodge Durango Forum

: better mpg with 3.55 or 3.92?


suprchgd99rango
06-22-2009, 09:42 PM
i have 3.55 in my durango and i was gunna steal my dads rear out of his dakota with 3.92. but then id have to get gears for the front diff since i have 4 wheel drive....he said id get better gas mileage with 3.92? i think hes a moron but maybe im wrong?

BAD
06-22-2009, 09:47 PM
depends on where you drive city/highway

suprchgd99rango
06-22-2009, 09:51 PM
well, idk what you would consider it but its a long main road thats only a 2 lane 1 lanes each way. and speed limit is 50, i usually do 60. and i occasionally go on highway, like once a weeka nd set cruise to 75. and i do alot of 10 mile rns to the stores. at like 45-60 mph.

suprchgd99rango
06-22-2009, 09:51 PM
and i do alot of back roads. and i speed on them. lol

02sxtkota
06-22-2009, 09:54 PM
ive always wanted to but dont have time to do both at same time. I would think for normal cruising u be alright but highway u would be turning more rpm's so i would think u wud use more gas. idk, thats my theory but could be wrong.

bwdakrt
06-22-2009, 09:55 PM
Your mpg will drop if you go to the lower geared 3.92 rear end.

suprchgd99rango
06-22-2009, 09:58 PM
thought so! thanks!

01Motorsport
06-23-2009, 12:09 AM
and i do alot of back roads. and i speed on them. lolMy kind of driver :funny:
Your city mileage will improve if you drive easy. However, no matter what, highway will drop over 65mph. A cheaper alternative is to slightly reduce the tire diameter to lower (raise numerically) the gear ratio. Superchips allows for 1/4" diameter changes.

Axiom
06-23-2009, 01:25 AM
City mileage will improve as it will be easier for you to get up to speed. Highway mileage will suffer slightly because you'll be turning a higher RPM. Don't let people scare you. The drop in MPG on the highway is relatively small and well worth the extra giddyup!

suprchgd99rango
06-23-2009, 02:16 AM
idk if i will do it tho. i would need a front 'rear' or new front gears because of the 4x4. i do not want to tear up either rear. and i figured mpg would go down all over. not just highway.

RebelX
06-23-2009, 03:05 AM
i run 3.92's. i average 18 on the highway and 13-14 in the city.

suprchgd99rango
06-23-2009, 03:13 AM
i got 3.55 and i average 15-16 highway and 13-14 city. lol

Primus
06-23-2009, 03:50 AM
The difference between the 3.55 and 3.92 is little in the drivevibility area and the fuel economy difference is small. It is probably LESS noticeable since you have engine mods as well.
The only reason you would need to consider going to 3.92 is for higher payload and towing to the max on your Durango. Then it would be worth the effort but if not, don't bother as the 3.55 equipped Durango's still can pull a decent load fine.

suprchgd99rango
06-23-2009, 03:54 AM
its mainly just for a posi and some different gearing.

wuba
06-23-2009, 04:59 AM
its mainly just for a posi and some different gearing.

tread jack.....are those American Racing Longboard rims on your truck? Where did you get them at, I was looking around 4 years ago and couldn't get them. Ok back to scheduled programing.

suprchgd99rango
06-23-2009, 05:11 AM
idk what rims they are. they are AR but idk what ones. they came on my truck and are now winter rims. if u werent so far away id say ill swap u those for wat u have now since u have 17s. i need atleast 17s for my rear disc setup.

SLT kota
06-23-2009, 05:45 AM
my milage dropped when I went from 3.55 to 4.10's.

Also if you want to put a dakota rear end into a durango you are going to have to do some welding. It isn't a direct swap.

EntreriDakota
06-23-2009, 06:36 PM
mathmatically the 3.55's will give you better gas milage.

if your milage drops when going to taller gears then its either the instalation of the gears or your lead foot trying to compensate for the lack of acceleration with the taller gears.

suprchgd99rango
06-23-2009, 07:13 PM
my milage dropped when I went from 3.55 to 4.10's.

Also if you want to put a dakota rear end into a durango you are going to have to do some welding. It isn't a direct swap.



no. its a r/t going into a 4x4 dur so they are both leaf under. .



and a 3.92 would have better acceleration then a 3.55

02sxtkota
06-23-2009, 08:18 PM
and a 3.92 would have better acceleration then a 3.55

yes and 4.10 better then 3.92 and so on, if its anything like with changing the sprockets on a bike u prob loose top end speed as well.

Hahns5.2
06-23-2009, 08:30 PM
It depends. I have a 5 speed with 3.55s and I cruise 75 at 2200 RPMs and get 21 MPG. Don't the autos have a deeper OD ratio? If so I would imagine the mileage would remain nearly the same since you're moving the engine into a more efficient RPM range.

024x4Sport
06-25-2009, 03:27 AM
I think it also depends on what type of terrain you're driving in. On flat ground, the 3.55s certainly are better for mileage at higher speeds (lower engine rpm), but I've found the opposite to be true during my occasional running around the Virginia mountainside. My truck pulls hills with the 3.92s that it otherwise would not with 3.55s, and constantly shifting up and down (plus constantly giving it more throttle trying to pull those hills) is brutal on gas mileage.

I would also imagine that you'd see better gas mileage while towing with 3.92s as well, but I haven't seen that one for myself to know for sure.

As far as the limited slip goes, I have to say I'm not very impressed with the factory Anti-Slip. It may be just mine, but it seems to have a hard time living up to it's namesake. If you need serious traction (upgraded engine, off-road rig, etc.), then you'll likely end up having to replace it with a better unit down the road. Since you can replace gear ratios at the same time you replace the differential (assuming 2wd), then it might be your most cost-effective route in the long run to just upgrade the differential you already have.

suprchgd99rango
06-25-2009, 05:33 AM
i have alot of hills too and i downshift alot. it also would be nice to punch it and go and not punch it and go slow. lol

Rob454
06-25-2009, 11:50 PM
I had 3.55s and I had no problem with them. Ive driven the same truck with both 3.55s and 3.92 and there was basically no difference. The difference is so minimal between 3.55 and 3.92 its almost not even worth it. You wont gain as much as you think. the only reason to swap to lower gears is if you tow more OR add larger taller tires otherwise save your money.

suprchgd99rango
06-25-2009, 11:56 PM
its free actually. besides time and labor. and the main reason would be to have a posi.

Hahns5.2
06-26-2009, 06:16 AM
I think it also depends on what type of terrain you're driving in. On flat ground, the 3.55s certainly are better for mileage at higher speeds (lower engine rpm)
Not always. Lower RPMs, even on flat ground, isn't always better, there's a reason we don't cruise around at idle, there's no torque there. My explorer I just sold had 3.27s, I could get nearly 2 MPG better if I stayed out of 5th under 60-65 MPH, so instead of 1500-1800RPMs, I'd run around at 1800-2200 RPMs and get better mileage and there aren't many hills around here. It's best mileage was still worse than my Dakota's best mileage because it was too damn tall geared, 3.73s would have done wonders for it. Granted it was only a 4.0, it still affects all engines in some way. I do agree deeper gears are definitaly better for hills and towing.

I'm happy with my 3.55s with 30"s and a 5 speed. 31s are too big for them though, even with a V8. Since I've got a low 4:1 1st gear my acceleration would suffer with lower gears and my highway mileage would also suffer since my OD is not as deep as the slushboxes. With an auto I think I'd want 3.92s.