Big Red 97'

My 97' 4x4 has a 4.10 rear end and I only get about 13 mpg. It will pull a freight train but the mileage is killing me at $3.45 a gallon. I was told that if I switched the rear end gearing to 3.55 that i could probably gain 5-6 mpg. I also thought of a 6 spd tranny. Does anyone know how much the rear end gear swap would cost me, or have any other ideas on what i could do.

C2O

if you're just after a whole axle swap you can find one for 6-800 which would be the quickest way unless you want to change ring and pinion. then you would have to change carriers also.
when i had 215inj. and the hx 35 i was getting 25 and even 27 at times with 4.10's yeah i got alot of slack on here a long time ago but thats what i got now im lucky to get 20 i average about 16-17 now

RowJ

I don't believe a 3.55 rear end will gain you 5-6 mpg. Maybe 2-3. You have an Automatic?
In either case you need to burn a lot of fuel to pay for a gear change out.

I believe 285's on your wheels would do almost as much for you. Haven't done the math but the net result of 285/75/17's should be close to maybe a 3.70 rear end??

RJ

Dr. Evil

Dont forget - you have a 4X4, so youve got to do 2 $waps. I too dont think you will see the milage increase you are looking for.

If you have an auto, maybe your torque converter isnt locking up?


I get crappy mileage with my 12V too and cant seem to find the problem.

mcoleman

If your only getting 13 on the highway with a 2500 I think you have some tuning issues. I would expect at least 17 mpg on the highway if your rpms are kept under 2000. How much tire are you running with that lift?

Dartmouth 12V

How is the truck used? If it is primarily running unloaded on the highway, making a swap might be worth it. However, 3.55's won't help you if you are doing a lot of stop and go and they won't help as much if you are towing a trailer. By changing gearing, you will be only really helping your highway mileage(which is important if this is all you do) at the expense of not pulling a load quite as well(if you do a lot of towing).

Changing tires is not a very good option. Your around town mileage will actually go down since you will have significantly increased your rotational inertia. Your highway mileage will go up if you keep to a tire that is reasonably narrow(both for smaller contact patch and less windage) and doesn't have too aggressive of a tread. However, bigger tires cost more and probably won't make up for the decrease in money spent on fuel unless you only run on the highway.

What speed are you running? The cheapest way of all to improve your mileage is to slow down. In going from 70 to 65mph, I pick up about 2mpg.

Remember, gearing up and going to bigger tires only helps when running at high speed and can hurt you in town.

winkle

Just reading your signature may indicate why your only getting 13mpg.

"black smoke belchin' head turner!!!!!
IF YOU'RE NOT JUST A BLURR IN THE DISTANCE YOU AREN'T GOIN FAST ENOUGH!!!"

Dhorn33

I have the exact same truck as you do (97 xcab 4x4 long box auto) that I just swapped to 3.54's last weekend from the 4.10's to try to get better mileage. Here is a breakdown of mine:

Used 3.54 gearset for front Dana 60 - $60
Used 3.54 gearset for rear Dana 70u - $80
Complete install kits for both axles - $325
Gear lube and additive - $30
Labor - $250
Front brake pads - $40
Speedo gear - $15
Total cost = $800

This was super cheap - if you went to a shop and had them intall brand new gearsets you are looking at a total cost of $1500+ for both axles based on my research.

YOU DO NOT NEED TO SWAP CARRIERS if you are going from 4.10 to 3.54 with the Dana 60 and 70u rearend! I have read in several posts on here that you do - but this is not correct. Check the websites for any gear vendor and you will see that I am correct - plus I did the swap and am using the same carriers. Both of these axles use the same carrier for 4.10 and numericaly lower gears.

I haven't towed yet and it has been snowing here all week so I cannot say if it is going to gain me any mpg but I am hopeful. The rpm at 70 is lower by 300 rpm so I am now at 1950 at 70 instead of 2250. I think the truck seems to have just as much power and it drives great.

ponygallery

I just ran from Post Falls, ID back to my home in WI and got 20 mpg going 70 all the way with 3.55s a 5sp tranny and the new Quadzilla economy chip. Your milage sucks.

torquefan

It can be done relatively cheaply if one is performing the labor himself, but to have a shop do it, one could easily spend 5 - 7 or more hours on EACH differential, depending on how difficult the setup proves to be, and how much time the technician wants to put in to getting it right. Don't know what labor rates are there, but they exceed $100/hour here. It adds up fast.

Don't forget the speedo will need to be calibrated too.

Tate

If you have a 5 spd, the 6 spd would be very narrowly better in terms of RPM. 5spd has a .76:1 OD, 6 spd has a .73:1 OD. Auto is .69:1 I beleive. To spend $3k+ on a 6 spd tranny, new clutch (unless you get the small input shaft model), cross member and driveshafts, you'd have to drive that truck for a long long time to make up any of the cost, if it even saves you money in the end.

WestTN

I'll be willing to bet the mileage change will fall between 0.5 and 0.6 mpg. There just isn't that much difference in the gears. That 3" lift is killing your mileage. Much more turbulence under the truck. Ovesize tires take another toll.

On my annual trip from west Tennessee to south-central Florida I held the speed to 65 mph. I had my full height camper in the bed and towed a 6x10 utility trailer. Average mileage was 15.2 mpg.

ScottN

If your wheels are bigger than stock (total outside diameter), then you will be recording fewer miles than actually travelled, reducing the MPG calculation.

Also, bigger tire patch = reduced fuel economy.

Lift = more turbulence.

Driving hard = great reduction in fuel economy.

I get around 17 mpg in the city, 21 on the highway - unloaded. (hand calculated)

need12v

But if you have a dana 80 rear, you do have to swap carriers to go to 3.55.

Speedo reads off the tone ring on the carrier, if you swap carriers then it should have the correct tone ring, if not, may have to go to dodge for the correct ring. Thats the easiest way to correct your speedo for the gears. Tires are a different story...

Dhorn33

He has a 97 - so the speedo is easily changed with a $14 gear that you change out in less than 5 minutes. If he has an auto it is a Dana 70 - if it is a 5 speed then it is an 80.