4.10 differential gear ratio questions.

[tod1188]

I have a comanche dana 35 rear axle with 4.10 gear ratio. I would love to install this but I have a few concerns.

 1- gas mileage/high rpm. I know 4.10 ratio won't provide high top speed but I do not care about that, I am concerned  about cruising at 55-65 mph and killing the gas mileage. I enjoy my Eagle and I plan to go on many road trips so highway must be doable. Losing a little mpg is fine but a lot is not.

 2- I understand that i must match the front and rear ratio for 4x4 driving. Is it possible to get 4.10 gearing in the front? Or will I need a new front differential and if so is that possible?

Reason for higher gearing: I have an axle, it is in better shape then the one in the eagle, and i want to pep up the driving experience. A little tired of Goldy, my eagle, driving like a truck.

[DaemonForce]

I prefer to swap parts with the 86 Comanche because it has the most common configurations I'm able to swap into the Eagle but I have the opposite situation. I have really tall gears(2.73) that cause a slow loss of strength up slight inclines and I somehow manage to get stuck in deep snow that gets caked really good between the wheels. Rather than do an insane amount of work by swapping out the front and rear differential gearing, I'm willing to place blame on a tired engine and lack of option.

At 177K with every idiot touching this car over the years, there have been so many power problems that would just instantly be solved by dropping in a new 4L or maybe 282. My Eagle is a daily roadster so it's disappointing to already have torque issues in places where I NEED to maintain speed. Thankfully the speed limit on the Oregon highways is lower than it is here in Washington so I can deal with it for now.

The deep snow issue, not so much. I tend to venture into areas that make it necessary to engage reverse and where I need a lot of power just to leave. For this reason, I'd rather look into getting a 2nd gear transfer case to multiply the power that I'm just not going to typically get from such a weak engine. It might be worth taking a look in the junkyard for a 81-88 Grand Wagoneer or something like that.

Failing all of that, the differential gearing needs to be changed. Obviously 4.10 is going to drive like a truck and that's way too low for a highway vehicle that rarely sees offroading conditions but the opposite is true with gears as high as 2.35-2.73 when there's a tired engine under the hood. 2.73 would probably be fine if I would just rebuild the engine but for what it's worth, 3.08 might be the ideal ratio for the Eagle. It's just high enough to make the highway tolerable and it's low enough to make offroading easy with or without a 2nd gear transfer case.

[carnuck]

On the Grand Wagoneers and Jtrucks, look at the front driveshaft. It has to be on the driver's side to work (80-91 FSJs) so that will avoid you looking at Borg Warner Quadratracs, but not the New Process Quadratrac NP219 which has AWD (4 hi open), Edrive (4 hi locked) and low range. NP208 is great for slippery zones but not for dry roads because it's always locked when in 4 high but it has 2wd and if you lose the rear driveshaft you can still drive (gently) in frontwheel drive in 4 high. I did that with my '83 Cherokee (80-83 Cherokee and Wagoneer limited are full size. '84 was the first Grand Wagoneer badge on a FS Cherokee with Wagoneer Limited drivetrain)
   Gear ratio needed will depend on tire size and whether or not you have O/D. With my 727 trans (from a Grand Wagoneer), the torque converter stall speed is lower and first gear ratio is higher so it takes more gas pedal to start moving and it's more sluggish. Add to that the 235/75/15 tires I have and it's horrible with 2.72 gears, so I have 3.08 8.25" rear end from a '91 Cherokee to put in and an Eagle non-vacuum matching front. That should up the take off power and later when I switch to an AW4 trans it'll still cruise fine too.

[BenM]

4.10 is available aftermarket, but it won't fit a 2.35 front axle except some of the early disconnect axles, identified by casting numbers.

That's pretty steep with stock Eagle tires, around 3400 RPM at 65 MPH. An overdrive like the AW4 would drop that to a peppy 2500.

In my experience, a stock camshaft drops off beyond 3800. Redline for a 258 is usually 4500, but it's not often one will want to go over 4000 in stock form. AMC installed very conservative camshafts in the sixes.

I've driven 3.08 gears, and they are reasonable in an Eagle. I think 2.73-3.31 is the range you want to stay in without overdrive, with an overdrive transmission 3.08-4.10 would be useable.

(I can only speculate on why AMC used the 2.35s, but I'd suspect it had more to do with EPA tests than real-world economy.)

[captspillane]

I've driven 3.08 gears, and they are reasonable in an Eagle. I think 2.73-3.31 is the range you want to stay in without overdrive, with an overdrive transmission 3.08-4.10 would be useable.

That is a spot on recommendation.

[carnuck]

4.10 is available aftermarket, but it won't fit a 2.35 front axle except some of the early disconnect axles, identified by casting numbers.

That's pretty steep with stock Eagle tires, around 3400 RPM at 65 MPH. An overdrive like the AW4 would drop that to a peppy 2500.

In my experience, a stock camshaft drops off beyond 3800. Redline for a 258 is usually 4500, but it's not often one will want to go over 4000 in stock form. AMC installed very conservative camshafts in the sixes.

I've driven 3.08 gears, and they are reasonable in an Eagle. I think 2.73-3.31 is the range you want to stay in without overdrive, with an overdrive transmission 3.08-4.10 would be useable.

(I can only speculate on why AMC used the 2.35s, but I'd suspect it had more to do with EPA tests than real-world economy.)

With the stock 205/75/15s I ran 2.37/2.35s in my '83 wagon with a 282 Mexican motor and a high stall 998 trans. It was definitely a freeway cruiser. I went to Canada and back every weekend after work to pick up my kids on friday night, return to Seattle and then bring the kids back sunday night so they could go to school monday. We did some camping and wheeling but I was pretty tired most of the time so we just visited, ate and watched TV)

I got 20 mpg highway and it floated along at 70 mph (posted limit here and sometimes a bit above) Bit sluggish in town though.

[DaemonForce]

The 282 is insane. What cam did you use? I'm looking at dropping the stock AM cam and going for this CompCAMS energy circus but I'm not sure what the stock specs are and how much of a difference it makes. I'm playing with the idea of experimenting with a single barrel carb somewhere in there as well. The idea is a stable but powerful highway cruiser.

[carnuck]

Baja beast in a stock Mexican motor. Pistons collapsed due to high compression, ping and poor casting but I got 50,000 fun miles out of it (no idea how many before. The PO of the Eagle and Mex CJ swapped it for the 4.2L because he couldn't get it running right with a cracked intake). I put it in my '83 Cherokee after the Eagle rotted away and killed the third 998 before the oil sender popped and destroyed it at 70 mph. Kept running but compression was really bad after that.