7moore7

I very much realize that this subject is touched on often, but I don't really have anything I need "fixed" I just want to make sure I understand what's going on.

2001 Jeep XJ limited, 185,000 miles, auto 4-4.5" lift, 33x10.5 BFG A/T. Commuter and weekend warrior.

I just put on the new tires about a week ago, next step is to work on the diffs (gearing and locker). I currently have the stock 3.55's.

Here is what I am a little confused about. I keep reading that 2200rpm or so is the ideal efficiency area for the 4.0 and you want to gear to hit that, especially if you're worried about mpg and commuting. What I'm confused about is that the jeep doesn't seem to think this is the case. When on the highway it'll shift down to 1600rpms or so as low as 45mph and up at my commuting speed of 55mph (and the overhead console readout reflects an increase in mpg's).

I do realize my speeds are faster here b/c the clock is assuming stock tire size. It seems to do pretty standard for jeep, around 14-14.5mpgs a tank, reading up to 21 mpg's for steady cruising at 55mph.

I think I just don't quite understand the shift pattern/overdrive. Is 4th gear OD? Is this the gear that locks the transfer case so the tranny is more efficient? Is this efficiency so much better that the jeep will switch out of the "ideal" 2200rpm range to get to that final gear? If I gear to hit stockish numbers (4.56 with my 33's) will I still get the same mpg's because I'll be in OD? It's just such a big jump in rpm's and my brain is having a hard time thinking it'll be more efficient.

I'm going to re-gear regardless so I can crawl better, but am considering one step lower just cause I don't fully understand this. Thanks for any tips.

7moore7

Re-reading this I want to be clear: the jeep will only shift to the final gear when it's not under load (once I reach cruising speed). It doesn't feel like it is acting up or shifting too soon.

JerrytheJeep

33's on stock gearing is going to mess up your rpm-power curve. You won't see as much torque and you'll stay in the lower rpms at higher speeds. As far as the transmission goes I can only say what I've observed. I generally only hear 3 shifts when I'm around town, and on the highway I hear a 4th if I'm not pounding on it. If I do pound on it, it goes into overdrive, regardless of what gear I am in. There's a thread on jeepforum or something like that called "everything you need to know about the aw-4" its very tech intensive and I can't make heads or tails of it because I'm a visual kind of guy. But maybe it'll help you.
Cheers

7moore7

I read through that little guy on NAXJA (thanks for that). Got some interesting stuff, although partially answers only one of my questions. And I totally understand that I'll need to swap gears.

I think it boils down to this question: If 2200rpm's is the efficient zone for this motor, why would it shift up when I'm cruising (to get in the 1600rmp range?) Is it the overdrive thing, or is the computer that dumb? OR is the 2200rpm thing not true

JerrytheJeep

it just wants to be in the highest gear possible. simply put a higher gear takes less energy to turn (because it's ratio is smaller) so if it can get to a higher gear it will. i believe that when your gearing gets corrected your rpm range will change a bit and youll see more rpms at those same speeds, getting you into that sweet spot.

hat's another thing, look into changing your speedo gear, there are a bunch of charts and calculators for it. look for a speedometer gear tire size chart. It'll ask for your gearing as well. Changing that will bring your odometer and spedometer back in line.

7moore7

Thanks for keeping tabs. Speed error correction is no problem, I've had to do that on vehicles in the past. I'm really not concerned because I'm in a jeep, I really only keep track of mpg's and speed to look for variance. It was just one of those things I was thinking of when driving to work this morning. I have a feeling it's the OD thing and not the lower rpm's.

I may have posted too soon on this, I'm finding lots of threads of discussion on efficiency vs speed vs load and there are a lot of things that seem to not make this cut and dry.

Lyferxb9s

I don't know about the Jeep but I have found in my wifes truck if we drive in 4th VS Drive when staying under 60 we get a decent more mileage out of it. I am going to start doing that with the Jeep too

ta2tony

How are you tracking mpg's when your speedometer is off? If your speedo is off due to larger tires, your odometer is off too. You are actually logging more miles than your odometer shows since its based off revolutions of the stock tire (with stock gearing). With a larger tire you are now traveling farther with each revolution.

As far as any variance you are seeing, its probably because you are burning a ton of fuel trying to turn those tires at 1600 RPM on the highway.

7moore7

It's all relative. It's easier to just look at the registered mpg's rather than calculate out the real mpg's every time.

So I know if I get gas at a different station and mpg's go down, it's poor gas. Whether or not the mpg's are accurate is a non-issue, only that they are consistent. If they are consistently reading 8.5% slower or less efficient than what is happening, for tracking changes I could care less that they are GPS confirmed accurate. If I were switching tire sizes and gears all the time it would be a concern. Not trying to argue that my readouts are off or that I should fix them by any means.

7moore7

I'm NOT burning a ton, that is part of my question. At 33's and stock gearing I'm getting near 17mpg combined mpg. That's after correcting cause you guys are so worried about it (new tires' circumference is 16.5% larger than stock). My readout is at around 14-14.5mpg. According to everything I'm reading it should be more efficient when not lugging it (properly geared), but then why would it shift down? It seems there is every reason for the Jeep to stay in the higher rpm's - better for the motor, more available power, more economical. The only thing 4th/OD has is the locking transfercase, right?

I do understand that I need to re-gear, I'm not arguing that. I do understand my ODO and speedo don't reflect real world, not arguing that either.

Rock Toy

Once you have achieved cruising speed and are no longer accelerating you don't need much in the way of torque or horsepower to maintain your speed so your on board computer will shift your transmission into overdrive until you hit a hill or something that requires more power then the transmission will shift down giving you more torque.

While 2200 rpm is decent for mileage, if you are at speed and don't need the extra torque to maintain speed, a high idle of 1200-1500 is still going to be more fuel efficient than 2200 rpms

yarom

Please correct me if I'm wrong - I thought changing the speedometer gear pinion (picture below) was necessary not only to show the correct speed but also for correct shifting, as the computer uses the data from that speedometer gear to tell the AW4 to shift up, down and OD based on some sort of lookup table...

7moore7

Yarom, I'll have to look into that, it would make a lot of sense. Especially in terms of estimated torque needed at certain speeds to account for wind resistance, etc.

OEJ

i'm in the same boat. i've got 33s and i plan on re-gearing.

Whats worrying me is that if i'm in OD doin 50 and running @ 1600 rpms; is that concidered "lugging" the motor. will it hurt the tranny by over heating it?

I drive in 3rd,manually shifting everytime i'm in the city. then when i'm on the road where i can do 50 i worry tht 1600 rpms will over heat the tranny causing damage over time. so i try to keep it at 17-2k.

So I don't mean to jack the thread but, should i just drive her like i did before the lift and tires? then when i need to have more power,just let the tranny decide?

what should i do? thanku.

7moore7

A couple of guys I wheel with are running 4.11's and 33 in mud terrains just to get that extra highway cruise edge and have no problems in the desert and on the rocks. This seems like the better route to go for me, plus the 4:11's have an extra spline so are just a touch stronger, and I can probably find them for cheaper in the junkyard.