mfpdm Posted January 10, 2008 Share Posted January 10, 2008 This is not about the MJ, but the math works for all jeeps. This is going to be long. :typing: I recently swapped the axles out of my TJ with D44's from a Rubi(4.10's and factory lockers). I had the 30/35 in it before with 3.07 gearing which with my 4.0 and 3 speed auto pushing 33's was generating around 2000-2200 RPM's at 65 mph. Jeep was slow off the start and bogged down on hills, but ran at highway speeds ok. With the swap, I gained a lot of pep in the low in but now am running about 3000 rpms at 65 mph. Needless to say my gas milage went into the toilet. I know if I want better gas milage get an economy car but I love the Jeep. So here are the options I have considered. I could go to 35's which would lower my rpm's and actually get better milage but then I would need to consider either a 1 in body lift or swapping out to 4.5 coils (have 3.5 now). My TW's drive shaft should still fit but may be short at full droop. Also would need to consider new shocks to achieve full droop as well. Maybe have to upgrade the brakes and steering to better handle the 35's. All in all about $600-1000 depending on the parts needed. Second option would be to swap out the 3 spd auto with a auto with overdrive. This should drop me down to 2400-2600 rpm's which is right in the correct powerband that I want. Would keep the pep in lower gears and reduce the highway rpm's and improve gas milage. Options on autos would be a 4 spd with overdrive out of an XJ which would bolt right up with little mods, but is computer controlled which would take a little more work to get it all wired up and working correctly. Plus the 4spd is longer and would make my rear drive shaft to long. With the TJ's every inch counts on the rear drive shaft and the front shaft would be too short. The cost of this would be $500-1000 range depending on whether or not I had to get new drive shafts made and the condition of the tranny. The other optional tranny would be get a 700R4 from a 4x4 chevy. They are 4 spd with over drive and have a decent first gear. Best of all they are not computer controlled which would be nice. Advanced adapters makes a kit to bolt it up to the 4.0 and an adapter for the tcase. I'm not sure of how much the overall length difference would be but if it is within an inch the driveshafts would be good I believe. I would have to get a new shifter for the tranny as well. The cost of this would be $1200-1800 depending on the condition of the tranny. I've seen a write up on this swap on 4x4 wire, however it was for a YJ. I think it would actually be easier with the TJ. So, now that I've bored you with the details, is there any other options or has anyone done a similar tranny swap. (Selling the Jeep is not an option :D ) Link to comment Share on other sites More sharing options...
Pete M Posted January 10, 2008 Share Posted January 10, 2008 Have you been switching your speedometer gears to make the speedo accurate with each gear or tire change? 4.10s with 33s should give you better mileage than 3.07s and 33s. RPMs of the engine is only part of the equation. More important than them is how hard your engine is working to maintain any given speed. Maybe your gas mileage went downhill because your Jeep was suddenly fun to drive? :brows: Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 Have you been switching your speedometer gears to make the speedo accurate with each gear or tire change? 4.10s with 33s should give you better mileage than 3.07s and 33s. RPMs of the engine is only part of the equation. More important than them is how hard your engine is working to maintain any given speed. Maybe your gas mileage went downhill because your Jeep was suddenly fun to drive? :brows: I forgot to mention that I have a speedo calibration thingy. Worked out well when I swapped out the 231 for the 241OR which doesn't use speedo gears. At 3000 rpm in a stock TJ, I would be running close to 80-85 mph. Jeep is definetly more fun to drive now. Just sounds like the engine is really working at 65 mph. If it just had that next gear to drop into it would be perfect. :D Link to comment Share on other sites More sharing options...
Pete M Posted January 10, 2008 Share Posted January 10, 2008 Honestly 3k isn't that bad. It should actually be working less hard now than before since 3.07s and 33s puts the engine real low in the sweet spot. Might just sound worse since you're so used to the previous tune the engine played. :dunno: My Dakota has 3.92 gears and little 28" tall tires and when in OD on the freeway it was usually around 3k. It always felt like the trans should shift again, but that's just the nature of the beast. 3k won't hurt anything. :D Link to comment Share on other sites More sharing options...
TNT Posted January 10, 2008 Share Posted January 10, 2008 Rework the fenders and run the 35's or do a 1" bodylift... Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 Honestly 3k isn't that bad. It should actually be working less hard now than before since 3.07s and 33s puts the engine real low in the sweet spot. Might just sound worse since you're so used to the previous tune the engine played. :dunno: My Dakota has 3.92 gears and little 28" tall tires and when in OD on the freeway it was usually around 3k. It always felt like the trans should shift again, but that's just the nature of the beast. 3k won't hurt anything. :D The rpm's I could live with, it's the 11-12 miles to the gallon that has me a little ticked. Following this " Good "rule of thumb" calculation is to mutiply .12 by your tire diameter. (.12 X 38" = 4.56)" to get what the right gear should be, I would be at 3.96 (3.91). Now following thischart I'm just a little above the sweet spot towards the power side. So I can understand why my milage went so far down(lost like 50 miles to the tank). I found this great link that has a spreadsheet that will calculate your rpm's based on your tire size, axle ratio, and tranny gears. I plugged in all my numbers and came out with pretty much what I am seeing in the jeep, however, I'm going to break out the gps again and verify my speedo. I ran the numbers for the 700R4 and in overdrive I would be running 1906 rpm's at 65 mph. I'm thinking that would be less work on the engine and therefore better milage, right? Link to comment Share on other sites More sharing options...
Pete M Posted January 10, 2008 Share Posted January 10, 2008 Spending tons of money to try and recoup a bit of gas mileage is rarely a wise move. My advice is to stop obsessing ofver the mileage and just go enjoy your Jeep. By the way, if I got 12 mpg in the Dakota I'd be happy. :( Too much dang power on tap. I just can't resist the call of the skinny pedal. :D Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 Spending tons of money to try and recoup a bit of gas mileage is rarely a wise move. My advice is to stop obsessing ofver the mileage and just go enjoy your Jeep. By the way, if I got 12 mpg in the Dakota I'd be happy. :( Too much dang power on tap. I just can't resist the call of the skinny pedal. :D Your probably right. I do have a tendency to over think things sometime. In the Navy we call that Nuking it. :cheers: Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 10, 2008 Share Posted January 10, 2008 The mileage "sweet spot" for me is 2400-2600 RPM which equates to 65-75 MPH on my stroker (31" tires, 4.11 gears). Sweet spot highway mileage is about 19-20 MPG; in town running around drops to about 14-15MPG because I have the same problem with the skinny pedal like Pete. :cheers: I did a recent lengthy trip and cruised at 2900-3000 RPM for nearly a tankful, mileage dropped to 17 MPG. The AW4 w. a 75% reduction O/D gear or similar I think is your answer, you need one higher gear for optimum mileage on the highway. You can't just say hit the pedal and enjoy when 93 octane costs in excess of $50 for a fill-up. Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 You can't just say hit the pedal and enjoy when 93 octane costs in excess of $50 for a fill-up. Bingo! That's exactly what started the wheels turning in my head when I filled up the other day and it was $50. This being my DD, that's a once a week event. The O/D for the 700R4 was .70 but first is 3.06 which will give a nice crawl ratio with the 4:1 tcase and 4.11's. Link to comment Share on other sites More sharing options...
TNT Posted January 10, 2008 Share Posted January 10, 2008 You can't just say hit the pedal and enjoy when 93 octane costs in excess of $50 for a fill-up. Bingo! That's exactly what started the wheels turning in my head when I filled up the other day and it was $50. This being my DD, that's a once a week event. The O/D for the 700R4 was .70 but first is 3.06 which will give a nice crawl ratio with the 4:1 tcase and 4.11's. Slap some 35's on it for now and then round up all the parts needed for the 700 swap. You'll need a crank sensor, a longer front driveshaft, a shorter rear shaft(unless you comp cut the fenders and move the axle back) a shifter and a properly beefed 700 trans. Probably a few other things too. Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 10, 2008 Share Posted January 10, 2008 If you're commuting from Chesapeake to Nof*&k, yeah, you need a taller final gear it seems with the gas $$ as it is now. I did it the commute the other way from Yorktown NWS for years while in the AO. 'Course the traffic is a whole lot worse now. Yeah, it's a dilemma, but obtaining the 2400-2600 RPM sweet spot cruise at 65-75 MPH the most effective and economical way is what you have to work out with your tire, diff ratio, and tranny final drive ratio IMHO. Course you already know that. :cheers: Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 10, 2008 Share Posted January 10, 2008 You can't just say hit the pedal and enjoy when 93 octane costs in excess of $50 for a fill-up. Bingo! That's exactly what started the wheels turning in my head when I filled up the other day and it was $50. This being my DD, that's a once a week event. The O/D for the 700R4 was .70 but first is 3.06 which will give a nice crawl ratio with the 4:1 tcase and 4.11's. Slap some 35's on it for now and then round up all the parts needed for the 700 swap. You'll need a crank sensor, a longer front driveshaft, a shorter rear shaft(unless you comp cut the fenders and move the axle back) a shifter and a properly beefed 700 trans. Probably a few other things too. Sounds like Tim has a workable solution. Link to comment Share on other sites More sharing options...
Pete M Posted January 10, 2008 Share Posted January 10, 2008 And pray-tell how much does this "solution" cost? Someone better be calculating the costs here and comparing the cost of extra fuel you think you're using each week to the total price of swapping part/tires. :smart: And what happens when you do all this and find out you're not saving anything? How do we know that your previous mileage calculations weren't off? I hate to be the devils advocate here, but I have a feeling this is much ta do 'bout nuthin'. :cheers: Link to comment Share on other sites More sharing options...
86FUBAR Posted January 10, 2008 Share Posted January 10, 2008 What was ur milage befor the swap and then after ?. the fix if deemed totaly necessary go 35s and trim. and as refrence my 05 rubicon unlimited with the 6 speed runs near 3k at 65-70 my milage is usualy 12-14 mpg , iv gotten 18 before but i got a lead foot Link to comment Share on other sites More sharing options...
TNT Posted January 10, 2008 Share Posted January 10, 2008 A front driveshaft from a XJ orZJ would probably be the right size too. Link to comment Share on other sites More sharing options...
TNT Posted January 10, 2008 Share Posted January 10, 2008 What was ur milage befor the swap and then after ?. the fix if deemed totaly necessary go 35s and trim. and as refrence my 05 rubicon unlimited with the 6 speed runs near 3k at 65-70 my milage is usualy 12-14 mpg , iv gotten 18 before but i got a lead foot The key to this is a light foot on the skinny pedal.... :D Self control is hard to do... :brows: I get about 2 mpg worse the when my wife drives.... Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 10, 2008 Share Posted January 10, 2008 The key is also accurate mileage readings, w. the correct speedo gear calibrated to your rig equipment, and verified with GPS or another reliable source (like a local cop car :eek: ). Don't look like gas prices are going down anytime soon, heck they never will, so unless you entertain thoughts of alternative fuel solutions, you gotta make the best of what you got, what you want, and weigh the pros and cons. If you're not concerned with $$, there is no problem. But I think most of us are. If you are concerned about like $1K to get the ideal ratio between performance and economy, I think the $1K will pay for itself in very quickly if gas prices continue as they are. Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 And pray-tell how much does this "solution" cost? Someone better be calculating the costs here and comparing the cost of extra fuel you think you're using each week to the total price of swapping part/tires. :smart: And what happens when you do all this and find out you're not saving anything? How do we know that your previous mileage calculations weren't off? I hate to be the devils advocate here, but I have a feeling this is much ta do 'bout nuthin'. :cheers: That's what it will finally boil down to is cost vs savings. However, I've come to realize that one thing leads to another with the jeep. Trying to get the perfect setup usually means when you think you got it right, you need just one more thing to get it perfect. I'm thinking that if I jump ahead this time, I'll miss a couple of ca-ching$ in the process. Slap some 35's on it for now and then round up all the parts needed for the 700 swap. You'll need a crank sensor, a longer front driveshaft, a shorter rear shaft(unless you comp cut the fenders and move the axle back) a shifter and a properly beefed 700 trans. Probably a few other things too. I'm leaning towards this way. I figure I can pick up a used set of 35's in good shape for about $250 and run the way I am until I get all the parts together. What was ur milage befor the swap and then after ?. Stock I was getting close to 300 miles per tank. After lift with 33's and 3.07 I was getting 250 per tank. After axle swap to 4.10 I'm down to 200 per tank. I really appreciate all the feedback. Things look different with someone elses perspective. That's also the reason I brought this to this forum instead of one of the wrangler forums. I love this place. :cheers: Link to comment Share on other sites More sharing options...
johnj92131 Posted January 10, 2008 Share Posted January 10, 2008 I have also wondered about reducing engine RPM on trips. Given that the AW4 transmission is used in Toyota Trucks, I have wondered if the 5 speed automatic transmission in some Toyota trucks could be used as a replacement for our AW4. The first gear in the 5 speed automatic is something like 3.52. That would allow the same off the line performance as with factory 3.55 gears with a 3.07 rear end. (2.80 x 3.54 = 9.94 and 3.07 x 3.52 - 10.80). Just the result of me playing with numbers I "think" the fifth gear in the Toyta 5 speed automatic is about the same as the AW4. something around .75. Ultimately, I would like to look at the idea of putting a Toyota 6 speed automatc transmission in to my Comanche. With the steep first gear and the double overdrive 6th gear, we could have the same off te line performance and better highway MPG. Has anyone looked at this? Link to comment Share on other sites More sharing options...
Pete M Posted January 10, 2008 Share Posted January 10, 2008 My guess (and this is just a guess) is that the Toy engine has a different bolt pattern and so their version of the AW-4 would have a different torque-converter housing (meaning there's a slim chance you could use a Toy AW-4 if you use a Jeep torque-converter housing). If this is the case, then you're probably not going to find a bellhousing that will match both the Jeep engine pattern and the toy manual trans pattern. :( Link to comment Share on other sites More sharing options...
mjeff87 Posted January 10, 2008 Share Posted January 10, 2008 **small thread hijack** FWIW, I'm a new TJ owner...bone stock 2.5L with 4.10's, 31's and an AX5. I'm a bit surprised at just how crappy these things are on mileage. I wasn't expecting too much, coming from a DD car that got 35-40 mpg, but it's only averaging about 17-18 (filling up consistently around 1/4 tank and 225-240 miles, it's been taking 13.5 gallons or so). I know it's like driving a toaster on wheels, but honestly was expecting about 20. I'm not crazy with the pedal, either. Were 31's stock on these things? I've got aftermarket aluminum wheels on it, and am thinking maybe the PO swapped tire sizes but didn't change the speedo gear (???) So far, I've changed the oil, tranny and TC fluids, cap, rotor, wires and plugs. Gonna do the diffs and give it a coolant flush, then everything will be new once again. ....now back to your regularly scheduled thread....thanks :D Jeff Link to comment Share on other sites More sharing options...
mfpdm Posted January 10, 2008 Author Share Posted January 10, 2008 **small thread hijack**Were 31's stock on these things? I've got aftermarket aluminum wheels on it, and am thinking maybe the PO swapped tire sizes but didn't change the speedo gear (???) Jeff Biggest tire that came on a TJ prior to the Rubi was 30's. They were mostly on the 4.0' though. So I would definetly say those aren't stock. I believe yours should have been in the 28-29 inch range or wantever the metric version of that is. If this is the case, then you're probably not going to find a bellhousing that will match both the Jeep engine pattern and the toy manual trans pattern. That is the big problem I've come across, finding something that will bolt up without any fab/custom work. I'm looking at the Rubi's tranny to see how it is wired up and if that would be a better choice to save cost. Link to comment Share on other sites More sharing options...
mjeff87 Posted January 10, 2008 Share Posted January 10, 2008 ahhhh, thanks! (this SWB world is all new to me). Wasn't making sense that the TJ was getting almost as bad gas mileage as the MJ :roll: (per the odometer) Gotta figger out what speedo gear I need and swap it out. :cheers: Jeff Link to comment Share on other sites More sharing options...
CEThomas Posted January 10, 2008 Share Posted January 10, 2008 That's exactly what started the wheels turning in my head when I filled up the other day and it was $50. This being my DD, that's a once a week event. I know the pain, If I drive the Ranger at 19mpg to work, it takes $55 to fill up at 2 tanks a week :ack: So I drive my Capri and keep thinking about a YJ with a little diesel for a DD :D Link to comment Share on other sites More sharing options...
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