4x4 Gear swap -> 2.73 Rear

[Four0Sport]

this website has tons of information on whats proven to work, then again none of this stuff was proven until someone tried it on their own vehicle. keep this updated bc the information gained from something like this whether it be good or bad could help countless others one day down the road.

 

[rudypoochris]

this website has tons of information on whats proven to work, then again none of this stuff was proven until someone tried it on their own vehicle. keep this updated bc the information gained from something like this whether it be good or bad could help countless others one day down the road.

Thank you. I agree.

I think once one realize there is too low and too high then it comes down to finding what is optimal. Maybe optimal is 2400rpm at torque peak, I don't think it is, but that doesn't make it impossible. Maybe optimal is right where it is now. Maybe optimal is 1650 @ 65mph. I don't know for sure. I cruise usually around 72-75 and I THINK that dropping my rev's to 1800 at that speed should net me better economy. Maybe not. We will see. This is a summer project though. I will source another M5OD from the yard, grab the gears, take them to the cutter, see what he can do, then piece it all together, then test it for ~1,000 miles (run it on a track somewhere to 110mph+ if I can find one), then disassemble and inspect. Might get lazy somewhere in between that whole process, mainly towards the end. Point is that it should take a while, but I will keep you all updated.

 

[hunterdan]

In heavier, underpowered vehicles, you need the proper gearing to keep the motor in its power band (where its most efficient). It also depends on the driving your doing. If your just driving through the city with minimal highway driving, lower gears will benefit you greatly, reducing the load on the motor making it work easier. Out on the highway, you want to be cruising just under the power band, this will be very close to peak efficiency, but will leave you with reserve power to help pass and maintain speed on hills. Look at all the little hondas, what rpm are they cruising at on the highway? They are usually at or over 3000 rpm. The name of the game is to keep the motor at its peak efficiency, which is in its power band.

Dan

 

[CopyKat]

Look at all the little hondas, what rpm are they cruising at on the highway? They are usually at or over 3000 rpm. The name of the game is to keep the motor at its peak efficiency, which is in its power band.

Dan

My 07 Civic cruses at 2000. Wifes old Cavalier was near 3000.

I had 35" tires on my old ranger years ago and was running 3.55 gears. In all honesty 5th was a gas robber. I got better milage when in 4th.

The broncoII I have now has 33's and 3.73's and it's a toss up on the milage between 4th and 5th. The wind plays a factor. revs 2000 rpm at 65 MPH

If I go 4.10's in the Bronco and stay with 33's the power band will shift me right back into factory spec (3.55's and 215/75 tires)

http://www.quadratec.com/jeep_knowledgebase/article-26.htm

 

[Hitchhikingmike]

The 5r55s are rated to handle 500 pounds of torque. Hence the "5" in 5r55, same with the 4 in 4r44 ford transmissions.

Nevertheless, undue shifting will shorten the life of the transmission (such as down shifting every time you see a hill), not to mention the added torque.

 

[rudypoochris]

In any case you guys bring up a good point. I know the engine is not efficient at those rpms. BUT there is an inherent trade off, as I understand it.

Yes, the engine is most efficient somewhere, at its torque peak at its power peak, somewhere around there. Meaning you put X amount of fuel in and you get Y amount of power out. The issue is that at 65mph or 75mph you need say Z amount of power which is only a fraction of what Y is at the motors peak efficiency. Basically if we were to simply run the motor at its peak efficiency RPM wise, we would have excess amounts of power and thus excess fuel wasted. So we can gear it long and move OUT of the engines peak efficiency. The point where the engine becomes so inefficient and where the power required meet, is the place where fuel economy starts dropping off. That is difficult to read and I am sure I didn't word it perfectly.

Main point is. At peak efficiency the engine makes too much power. Yes, your getting more for less fuel, but you are still getting much more than you need and consequently you are using much more fuel than you need to be. Torque peak yields ~220ftlbs which is the equivalent to 100hp which is approximately 3 times the power one needs to maintain 70mph in a stock Explorer. At a certain point lowering the RPMs the engine will become so inefficient that it will start consuming more fuel for the power you are requiring. Trying to find the peak in between these two. I have a feeling this data could be obtained with a chassis dyno and a fuel meter, or some calculations with just a dyno graph... Anyone got a stock dyno graph that goes as low as 1500rpm?

 

[hunterdan]

My 07 Civic cruses at 2000. Wifes old Cavalier was near 3000.

I had 35" tires on my old ranger years ago and was running 3.55 gears. In all honesty 5th was a gas robber. I got better milage when in 4th.

The broncoII I have now has 33's and 3.73's and it's a toss up on the milage between 4th and 5th. The wind plays a factor. revs 2000 rpm at 65 MPH

If I go 4.10's in the Bronco and stay with 33's the power band will shift me right back into factory spec (3.55's and 215/75 tires)

http://www.quadratec.com/jeep_knowledgebase/article-26.htm

I obviously haven't spent much time in newer 4 cylinder cars. At any rate, the power and torque numbers you are getting are flywheel numbers, not actual rear wheel HP or torque numbers. The 4.0 OHV was rated somewhere between 150-160 HP through the years, and torque was somewhere around the 220 mark that you have been saying. At the rear wheels, the truck is putting down probably about 110-120 HP and 180-190 ft-lbs of torque. Considering the age of the vehicle and its aerodynamics its got to work a little bit harder. The motor probably isn't quite as strong as it was when it was younger. So that 110-120 hp might be more like 100-110 hp. Now look at the aerodynamics. It really is not all that aerodynamic. So it will require a bit more power to maintain speed while cruising. Consequently, the faster you go, the more aerodynamic drag you place on the vehicle. I think my point is that I don't think you will have enough power to push the vehicle at those speeds with those gears. The 3.08's will be better than the 2.73's. What ever you decide to do, I wish you luck.

Dan

 

[Burns]

Either way i feel that its a very bad idea but cant wait to see it happen

 

[rudypoochris]

Just for fun guys, here is the aerodynamic drag I calculated for the 1st gen explorer at various speeds. First value is in HP, second in lbs of resistance, third is % change from before.

10mph - 0.1hp - 3.1lbs - N/A
20mph - 0.7hp - 12.3lbs - 297%
30mph - 2.2hp - 27.8lbs - 126%
40mph - 5.3hp - 49.4lbs - 78%
50mph - 10.3hp - 77.2lbs - 56%
55mph - 13.7hp - 93.4lbs - 21%
60mph - 17.8hp - 111.1lbs - 19%
65mph - 22.7hp - 130.4lbs - 17%
70mph - 28.3hp - 151.2lbs - 16%
75mph - 34.8hp - 173.6lbs - 15%
80mph - 42.3hp - 197.5lbs - 14%
90mph - 60.2hp - 250.0lbs - 27%
100mph - 82.5hp - 308.6lbs - 23%
105mph - 95.5hp - 340.3lbs - 10%
110mph - 109.9hp - 373.4lbs - 10%
115mph - 125.5hp - 408.2lbs - 9%

I have hit between 110 and 115 before, which seems to jive. Compare 55mph cruise with 65mph cruise. That is where is seems the aerodynamics really start getting "bad". Even a little change to Cd or frontal area will really change these values. For instance adding .05 to the Cd and bumping up the frontal area by 3ft, will result in an HP drag of 27.9hp at 65mph, compared to the 22.7hp for a stock bodied Explorer. That chart really took a long time to type. :/

 

[DeRocha]

DeRocha, just out of curiosity were you running the 5-Speed or the auto?

I was running an A4LD automatic. A 5 speed Manual transmission will give you better gear selection and will be more tolerable than an automatic.

If you are serious about running 2.73s then just bite the bullet and put then in the rear and see how they perform (disable the 4x4 system to prevent driveline damage). Yes it will cost a few $$, but like all research you have to start somewhere.

 

[rudypoochris]

I am first going to see if a new OD gear can be cut. I am reasonably happy with the other 4 forward gears.

 

[freeridejunkie87]

best thing for good mpg is taking care of your stock vehicle...making sure everything is to OEM specs. some mods like said before electric fans, intake, headers&exhaust, if you want to spend some money electric water pumps like on race trucks haha. keep tires inflated properly, windows up, drive steady, use proper weight oil. and keeping your speed down, like at 65 mph, 75pmh is not cruising speed.

think of it like a bicycle, if you start in too high of a gear and struggle everytime you start off, youre going to be tired as heck in a very short amount of time. thats what youre doing to your explorer... but if you have proper gears you can ride your bike all day and feel good (better fuel economy hahaha)

taking away any extra load on the engine is going to help out economy.

not to say dont do your experiment, but everyone here was just trying to be helpful because they speak through experience, and are just trying to save YOU time and money and frustration.... they werent avoiding your questions it was just a matter of trying to help out



by all means, please please please try to make an explorer get 30mpg it would make everyone here SOOOOOOOOOOOO happy! and then everyone will be coming to you for advice




oh and your 4.0 wont handle much boost, the 4.0 sucks b-hole
if you want turbo'd 4wd ford... go to europe and get a focus RS oh and you'll have 30mpg!

 

[rudypoochris]

oh and your 4.0 wont handle much boost, the 4.0 sucks b-hole
if you want turbo'd 4wd ford... go to europe and get a focus RS oh and you'll have 30mpg!

Hehe. I haven't looked into it. I know a big let down on the 4.0L is that the push rods are spaced like it was a 2.8L, this really kills valve area since they are spaced so close together. That is one of the main reasons the engine isn't very strong despite the displacement. As for turboing it. Well... we will see! I wouldn't want much over 230hp anyway, which can be achieved at quite low boost.

 

[calmac06]

I know for sure driving around in 4th gear (equivalent of switching to 4.11's in 5th) would kill my economy and eventually my engine. Would you drive your Explorer around at 65-70mph in 4th all the time? Then again there is such thing as too low or too high.

Just wanted to say that my poor ass has been driving around with an inoperable OD in my 5R55E from September until now (almost 16,000 miles)!! Absolutely no problems from the motor yet, it doesn't complain a bit.

Before I bought my Explorer I had an '87 Ranger 2.3 with a 5 speed... That truck was so slow that my driving habits that carried over to my Explorer (for the first week, till I got used to the power) netted me 20.3 mpg without overdrive, in need of a tune up. Now that I'm used to my truck, I take off much faster than I did with my Ranger. (Now ~17 mpg, in a good state of tune) That's a lot of the problem with fuel economy... just pay attention at how fast almost everyone takes off from a stop, we're all guilty of it. Just my .02

 

[goliath]

I know for sure driving around in 4th gear (equivalent of switching to 4.11's in 5th) would kill my economy and eventually my engine. Would you drive your Explorer around at 65-70mph in 4th all the time? Then again there is such thing as too low or too high.

I run 4th gear pulling my 3000lb boat in ca all the time. i dont use 5th because it lugs everything and i get terriable fuel economy - 2400rpm and 10mpg. 4th gear manual tranny, 4.56 gears with 33inch tires turns 3100rpm at 65 and i get 15mpg pulling the boat. ive logged thousands of miles like this and the truck has over 200K so killing it isnt going to happen. if anyone has seen me wheel they know i try

 

[rudypoochris]

The demand on the engine while pulling a boat compared to me just traveling at 65+mph is sort of a different thing. You probably NEED to be in 4th to move that thing and maintain that speed. The gearing, tire OD, etc. etc. is very different that that point.

I don't think the engine is going to explode if you drive 4th all the time, obviously not. In my situation though I wouldn't cruise 4th at 70mph all day, that would be stupid. The over drive transmission has been one of the very large contributors to our increased fuel economy and reliability.

 

[CDW6212R]

This seems to be about fuel economy, rhetorical.

Hundreds of other threads have gone over this. Al is well beyond most everyone here. He has shown by example what various changes do. My best suggestion is to work on many of those things first. Tons of typical efficiency upgrades will gain a whole lot more fuel mileage than any gearing change.

These are Explorers, they weigh 4400 pounds or so, they are nothing like a Mustang or any other efficient vehicle. The gearing is absolutely critical for efficiency, just as it is for towing and big 4WD tires etc. A 2.73 rear gear is not going to work like it has or should in countless older cars that weighed far less, OD or not.

Here is my example, learned about ten years ago. I owned a 91 4WD Explorer(3.27's) and a 93 4WD Explorer(3.73's) at the same time. Those were identical engines, weight, trans, and size. The 3.27 Explorer gets poorer mileage in town, and on the highway. It simply uses more gas fighting the poorer gearing. My 93 Explorer did as well as anything I have had on my mail route, better in the city, and over 21mpg on the highway(1100 miles to SD). I took equal care of both, the conclusion is very simple. The 3.73 gears are better for mileage in the 1st gen 4WD Explorers.

The engine needs to be near its power band, 1600rpm is too far away. The best fuel economy will come when the least throttle is used, and rpm is a few hundred rpm below the peak torque. Stock engines will have PT in the 2500-3000rpm range, so shoot for something around 2200-2400rpm at cruise.

Using any real throttle while cruising to gain speed or maintain speed, that kills mileage. You may think that the short second or two with a little throttle is no big deal, that is wrong. I have watched the affect of driving on flat roads, versus a little throttle needed to get up a slight hill, versus letting the speed drop on hills.
I have owned a bunch of vehicles, I keep all fuel mileages for every tank of gas, for all of my vehicles. I know when something is changing, it points me to when maintenance is needed. I also have had several with OEM message centers, showing instantaneous fuel mileage.

What kills fuel mileage is having to use more throttle than 5% or 10%. Learning this takes experience, and having a tool to monitor the MPG with. I have been where you are, you "know" that you are right, and everyone else is wrong. You are unstoppable in that thinking, and you will see the error in that thinking, soon.

We are only trying to save you a bunch of money. I would suggest doing the hundreds of dollars of other things first, you may be happy with 20mpg all around mileage. If that isn't enough, swap to 3.73's(or up one or down one), depending on your new goals and power levels. Good luck,

 

[rudypoochris]

Thanks! I am working on seeing if I can fit a fuel meter on the rail to count the flow. That way I can take it to the chassis dynometer and with the dynometer graph from 600 to 4750 rpm plus fuel flow sampled at these rpms, I can build a BSFC graph based on RPM. That would be sweet and tell me a lot. With that I could figure out exactly what RPM is most efficient for what speed.

I tried calling Ford corporate so I wouldn't have to do this, but they seem to think the dealers would have this info... Right. Ha.

I don't know I am right, otherwise I wouldn't even ask or type here. It would be pointless. I THINK I am right, and I know that running a 4.11 lowers my mileage, since I have simulated that by running my current gears in 4th. I will do it again though if I can get the fuel meter stuff. It is $350 for the most basic 1 sensor and read out. Unfortunately if the rail is not dead end, then I will need to run a twin engine setup and calculate the difference, this means another sensor and closer to $550, which I am not willing to spend at this time. We will see.

 

[CDW6212R]

Find some kind of aftermarket message center, those all take a fuel signal from the PCM and convert it into a mpg display. Those are not accurate enough to predict empty, but likely within 10%. When you watch the mpg display change from 30mpg while cruising to 14mpg with slight throttle, you learn to let off of the gas.

 

[Jack Flash]

I can't believe that I've read 3 pages of this...

I will venture a guess that I'm quite a bit older than all or most of you and have tons of practical experience that the OP is trying to gain from formulas and a computer simulation. Not knocking any of you...just taking out a few minutes before a pool tournament to offer my 2 cents worth.

I am sure that somebody still makes an overdrive unit that goes between the transfer case and the rear differential. I remember a company, Hone, that made these units. One type was hung from crossmembers and required fore and aft shorty driveshafts. The other type was a unit that bolted directly to a 9'' Ford differential.

Finding the crossmember type would be the best of both options. You could test your theory of 2:73 gears without changing gears. With the unit not in overdrive, you would still maintain front and rear ratios being equal. Just a thought.

Now some advice (and don't take it the wrong way): get your vehicle on a chassis dyno. You will be surprised how little horsepower is actually at the rear wheels. Community Colleges in my part of the USA have chassis dynos and will test vehicles for a nominal fee. You will find that all the computerized formulas can be thrown out the window. Most theories are written by engineers with close to zero practical experience. One thing that has not been discussed here is torque. Horsepower is fine providing you are on a flat surface or in higher rpm ranges. Go up an incline and that shallow 2:73 ratio will have you shifting down a gear just to maintain forward motion. Check the factory numbers for torque and horsepower. See at what rpm they are calculated. Slightly below that rpm is your best mpg.

I'll check back in a few days and see if anyone has done any homework after reading my Junkyard 101 lesson.

Remember: speedometers & odometers are inherently WRONG. They are close (within 15%) at 55 to 60 mph. Tire size, clutch slippage along with wind resistance make your speedo and mileage vary somewhat. If I believed that speedos were right, why does my ranger with 3:73's get 22 mph, my EX with 3:73's get 16 mpg, and my EX with 3:08's get 20 mpg ? Figure that one out !
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