I would like some feedback from others that may have done this or have other suggestions. After tossing in some NGK V-Power FR5-1 plugs, I decided to test a few theories. Alongside the hotter plug, (had autolite 3923s previously) also grabbed some PEA injector/fuel system cleaner just to clean up some from the previous owner. Specifically used Gumout Regane high mileage, thoughts on this product? I sadly didn't think to scope the cylinders as I changed the plugs, so that's out, but anyway back to the math.

I have read this thread a few times and decided to do up the math specific to my truck and check some results. My truck is a 1998 Dakota regular cab, 3.9L magnum with the 42RE auto trans, 3.55 rear diff, and 235/75/r15 tires, which stand at 28.9" tall. Props to all those who contributed to the aforementioned thread and the info held within.

Calling upon the Mathemachicken in my head, I consulted magnethead's findings and formulas to come up with this equation to find out the speed in top gears for "efficient" piston travel.

4th gear with convertor locked up:
= (2175 / 0.69 / 3.55 ) * (28.9" * 3.141591 / 12 * 60 / 5280)
= 887.93 * 0.08598
= 76.34 MPH

3rd gear with convertor locked up (tow/haul mode):
= (2175 / 1.00 / 3.55 ) * (28.9" * 3.141591 / 12 * 60 / 5280)
= 612.67 * 0.08598
= 52.68 MPH

Without accounting for wind resistance, I took these values to come up with the scheme: <50mph, OD Off. 50~55+ terrain dependent (hills vs flat), >55 OD On. Interstate I run at 75MPH. This keeps my RPM range right between 1800-2100 depending on traffic. My route to work is about 26 miles, 8 of which are interstate.

8 / 26 = 0.3077 * 100 = 30.77% Interstate @ 75MPH OD on
The other 18 miles, or 69.23% is basic roads, between 35 and 55 (about equal parts 35, 45, and 55, however, traffic likes to stay at 45-50) so average @ 50MPH OD Off
So when it comes down to it, I'm actually running most of my trip with OD off, higher RPMs, right? Most people would say "LOWER RPM BETTER GAS MILEAGE" but the math actually works out. Engineering at it's finest. Also addressing takeoffs: Moderately hard, people around here are rude and race at every moment, so slow takeoffs can cause dangerous situations. However, I still do not floor it and race light-to-light, shift-point is about 2700-3000 RPMs.

Despite running at a higher RPM, it's basically "idling" at a higher speed, pretty much just resting my foot on the gas to hold it instead of having to push more air (thus more fuel) to lug the engine in OD, which at 45-50 when the converter locks up, is about 1400-1500 RPM. This is fairly luggy in the 3.9L V6. So: Effortless higher RPM vs Luggy low RPM, additionally, with OD off, keeps the piston speed at that efficient 1200fpm. Driving this way I found I have better throttle response, more readily accessible power, etc as it doesn't have to downshift. Also when using cruise, even on hills, it's able to hold constant speed without dropping speed, erratically downshifting to race up speed, or slam the gas to the floor, the power is already there to just carry the truck. Plus, it takes some mileage wear and tear off the OD unit, which as I understand, is usually the weak link in dodge autos.

Now to the point of it all: MPG numbers. What does all this gibberish mean, mathematics, engineering, piston speed, RPMs, etc. How does it actually work out? Better actually. Fuelly currently has me an average of 18mpg, I constantly float between abut 17.6~18.2 or so mpg. This tank, with applying the above principles, I fueled up and achieved 19mpg. That's almost a 1mpg gain, keeping in mind almost 70% of my drive is overdrive OFF 50mph @ 2100RPM.

End result: Actually using the engineering and maths associated with it equates to more available power, better performance, less wear, and improved gas mileage. I'll continue driving this way through another tank of gas to rule out the cleaner having affect.

Footnote: Was it the keeping piston speed in the efficient zone? Was it the fuel additive? Who really knows, I'll continue the driving style to see what happens. I can at least conclude that turning OD off under 50 for my setup does not adversely mileage, and saves shifting and miles on my OD unit. It was hard to force myself to change regular habits, but I'll keep it up. If anyone wants to try it out for themselves, please post your findings here. Just account for tire sizes and rear ratio in the equation above, and read the thread I linked earlier, it's full of neat info. I'll add more information if I can think of it or if someone asks.

 

Being in ATL, I. spend too much time idling in rush hour traffic (0 mpgs) to bother with mileage calculations anymore. Back when I did, I used to get anywhere between 15-16 mpgs with this 4.7. Cruise control is for the once in a blue moon long trip...can't recall the last one. LOL.

Hard to believe that higher RPM results in better fuel efficiency, but if it works for you, that's great. Seen lots of threads with people swapping out 45RFE TCMs with 545RFE, so they can gain 5th gear and lower RPMS in overdrive...and they do that to save on fuel, parts wear and engine noise at highway speeds. Yeah you might save "wear and tear on the OD unit", but that's at the expense of your entire drive-train and peripherals running at higher RPMs, no?

 

I would actually think that being in 3rd (1:1 direct drive) would be better on the drive train, as it's always what's recommended for towing. In 4th, the ratio is 0.69, causing the transmission (the 'weak' point) to spin faster, along with the rest of the drive train, negating the reduction in RPM. At 1000RPM 1:1 the transmission is spinning at 1000RPM, at .69:1 The transmission is spinning 1 time for ever .69 engine revolutions, Or Engine: 690RPM Transmission: 1000RPM. A difference of 310 RPM extra on the transmission. This is why when towing you don't want to use OD, faster, more stress, more breaking. So I would actually want my engine to run a little faster effortlessly (until it overruns the efficiency zone) than putting a 31% extra load on my transmission AND drive train. Engine RPM != drive train RPM.

 

Towing = greater mass = load. What's recommended for towing is different from unloaded, because towing creates drag. The tranny would keep downshifting to match engine speed with the rest of the drivetrain if you leave overdrive on. Then you have the benefit of engine braking to keep your brakes from being overloaded too. Without a trailer and at high speeds, you have only the mass and momentum of the car, so it's much easier to move the mass and a higher gear makes sense. Keeping your RPMS low is a good thing. If you're right, all the automakers and their engineers are wrong, including those that designed 5th and 6th gears on manuals. OD is there for a reason.

 

Overdrive is added to keep the engine in the efficiency zone at highway speeds. Yes, keeping the rpms low is good, but too low is bad. That's why I mentioned piston speed/travel. If the secret catch-all was low rpms, then eve 4 cylinders would run low rpms, but they don't. There has to be sufficient power to friction and weight. That's why gearing is as it is, because the design and engineers are right. They need a gradual reduction to keep the engine in the power/efficiency area. I was applying to straight efficient speed with my test, not counting wind drag or weight. At no point did I call the engineers wrong. Give the overdrive wiki a read, it's an interesting read, also check the thread I linked in my original post. It goes deeper into the efficiency dealings than I did here. That's what this entire test was about, keeping the engine at efficiency speed. I'm sure taking rpm into account if it's making sufficient power will help, yes. Say keeping it at 65 instead, that reduces piston speed and friction. I was doing a purely mathematical representation and showing my findings.

It's not a post saying not to use od, just testing a better "when" to.

 

Okay...from the Wiki article you mentioned... First paragraph.... "Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute (RPM), leading to better fuel consumption, lower noise, and lower wear."

And...

"There is therefore one specific gear ratio at which the car can achieve its maximum speed: the one that matches that engine speed with that travel speed.[1] At travel speeds below this maximum, there is a range of gear ratios that can match engine power to air resistance, and the most fuel efficient is the one that results in the lowest engine speed"

 

Yes, thank you for cherry picking information to aggressively prove that overdrive is a good thing... I don't want this thread to become a back and forth... :/

As far as my ignorance of the (5)45RFE, I did some studying to find it's infact, people putting the 545RFE TCM onto the 45RFE to "convert" them, as the 45RFE only has 1 OD gear at 0.75:1. Using the planetary gears the 545RFE reprogram to give another OD gear at 0.68:1, the same as the 42~6RF/E series. Checking several forums, it appears to shift into 5th at 60-65 MPH. The same gear ratio my 42RE shifts to at 40-45 MPH, which, causes me to lug on anything except very flat/downhill. Since we're cherry picking information from the overdrive wiki...

"The impetus is to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that modern automobiles tend to have larger numbers of gears in their transmissions. It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances i.e. where high (numerical) differential gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vehicles, often with a small number on the axle gear reduction, but usually only engage at speeds exceeding 100 kilometres per hour (62 mph)."

With my previous statement of people claiming the 545RFE shifts at around 60 MPH, confirms my findings above. Chrysler engineers moved the shift points around to keep the RPMs in the efficiency zone for fuel mileage and placed the OD accordingly. Which is what I'm trying to do. The problem is my 42RE lacks "4th" at 0.75:1 to fill in the 50-60 gap, which the 545RFE has. I assume the 545RFE shifts into 4th at about 40-45, then 5th at 60 (maybe as low as 55 with very light throttle). That's why above I've said I use 3rd until 50-55, then 4th (OD) above that.

Take it as you will, I'm not doing anymore back and forth, there's no point in it, we know that OD is a good thing and engaging it faster means fuel savings, if the engine is making efficient power. I want to get my thread back on track, since obviously Chrysler engineers already put my applied math into the 545RFE, it seems to apply to those with the 4XRE/RH series only, since we lack the median 1st OD.

 

Skree, aren't all thread discussions back and forth? Is it really cherry picking when I quote the basic principles from the opening paragraphs, of a page that you suggested that I go read?

I didn't take the discussion as a negative, so I didn't get worked up about it. In fact, I thought it was a healthy discussion and that you've explained yourself very well regarding the 42RE's disadvantage... at least in your latter posts. Obviously with the 4.7 and 545RFE, I don't have your setup and am not able to feel the lugging in overdrive that you detailed, which no math can describe. If running at higher RPMs will work out in a 1 mpg gas savings for you and you feel it's better for your drive-train because it's not lugging, then do what you feel is better for you.

My initial reaction was in part based on conventional wisdom and based on my own observations the few times that I've run with overdrive off (while not towing). My goal is reliability and longevity when it comes to this 4.7L and I see running higher RPMs as disadvantageous to that. As I was saying, my traffic situation being what it is, I don't worry much about mpg but even with that, my mileage is within average of what most see on the forum, even running in rural areas. I actually run a cold plug (NGK V-Power ZFR6F-11G), due to a small HG leak issue that I overcame a few years back and I'd rather keep cylinder temps to a minimum if i can. Keeping revs low ties in with that.

 

When I said 'back and forth' I was meaning nitpicking, there's no reason for that, back and forth discussion is what we're after. Only quoting parts that support your side instead of going through the entire thing is basically cherry picking. Especially since I was able to get my side out of the same article. Anyway, it just seemed like you were getting a bit aggressive with it like I was suggesting OD was bad to use. We reach an understanding that we both agree OD is a good thing to have, it's just when to apply it is the basics of this thread. We have similar goals of reliability and longevity, just as a personal opinion, I'd rather work on the engine than the transmission, especially with the reliability that's well known with the 5.2L and 3.9L LA/Magnums and the "fault" prone transmissions they put on them. From what I've heard, the design of the 545RFE is a bit better reliability and with the further expansion of gears, reduces the stress on it. If I had that, I would be inclined to balance them out a bit more. Of course, in the city driving you have, low RPMs is great, earlier OD is fine, since you're not pushing the truck against wind, etc.

I'm on the opposite end, cruising. With the higher, sooner OD, median RPMs are a bit better and keeps extra wear off my transmission, while just doing minimal extra on the engine. At the same time, the extra RPMs keep the cylinders cleaner at the lower speeds and in the 'efficiency' zone so the gas mixture burns more completely, less deposits. That's basically what "my" side was, I was just taking in mathmaticals and applying without taking the other factors in. It gives me a baseline to work with to factor in things at a future date to see what has what effect. I'm sure if I had a 5.2 instead of the 3.9, despite the efficiency zone being the same (same stroke, same piston speed), it produces more power, so I could lower the piston speed (RPMs) and get the same results. That's also why the formulas were tossed in, figuring out at what speeds keep it at the efficiency zone, which engineers found between 1000-1400FPM mean piston speed, those calculations, thanks to magnethead, are a middle ground of 1200FPM. In the linked thread, they do speak of the 4.7, but I didn't pay too much attention to that section, as I don't have the powertech series.

Guess it comes down to the fact we were on the same side, just looking at different angles. Although, if you do mostly stop-go city, turning OD off might help you a bit since you're not often getting really into OD speeds, but it shifts more often. Might save a bit of wear and tear on your drive train to turn OD off in say, 35-40 zones only, so the torque converter locks up in 3rd and gives you the 'manual' trans effect. It'd pull your RPMs down, and you'd get direct transfer of power without TC slip (since city driving I wouldn't think you reach lockup speed in OD). Especially if you're running larger tires, slippage == heat == bad for transmissions. Couldn't hurt to try it for a tank and see what it does.

 

Typed up a worthy response and lost my text. LOL. Anyway, I'm good with the way the 545RFE engages OD. It's actually a pretty robust unit. If I may share...

https://www.allpar.com/mopar/transmissions/545RFE.html