MPG Gains with OD trans?

[drajj5]

I am thinking of doing a five speed swap. So my question is,
" Is there enough gain in MPG to offset the cost in the swap? "

For those that have already done this swap, please give your advice..... It doesn't matter weather you have done a manual or automatic swap.

Thanks for your input!;D

 

[chloichina]

I have a 60 pickup with a .68 OD and I get around 26-28 MPG with 30 inch tires. 3.50 rear gear and the truck weighs around 2800 lbs. As long as I don't run it hard and drive like my wife wants me to. %)

 

[broncnaz]

The only way your going to gain mileage with a OD swap is if you have to low of gearing now. Which is not the case for very many people with broncos. Figure most people run larger tires and stock axle gearing which already gives them a slight OD. To really do a OD swap axle gearing also needs to be changed. Thats one reason my ZF tranny is still sitting in the shed as I would have to swap axle gears to use OD. All in all after the swap I wouldnt expect to see more more than 1-2 MPG increase if your lucky. You best bet for increasing mileage is to get the engine tuned correctly. My other suggestion would be to get a camshaft matched to your driving needs so the engine is in the powerband when you need it.

 

[drajj5]

I own 2 broncos and am considering converting both to Manual 5 speed O/D.

77' 351/C6 with stock 3.50 gearing (was standard trans when stock)
33x12.50 A/T tires

69' 289/3 speed stock 3.50 gearing
P235 75R15 A/T tires


This is a his and her's Bronco's both are daily driven. Mine seem's to run at a slower rpm down the highway, But I want the git-up-and-go that her three speed has... Her's seem's to want another gear when traveling down the road-has plenty of power there for another gear. 65-70mph motor would be happier with O/D at these speeds.

Hoping that I will get input, from those who have performed the O/D swap.

 

[73stallion]

i didn't notice any when i went 5 speed, and i have 4.11 gears with 33's. only way i figure i'll see any is with EFI, which is on my list down the road. i can get about 12 or 13 highway if i stay around 65mph right now.

 

[blazinchuck]

i took a trip last week...usually i would get 11.5-12mpg ...i got 13.7-14 this time.
i have 5.0 EFI 3.50 gears/33" tires. before i had a C4 getting the 11.5

 

[AZ69EB]

I have the zf 5 speed, 4.88 gears, and 33" tires. I can cruise at 65 mph at 2500 - 2600 rpm. I can pass anyone if needed without the engine screaming too much.

As for gas mileage, it is in the mid to low teens. I really don't know and don't care, as it is a recreational vehicle.

I just drove it from Phoenix to Borrego Springs and back last week for the 45th EBR event. This was about 650 plus miles road trip without the wheeling mileage.

It drives down the highway great.

Now, I want lower T-case gears. It never stops.

 

[Broncobowsher]

Same Bronco
Started as warmed up 351W/NP435/4.11/33s
Ended with the same 351W/4R70W/4.88/33s
I gained maybe 1 MPG with the huge drop in RPM.
Will it pay for the OD, NO!
But I did gain lower gears that made it a lot more fun on the street and the trail. The lower highway RPM made it more enjoyable to drive on the highway. Having driven C4 Broncos I found it a lot better overall driving experience for being automatic overdrive vs. non-overdrive. It allowed the ample torque to drive the Bronco at lower, more comfortable RPMs while still giving very good low gear acceleration. Something the narrow ratio spread of a non-OD can't do.

For a stick shift I remember the old 3-speed in the first Bonco, and the NP435 in the one mentioned above. Neither one would have appriciated a complete extra gear. That is an overdrive alone just would not have worked. Regearing the axles would have been needed to have been able to use the overdrive in both cases. Most Broncos I have been in could not really use an OD right out of the box. As much as you think the engine is screaming, another gear gear would be too much.

 

[broncnaz]

77' 351/C6 with stock 3.50 gearing (was standard trans when stock)
33x12.50 A/T tires

69' 289/3 speed stock 3.50 gearing
P235 75R15 A/T tires


This is a his and her's Bronco's both are daily driven. Mine seem's to run at a slower rpm down the highway, But I want the git-up-and-go that her three speed has... Her's seem's to want another gear when traveling down the road-has plenty of power there for another gear. 65-70mph motor would be happier with O/D at these speeds.

Hoping that I will get input, from those who have performed the O/D swap.

Doubt neither will see any benefit from a OD swap alone. Your should run less RPM's down the hiway as the 33in tires give it the slight OD. You also have to consider that the C6 can use up as much as 60 hp alone and the bigger tires and 3.50 gear also take more power to turn so that may be where your 351 is falling short in the power department again a camshaft matched to your gearing and driving needs would be a bigger help.
As for the 289 powered bronco with its smaller tires well thats what the smaller tires will do with those axle gears want to do a test put the 33in tires on it see if its still seems like it wants another gear. The tire swap alone will be like you put in a OD tranny although OD would probably lower the RPM about 200 RPM more.

keep in mind that 289's like to rev a little so really you need to keep the RPM's up to stay in the powerband with them where with a 351 you will or should have more lowend torque. IMO in either case axle gearing will have to be changed to really run a OD tranny and see any gain or at least stay the same.

 

[okorangebrnco]

No luck for the price off set, but I did go from 6-8 to 11-12 possibly a little more if I really calculated it. 351w & NV4500

 

[blubuckaroo]

You ar gonna get your best efficiency when the motor is running at its maximum torque RPM. If thats around 2800 RPM then that's where you want to gear for. If you want the best milage you can get, set up the gearing so it is happy at around 60 MPH. Anything faster than that and you really couldn't be concerned about the mileage anyway.

 

[Broncobowsher]

You ar gonna get your best efficiency when the motor is running at its maximum torque RPM...

The missing information in that line is "when running at full throttle". The peak effiency line you have quoted comes from industrial engines running at full load.

Take and engine and set the throttle at something like 25% and do a dyno curve at the fixe throttle setting. The torque peak will now be down at something like 1800 RPM. The peak value will be down as well, but that is expected at part throttle. At that throttle setting the peak effiency will likely be at 1800 RPM as well. so part throttle cruise at 1800 will be more efficient then part throttle cruise at 2800.

Fuel economy (effiency of an engine at the simplist of that) has many variables. Trouble is when a good reference material gets dumbed down to a simple statement which is very good in its original content suddenly isn't as correct when extra variables (like part throttle) are put into play.

If the peak effiency line were true my daily driver would get better gas mileage at 60 if I left it in 3rd gear on the highway. While it is a blast to drive that way, throttle response is awasome when cruising at the torque peak, fuel economy drops a third. That is why they give me 4th, 5th and 6th gears. 3 more options that each get me better gas mileage. I have also had truck running larger tires on stock gearing. In that case the overdrive was too tall, lacked driving force in OD, I could sometimes get better mileage by dropping a gear on the highway. Running in OD at high load at too low of an RPM being too far away from the efficient zone was worse then the extra RPM.

 

[blubuckaroo]

I'm just trying to say that many Broncos are geared too high for good economy because their gear ratio is too fast for the aerodynamics of the car.

 

[drajj5]

So.... from reading the post(s) above.

There is "Zero Gain" in MPG in the overdrive swap. I certainly would not have guessed that! I guess the only practical reason for the swap is to gain the lower initial gears that a typical 5 speed has.

I think that my wife will retain the factory 3 speed in her bronco

As for mine, that C6 has got to go. I know there is power in the engine however a ton is being soaked up through the automatic trans. I want to do the 5 speed swap like so many others on this board has done but am now reluctant to do this from what has been posted in this thread.

I have a np435 in the barn I may install just to get that power back, like my wifes 289 has.

thanks

 

[Broncobowsher]

The gains are not in the fuel economy, they are in drivability.
With a 3-speed you have a 1st gear that is a bit high. You run a little deeper axle ratio then ideal so you can pull grades in top gear and to help take up the lack of a deep 1st gear. So at any given speed you have 1, maybe 2 gears you can be in.
The NP435 simply adds an extra deep gear in front of the 3-speed gearing.

Now lets look at a newer 5-speed. you can gear a little lower in the axle. The lower gears of the transmission are easier to drive this way. At highway speeds you drive at a lower RPM in OD. But, you have the option to downshift a gear. Need to pass, pull a steep grade with a load, drop out of overdrive into 1:1 and the extra gearing will wind out the motor for that extra power. then back into overdrive. That is where the overdrive shines. Back when I had the NP435 and 4.11s I could cruise the highway, but I only had one choice in gear, top gear. If I lacked power on a grade or wanted to pass someone it had to be done in that single gear or slow down enough that I could drop a gear and really listen to the engine scream. After the 4R70W swap (went to automatic as well as the overdrive) and even lower axle gearing I could cruise at lower RPM and be more comfortable. But when I wanted to pass that semi on a 2-lane road I had a gear I could downshift into in order to get the passing power I wanted.

So there are gains that can be had. fuel economy is minimal, overall driving is experince is a lot more. Depending on the starting point setup as the reference point for fuel economy gains. If you have 4.56 gears and stock height tires then an overdrive will be great. Few of us are that far off as a starting point.

 

[drajj5]

The gains are not in the fuel economy, they are in drivability.

I follow you, however I was looking for the best of both world's(economy/drivabilty). I chose this type of automobile based on its simplicity, 4 wheel drive, and came with zero car payment.

Perhaps there is some magic sweet spot that has been done through gearing/transmission's that could warrant 15mpg. Which seems to be industry standard on most 4x4's sold today. My buddy's 08' toyota just squeeks by the above number, and it's fairly 'new'.

Both our Bronco's will be a part of our lives for many years to come....Mine gets played with, and hers gets groceries. Either way we have yet to get stranded in inclimate weather;D

You seem to have found the sweet spot in your rig, albeit using a auto trans. It goes to show that your mpg gains however miniscule, was the result of the modifications you have made.

I realy wanted to copy
nv3550 Tech article by Larry C. (MarsChariot)

But must convince wife that it is worth the $$$$

 

[okorangebrnco]

Like I said, I gained MPG with my swap, but I also had a C4 with 35's and 4.56's. At 65mph I was screaming at 3000rpm. If I were doing 55mph I could get almost 11mpg. It will all boil down to your gear choice and what trans you opt for. If you are very conservative in driving, you should be able to get 15mpg in your Bronco. Just have to play a numbers game and look for ideal running conditions.

 

[AZ69EB]

Another option for the 435 is a range box in front with the choice of an overdrive or under-drive gears. Most people pick the overdrive option unless you need to crawl.

The overdrive gears will let you split the gears more efficiently and provide a true overdrive while still having one of the lowest 1st gears available. The range box is all gears and is rock solid with another shifter leaver.

Now you have the standard 4 forward gears and reverse or an option for 8 forward and 2 reverse gears. You can split each gear.

The 3rd gear, then 3rd gear with overdrive, 4th gear and 4th gear overdrive is great for passing and cruising. Most people complain about the large jump between 3rd and 4th gear in the 435. This solves this problem efficiently.

It also keeps the drivelines shorter than some of the other options like the ZF5 or 4R70W. You still have room if you want to add the 203-range box behind the 435 to under-drive all gears again for crawling.

The gear choices would be crazy at that point, but impressive for both on road and off road activities!!!

We have two members in the AZ Classic Bronco Club with the range box in front of the 435. One has a D20 and the other have an Atlas behind the range box / 435 combinations. Both love it.

Just another option to think about.