I am trying to justify swapping my C6 for a ZF. I was hoping that such a swap might produce three more mpg on the freeway.

Does that sound reasonable?

Would anyone venture to guess as to whether I would see a change in mpg around town.

 

On Highway depends on what is your RPM at 70 MPH with the C6, so we can calculate your new RPM at same speed then will know.

At city driving under 50 MPH or in traffic you will notice a MPG reduction since auto trannys are more efficient than manual

If you want more MPG get an auto-tranny with Overdrive OR you can get and external overdrive to attach directly to C6

 

Auto is more efficient than manual? 1st time I heard that. C6 is not an efficient tranny, bulletproof almost, but not efficient.

 

Speaking from the "pure energy usage" point of view, yes manual are more efficient BUT...

From the point of view of fuel usage, in some driving conditions Auto-Tranny is more efficient for 2 main reasons:

1° Auto has T-Converter, it makes a better job to put and keep in movement the truck, than a simple clutch.

2° If you are driving for example at 50 MPH and you release the gas pedal, Manual tranny keep the engine "locked" with rear axle, then RPM still high, of course; there is a breaking action from the engine BUT...

the C6 Auto tranny can let the engine RPM be reduced to almost Idle then extend engine life and save fuel, almost no breaking action.

To get a Real World comparison and see an important difference between both trannys, you have to drive long distance at constant speed or spend several days driving in heavy traffic...

 

New 4, 5 or more speed OD transmissions with lockup computers can beat a stick in the EPA cycle, and will beat them against many drivers in the real world.

The automatic is always in the highest gear it will pull, and the lockup gives you the same direct as a clutch.

A C6?... Maybe in some very limited situations, and highway running would not be one of them. (vs. an OD 5 speed manual with the same axle ratio)

 

don't know, but the auto dump trucks at work do better on fuel mileage than the 10 spds, guess there is no beating auto verses human shifting for economy. as far as pulling a load, the 10 spds blow the auto's away in power, and get-up-and-go

 

Yes right! just because the C6 maximum gear ratio is 1:1 and the typical overdrive is about 0.70:1

If you want make a real comparison between both trannys driving on highways, then just drive the the manual in 4th gear which is 1:1 >>> same as C6. Then you will know the difference is not because one is auto and another is manual, the difference is because the overdrive is NOT present on a C6

Remember my initial post was initiated ASKING what's the actual RPM at 70 MPH to calculate new RPM with an overdrive (Manual or Auto) then I can estimated if the 4.9L will be running in a good RPM to get better economy.

In my experience with F150's I can say the best economy we got here (Venezuela) was about 17 MPG with this configuration:

4.9L carby + AOD tranny + 3.55 axle + 235/75R15 tires + 91 Octane fuel (stock truck) this put the 4.9L running about 1.800 RPM @ 70 MPH

I'm sure the configuration of "F-250 Restorer" will increase the MPG on highways with an Overdrive, no mather Auto or Manual.

 

I have a Bronco with the M5OD(Mazda) and EFI 300 with 3.08 Diff. When I'm driving at 65-70mph, I'm turning about 15-1800rpm in 5th.

Not really certain as I'm still studying the differences in power/economy compared to throttle and engine speed. But my simple logic would be that the slower the engine turns, the less fuel needed to turn it. Unless you are stressing the engine at a low RPM/high gear ratio and bogging it down. Also, I would take into consideration that fact you can "coast" alot better with the Manual that with an Automatic. I do that alot when driving around town as there's no reason to be running the engine at 12-1500 rpm when you're coming up to a light in traffic that's most likely change or when you see the cars have yet to proceed and you';ll drive right up to them unless you begin to decelerate. I kinda took tips like that from Hyper-Milers. I know my Bronco can coast at or near speed pretty well for quite a distance. I can also remember my fathers old '67 F100 with 3"on the Tree" that seemed like it would coast forever on the highway.

You just can't really do that with an Auto without risk to the transmission over the long haul. I've done it a few times with Autos and just hate the way the transmission re-engages from Neutral when you're still moving. Some of the newer computer controlled units can do it smoothly. The older valve driven units fell and sound like they're slamming into gear.

 

javi 2001. I think you need to go back to school.

Real world experience has shown consistantly that manuals out perform automatics in so many ways.

1)You do not burn any more fuel in a properly tuned carb or EFI engine when you let off the gas in gear. The fuel is sent based of of airflow not vaccum. you are proboly sending about 110-125% of your idle fuel flow with the butterfly closed and the engine in gear. Your kickdown will hold your butterflys open slightly to prevent backfires on an auto.

2)You loose an accepted minimum of 50 rpm on a new torque converter to fluid slosh. on a realy worn out tranny or engine it can be upwards of a thousand rpm. A typical automatic will have an rpm loss of 300 rpm at 100k. I belive ford offered a C6 with a lockup but I highly doubt F250 has one.

I need you to explain to me why a torque converter does a better job of coupling than a simple clutch. That one is too incomplete to even fantom where your thoughts lie Torque converters slip? Clutches dont? What is your logic?

You realy did not think this through did you?



F250- Depending on your final drive ratio you do stand to see an increase. If you have a high ratio of say 2.79-3.30 I would honestly belive you would see better milage and driveability out of a four speed trany like a T18. The OD would turn your engine a bit to low and lug it hard especialy against hills and wind. Hurting your milage therin.
3.55 and lower ratios would probobly be a boon for you. A 3.73 with a .75 OD puts you at about 2.79 total final which is agreat unloded crusing gear for a light truck.

My 96 i recently bought is pulling in the 18.5 avg at 55mph in OD, thats with a 3.08 rear. My old 96 with a 3.55 rear pulled 20.3 avg at that speed. Same 4.9, same tranny.

 

I'd agree with all of the above. My current trucks are basically identical other than the '94 came with a M5OD and the '95 with a E4OD. The auto is ext cab, long WB, bit both have the same EFI 4.9 and the same 2.79 axle. There have been several other 300's in our family over the years, all carbed, all sticks. My '95 has always gotten the worst mileage of the bunch at 15/16. The best I got out of the M5OD truck was 19 on long hwy trip. Typically it got 17, sometimes 18. The best was my dad's '82 carbed, 3-on-the-tree with a 2.79. Weak for towing, but consistent 21, occasional 23MPG. My brother had a nearly identical one with the granny 4 spd. that got around 20/21 with 3.55's. We've both seen changes to lower gears result in the same or higher MPG's simply due to the fact that many engines (the 4.9 being one of them) really don't like to be lugged. Even turning more RPM, sometimes the efficiency is enough greater to offset it. I think in the case of F250 Restorer's truck he would still see an overall increase in mileage however. If nothing else, the ability to go into OD at highway speeds would allow higher/more comfortable speeds at the same RPM. Besides, having more gears is never a bad thing IMHO. The mazda tranny in my '94 took a dump (thanks crappy rubber plugs) a while back. I couldn't find an affordable M5OD or ZF, so I put in a Free-to-me 4spd w/OD out of an early 80's. Couldn't find a bellhousing with the built-in bracket for the slave cyl. except a new one for $211 at the Ford dealer, so we rigged a bracket which has worked great. The only problem is that the 1st gear of the tranny is so high that I have no bottom end power now. I found a complete 3.55 rear for cheap I was considering buying, but I think I've found someone to build the Mazda tranny fro cheap, so holding off. I may get it anyway as the low 1st along with a 3.55 would make a better puller than it used to be. If you put a ZF in with the lower gears it should pull better than now and pick up MPGs/highway cruising speed as well. Win/Win situation in my book. Ditto the TC info. All slip some, whereas a clutch doesn't (unless broken ). If you decide not to bother, just ship me the ZF. I'll put it to good use!

 

BTW, despite my love of EFI and all of its' benefits, the EFI 4.9s seem to consistently get worse mileage than the (stock) carbed versions. Theoretically they should bet better, but my theory is that the EFI keeps the mixture within "safe" limits, while many of the carbed trucks were tuned extremely lean. In a post I left a few days ago I recounted the destroyed pistons in my dad's 82 with only 130,000 miles. After rebuild I richened it up a bit and no further problems. It is something to think about, however. Also, I heard something several years ago that I've always wondered if it was true, but never got validation: I heard it said that an engine's torque peak was essentially it's efficiency peak. Don't know if there's any truth to that, but it seems to make sense. If this IS true, however, a modified engine with a torque peak at a higher RPM should get better MPGs at a higher speed (or with lower gears) than a stocker with higher gears. If anyone has any input on this theory LMK. I kinda HOPE that's the case as I've just finished re-building/modding the 4.9 in my '95. I ran into some issues that I think are resolved now and hope it will be making it's 'maiden voyage' this afternoon. Since I hardly ever tow anymore, and frequently cruise at 80MPH, I'm hoping maybe I'll pick up a bit of mileage. That said, the same mileage with a little more fun wouldn't be bad either! Any other real-world validation of lower gears resulting in better mileage would be great. Also, while there is plenty of other stuff out there, I wrote a simple VB program a few years back to calculate RPM/speed based on gearing and tire height that I'll send to anyone interested. Based on simple formula here: RPM x Tire Height(in inches) / 336 x Final Drive Ratio.

 

Ok, I'll give you my opinion, since everyone else seems to missed a point or 2 in this dissusion.
Yes, it's very likely you could gain MPG by going to a ZF from the C6. It could be as little as 1 MPG, or more than 5 MPG. It's impossible to predict with any accuracy. Driving habits and terrian have a large role on MPG.
The list of mods to the C6 you have posted in the past tells me that it was built for massive strength, and should be capable of living a very long life, so the relatively low miles you have put on it would likely yield a decent price if you were to sell it after doing such a swap, possibly even bringing enough funds to recoop the money invested in doing the swap.
The real question here would be, will the MPG gained be enough to justify the hassle of performing the swap, considering the hydraulic clutch and other fabrication work needed to put it into the earlier model truck.

A C6 (or any decent auto) has one advantage over a manual. Heavy loads are easier to get moving, since the TC is designed to have some slippage in it, as well as having the ability to multiply the engine's torque while untracking a heavily loaded vehicle.
Slipping the clutch to get a heavy load moving will eventually wear the clutch out. On the other hand, a TC will generally outlive the rest of an auto trans, but should be replaced when the trans is being rebuilt.
Speaking of the TC, did you use an aftermarket TC, or the stock low stall speed TC for the 300? If you have a higher stall speed TC, you won't get as good MPG as it could, with the lower stall speed specified by ford for the 300 and diesel engines.

 

Thanks, everyone, for all the good info. I will speak to my builder regarding the TC.

I should have mentioned my rears are 3.31's.

At this point I don't know if I will do the conversion, but it is educational to hear the pros/cons.

 

I don't know how traffic is where you live. I do know if you have alot of stop and go and backed up freeways standard shift can be a PITA.
I once drove my 4 speed 81 through ungodly Atlanta traffic to help move my daughter back home. If I lived there I would not have a standard shift !

 

I have gotten about 17mpg summer with my 85 I have used for the past few years. That is at 55mph though. In town and winter highway driving is about 13mpg.

This is a truck with 3.55s and a NP 4spd. I have the entire engine biased for torque under 2k rpm.

I think the EFI 300 is great for driving. Hauling and towing the carbed engine is far superior though. My Old 96 got about 20mpg at 55 this new one is about 18 mpg.