1) never tow with overdrive? (why not is it bad for the eng or transmission?)

2) overdrive saves gas? (yes it does anytime the rpm is lower less gas is used to attain any set speed than higher rpm's) so why don't they make 12,20 gear's in a transmission's to keep rpm's low so when i drive @ 70mph i get better mpg? (they do for tractor trailers)

the government standerd is 55mph for mpg so manufactures care nothing about mpg anthing over 55mph.

3) I have a 89 f-250 with a jasper manual 4 speed with overdrive (5thspeed)
the rpms are not consistant at 45mph 1000rpms at 55mph 1100rpms great but some times in overdirve at 55mph the rpms are 3000 wtf is this over drive a gear or must my rpm meter way off how is this possible?

 

Overdrive has no mystical magical voodoo about it, so you can just say a five speed tranny. Just so happens that the first three gears are underdrive, fourth is directdrive, and the fifth is overdrive.

1. For automatics, only lock out overdrive if it hunts between the top two gears, which usually happens at lower highway speeds loaded down. This hunting creates heat which is a killer. For sticks, if the motor has the torque to hold yer speed in OD, keep it there and save gas.

2. Money, those trannys are usually 5-8 speed units with a secondary tranny behind it with two or three speeds to get the bigger batch of net gears. This is usually overkill for normal light pickup drivin', and for those who need more Gearvender is a popular unit aftermarket. Got a couple thousand sittin' around? Go fer it.

3. You didn't say what your rear gears are, tire sizes, or motor. Also if you have the 302 and a factory five speed, it would be the M5OD-R2 tranny, and not the ZF-5 S42 that came with the 300/351/460/7.3L. It matters to know which to determine the ratios at a certain speed.

Adrianspeeder

 

2) Lowering the RPMs does NOT necessarily save gas. Assuming the truck's weight, drag, rolling resistance, speed, & all other factors are the same, it will burn virtually the same gas in either D or OD. But over a VERY LONG haul, and given specific conditions, the engine might be SLIGHTLY more efficient at a particular RPM & load, so one gear might show a TINY improvement over the other. It could go either way, though: D might be better than OD.

In a higher gear, you turn fewer RPMs to go the same distance, but the engine has to produce more torque (more load = more gas), so it washes out.

3) Your clutch might be slipping.

 

I respecfully disagree the less time's you suck, squease, bang, blow. the less fuel/air you use peirord.

If i remeber correctly from 20+ years ago, the term is volumetric efficancy you are confusing with the simple fact higher RPM at set speed uses more gas. describing load and fuel effcancy like flooring it to maintain speed up hill you will use more gas than maintianing speed rolling down hill yes all this is true.(Load changes)

however on a level road with a constant load and speed:
in 3rd gear at 55mph and 6000rpms= 4 gal of fuel per hr=SP= 5
in 4th gear at 55mph and 3000rpm's= 2 gals of fuel per hr =SP= 2.5
in 5th gear at 55mph and 1000rpm's= 1 gal of fuel per hr=SP= 1.25
i wish 6th gear came stock=1/2 gal per hr =SP=.625


the extra fuel used in 3rd at 55mph gives you more SP(spare power) or unused energy or torque all this at set speed(55) flows out your exaust pipe and your wallet.
you may increasse the RPM power/acceration untill red line then engine explodes other side is when the SP value to 0.000000000 engine stalls

all the numbers are not real world they are just an example for ease of explanation

I think you are talking is strictly about the engine, what i call the sweet spot. this is an rpm and set load where the engine has the best air/fuel to power ratio or the highest volmetric efficancy.

usually around 2000rpms you would need the results from a dynomomerter test find it you cannot mesure this without a dyno test stand.

so then to get the best mpg you keep your engine at 2000rpm and shift gears for speed changes but the kiker is you have to shift and keep the SP as low as possible or esle fuel/power is wasted.

engines for large electric generator's use set Rpm at or close to there highest volmetric efficancy.

all this i have not thought about in many years please correct me if i am wrong or misleading anyone.

 

Modular engines don't apply to the same rules as older pushrod larger cubic inch engines. These engines need rpm to make torque. The closer you keep them to volumetric efficiency the more efficient they will be. Danco? Is that Danco of Cincinnati?
Alan

 

I am danco of NY/VA new to these fourms.
i think the rules are the same a modular engine is in the way it is built in the factory.

 

What I am saying is that keeping a 351 running at 1500rpms is good for economy, keeping a 4.6 running at 1500 is bad for economy. The load is too much and you have to keep your foot in it to get anywhere. Everyone kept telling me not to change gears because it would hurt my economy too much. I changed from 3.08 to 3.73 and around town economy has increased. Even on 55mph backroads it is about the same. The only loss I see is on the highway.
Alan

 

i am not fully follwing you,

keeping a 4.6 running at 1500 is bad for economy. changed from 3.08 to 3.73 and around town economy has increased.

does the new gears lower or raise the engine RPM at set speed?

(i use a handheld gps to test speed, i realize the speedomerter being off due to, tires, gears.)

 

I changed my speedometer and tire size to get perfect speedometer calculations. The lower gears (numerically higher) increase rpms. Check out even a stock 4.6 on a dyno at 1500 rpms, they make very little torque there. Around 2000 they wake up quite a bit. The 351 and 302 engines made considerably better low end torque. The 300 I6 was even better but it did not make as much horsepower.
Alan

 

so you and I agree. that if you drive and keep your engine rpms at or below 2000 you will see a mpg increase regardless of road speed you are doing.

"The only loss I see is on the highway." I asume your rpms are at 3000,4000
5000 on the hwy.

 

Do you suppose that by changing the gear, you have altered your driving style around town? Meaning do you open the throttle less than when you had the 3.08 gears? That would explain the difference in fuel mileage.

 

The bottom line is that there is no direct correlation between engine RPM and fuel consumption because there are too many other variables in between the two. Running a truck engine at 6000 rpm with no load takes WAY less fuel than running that same engine in a 5000 lb truck pulling a 9000 lb trailer at the same RPM.

As was already mentioned, there is no easy wya to quantify the connection between fuel consumption and any other operating condition. However, a halfway decent seat-of-the-pants way to gauge it is to try another gear and see... In general, the gear that requires less go-pedal to maintain the speed will net you less fuel consumption and, therefore, more gas mileage. (there are, of course, always exceptions)

-Joe

 

there is a direct correlation between engine RPM and fuel consumption and consistant LOAD or work preformed per gal of fuel. it is not to complicated or some sort of magic.

think of your4 cycle internal combustion engine as a pump an air/fuel pump.

the faster you hand crank it over it turns, inlet side you suck in air/fuel--outlet side, faster you exaust. more rpm more fuel/air pumped.


now back to it being an engine. load comes into play work needs to be preformed.

inside your combustion chamber you have a fire cracker when it blows it pushes the pistion down turning a crankshaft. if no pump is engaged on the crank it turns fast(no work preformed). next firecracker you engage the clutch to the pump (work prefomed) it turns slower due to work slowing it down.

it is exactly the same size and type of firecracker the first one was wasted in rpm not used to its full potental.

the second did the work.

if you increase the load (pump mud now instead of water) 1 fire cracker does not move the pistion down.(stall or slow down) so you step on the gas and now two firecrackers drop into the pistion blow and move the crank.

obviosly the firecracker is fuel.

now work preformed must maintian 55mph on level road pulling a 5000lb trailer with a staedy 10mph headwind. in other words a steady load at a steady speed 55mph.

in 3rd gear at 55mph and 6000rpms= 4 gal of fuel per hr,lot's pump out exaust
in 4th gear at 55mph and 3000rpm's= 2 gals of fuel per hr
in 5th gear at 55mph and 1000rpm's= 1 gal of fuel per hr

the lowest rpm you can acheve 55mph with constant load you will have the best mpg.

now yes volumetric efficancy has a role in this so does the size and weight of your flywheel. and you engines (bhp) (Brake horsepower)

however I still stand by that if you drive and keep your engine rpms at or below 2000 rpms you will see a mpg increase regardless of road speed you are doing

 

Absolutely not, I drive ***** to the wall all the time. I just get to the speed limit faster and can ease out and cruise more.
Alan

 

Ya'll are missing an important point. Anytime a GASOLINE engine is at part throttle, it is operating under REDUCED efficiency. Because your throttle body is having to choke the engine to keep power output levels low.

Take a 4.6 and a 5.4 for example. The 4.6 will always get better fuel mileage under the same load positions, because the throttle body will be open more, and you will have a higher initial pressure in the comubstion chamber. Hence less drag.

So, theoretically, if you changed the axle ratio to the exact ratio that would allow the engine to run with the throttle body completely open, you would be getting MAXIMUM efficiency. In effect, with a very high gear ratio, the 5.4 would get practically the same fuel mileage as the 4.6

That's why aftermarket air filters don't increase fuel mileage. They have nothing to do with it. The throttle body dictates effeciency. That's also why, when you're pulling a trailer, efficiency/lb increases. Cause you open the throttle body more!