RPM in relation to speed

[Ozwynn]

Solve this argument

and post a link to go with your solution

My brother and I almost came to blows over this

RPM is constant at any given speed. 1700 RPMs at 55 in your top gear will always be 1700 rpms..... wether you got 1 pound or 50,000 pounds on that engine.

he claims his RPMs jump 200 when pulling his trailer with his Tundra. I am telling he has a problem because it is not supposed to do that. either his transmission is slipping or the TC is unlocking.

 

[Tominator]

IMHO, you are correct...this is assuming a standard transmission where there is a direct correlation to RPM v. Speed because of a mechanical drive. That is no loss to friction.

The Tundra is probably an automatic. The computer varies the RPM based on load. There is no mechanical hookup. The Tundra's computer can increase the RPM to offset the perceived load and.

No link....just my own theory. Being able to program load V. shiftpoint is rather commonplace today.

 

[Ozwynn]

it could not do that unless the torque converter is unlocked


but anyways I would need a link.... I showed him. I hooked my 1 ton up to my 5th wheel and took it down the highway... and then we did it empty..... then we used a lighter trailer (his) 55 mph in all 3 cases with an E4OD was 1850 RPMs...... he still says I am wrong because his dodge at work (2012 5500) has significantly more RPM when heavily loaded and that is a 6spd automatic.....


like 3000 rpms ....... I didn't know a cummins 6.7L revved that high

 

[adsm08]

And if the computer senses too much load it will unlock the converter. It has the option of doing that you know.

 

[Ozwynn]

yeah I know, and that is what I am trying to tell him.

 

[exbass94]

Explain it to him this way: "What do gear ratios mean?" If the transmission's overdrive gear has a 0.86:1 ratio, that means (assuming the TC is locked) the driveshaft WILL spin 0.86 times faster than the engine. PERIOD. And, if he has say, 3.73 gears in the axle, that means the wheels ALWAYS spin 3.73 times slower than the driveshaft. PERIOD. Therefore, if the wheels are spinning at a given RPM, the driveshaft WILL always spin 3.73 times faster, and the engine WILL always spin 0.86 times slower than the driveshaft.

 

[Ozwynn]

I am just going to take his truck and get it looked at ........ it is easier that way...... I am also going to put a transmission cooler on it for him because it will also be easier that way.

 

[Ozwynn]

I am just going to go get his truck looked at. It will be easier that way so I know how long it will before I have to help him put in a transmission.

 

[Jraymer360]

What i don't understand is how load decreases gas mileage.

is it because it takes more rpms to get to the same speed at the same amount of time vs. empty?

or because of longer acceleration ..or what.

i don't see how a 10k pound vs a 2k pound car with the same drivetrain ( completely same drivetrain) would get different gas mileage at the same rpm.

well i understand that a heavy car would get worse gas mileage but i don't understand why. its not like a 10,000 pound car is dumping in more fuel than a 2,000 pound car at the same rpm?

 

[Ozwynn]

yes it is. the more you weigh, the more power you need to move. takes fuel to make power.

so you might be turning the same RPM at a given speed in a given gear but you are using more fuel to turn that RPM.

 

[Insanejughead]

Explain it to him this way: "What do gear ratios mean?" If the transmission's overdrive gear has a 0.86:1 ratio, that means (assuming the TC is locked) the driveshaft WILL spin 0.86 times slower than the engine. PERIOD. And, if he has say, 3.73 gears in the axle, that means the wheels ALWAYS spin 3.73 times slower than the driveshaft. PERIOD. Therefore, if the wheels are spinning at a given RPM, the driveshaft WILL always spin 3.73 times faster, and the engine WILL always spin 0.86 times faster than the driveshaft.

You're thinking backwards.


Let's say (just for ease of numbers) that a five speed transmission has the gears laid out in these ratios:

1st = 4:1
2nd = 3:1
3rd = 2:1
4th = 1:1
5th = .85:1

In 1st gear, the engine turns four times for every one turn of the driveshaft.
Three turns of crankshaft to equal one of the driveshaft in 2nd gear.
However, when you get to overdrive, the engine will turn slower than the driveshaft. Otherwise, you don't have overdrive.


(I just wanted to get that out of the way to save confusion for whoever comes across this.)

 

[exbass94]

You're thinking backwards.


Let's say (just for ease of numbers) that a five speed transmission has the gears laid out in these ratios:

1st = 4:1
2nd = 3:1
3rd = 2:1
4th = 1:1
5th = .85:1

In 1st gear, the engine turns four times for every one turn of the driveshaft.
Three turns of crankshaft to equal one of the driveshaft in 2nd gear.
However, when you get to overdrive, the engine will turn slower than the driveshaft. Otherwise, you don't have overdrive.


(I just wanted to get that out of the way to save confusion for whoever comes across this.)

Oos, I meant to say the driveshaft spins faster than the engine. I edited my last post.

 

[exbass94]

What i don't understand is how load decreases gas mileage.

is it because it takes more rpms to get to the same speed at the same amount of time vs. empty?

or because of longer acceleration ..or what.

i don't see how a 10k pound vs a 2k pound car with the same drivetrain ( completely same drivetrain) would get different gas mileage at the same rpm.

well i understand that a heavy car would get worse gas mileage but i don't understand why. its not like a 10,000 pound car is dumping in more fuel than a 2,000 pound car at the same rpm?

Actually, yes the heavier car IS dumping in more fuel at the same rpm. It has to work harder and use more fuel to maintain that rpm than the lighter car.

 

[doorgunner]

what happened was.....your brothers truck tires squatted down 1&1/2 inches with a load......making them shorter....causing an increase in rpmmmmmms

 

[85_Ranger4x4]

what happened was.....your brothers truck tires squatted down 1&1/2 inches with a load......making them shorter....causing an increase in rpmmmmmms

If it isn't the tranny and it isn't spinning tires that is about all you are down to.

If he is pulling enough to unlock the tq regularly he needs to kick it out of od or he will burn it up when it starts hunting through the gears. It is completly normal for a load to make it unlock the tq, and even per the manual it will specifically say to put it in drive when towing a substantial load.

I run all over with my '150 and tractor in tow in direct, it pulls it pretty nice. It would hunt like Elmer Fudd if it was in OD.