fuel vs rpms

[Shannow]

Impossible to tell from a desktop.

For example, a particular platform was available in Oz that had everything from a 1.9L 4 to a 5l V-8, and I owned botht the top and bottom aend, and most of the middle at various times. On the highway, all of them returned the same (within 1MPG) mileage.

In your case, the engine is loaded, at low RPM you will have the throttles far more open, and have better cylinder filling and theoretical efficiency, and lower frictional losses...that's why they do it.

Drop a gear, the revs go up, presumably at a lighter throttle (power equals torquexRPM, so more RPM, lower torque, more throttling), and should therefore have lower efficincy, and higher frictional losses...not much difference., but probably more fuel.

 

[meep]

I’ll add this… nobody has mentioned pumping losses, which is definitely rpm sensitive. under real load, however, pumping losses arent as big of a factor because cylinder pressures are higher. so, if the engine is loafing because it’s an aero load like a pop up trailer or bass boat, the downshift could have some small impact. Pulling a box trailer or RV at 55, the difference in mpg is smaller.

note - if pulling a heavy load, OD gears may need to be avoided anyway due to gear stress and heat loading internal to the transmission.

 

[brages]

This graph varies for different engines, but generally if you are following a line of constant power, the lowest consumption will be lower rpm and high but not max throttle.

 

[Cujet]

On some vehicles, the instant MPG display is accurate enough to run these experiments.

Higher RPM is almost always associated with more fuel use. But often by less than one might expect.

 

[Bailes1992]

There's other things to consider too. I tow a 1500kgs caravan with my 1.5 diesel Dacia Duster with just 115bhp.

Cruising on the motorway at 60mph I can use 6th (~2000rpm) or 5th (~2600rpm). 6th gear is comfortable until I get to inclines where I have to drop into 5th. However, staying in 5th my oil temperatures are usually a lot more stable, boost is less and so are EGT's. So which one is actually causing more wear?

 

[MSCH]

Just a general question. Let's say I'm towing in 7th gear at 65 mph, holding steady on a flat road. I feel the engine is lugging at that speed (just a feeling of course), so I want to drop a gear to raise the rpms. I use the gear limiter only to lock out 7th, and immediately the truck downshifts.

I was going straight, wind doesn't change, no difference in elevation, no difference in throttle input, all I did was use the computer to force a downshift.

Does my truck now use more gas at those slightly higher rpms to hold my speed?

I can tellya alot of it depends on load. for instance I had 2 different Chevy trucks, both with a 350, one had 342 gears and the other had 410's... highway driving empty the 342 truck got about 17 mpg, the 410 truck maybe 12 or 13 mpg.

but put a windsucking parachute shaped trailer on the back and the truck with 342 got 6-7 mpg but the 410 truck got 9 or 10 mpg.

it has something to do with running the engine at the torque peak versus running the engine well below torque peak with a heavy load..

same applies even with diesels... they might be able to pull just about anything in the top gear, but you'll see boost pressure and coolant temps at the top end of the good range whereas if you shift it down one gear and run the engine faster, boost pressure and coolant temps will probably go down and fuel economy wont change or may even improve

 

[Srt20]

I have no facts to back this up, this is just my experiences and opinions…

Most if not all of the gas engine vehicles I have ever towed with get best mpg while towing in the 2500-3200rpm range. Pickups and large SUV’s. Some turbo, some not.

Almost every diesel pickup, GM, Ram, and Ford, has gotten the best mpg 1800-2100rpm range. All turbo diesels.

IMO, it takes X amount of power to move a load at the given conditions at a certain speed. Also IMO it takes more fuel to make X amount of power at a lower rpm than it does at a higher rpm. Lower rpm, more throttle input, more cylinder psi. Higher rpm, less throttle input, less cylinder psi.

Again, just my opinions

 

[hemioiler]

Thanks everyone for your comments, especially the graphs are very useful (in theory anyway).

Basically my thoughts on this are: while it can hold 7th (first overdrive) in this scenario, it's not very happy about it. It will be quick to downshift, and it has no power in reserve. In 6th it's now in direct drive, better for the transmission, and it will usually hold 6th no problem until it encounter bigger slopes and hills.

So I've preferred 6th so far, but was just wondering if there was any significant mpg cost associated with this, and it looks like there probably isn't anything that one could reliably measure. My personal guess being less < 1mpg as I can't see any difference based on dash readouts (which isn't accurate of course, but still.)

Perhaps if the scenario was changed to a camry unloaded getting 30 mph, and then dropping 1 gear etc, there might be more of a difference.

But while towing, the other variables in this scenario like weight and aero profile of the trailer are so large and dominant that any possible "loss" in mpg by the increased RPMs is completely overshadowed by them.

 

[Srt20]

I want to also add, wind by far makes a much bigger difference than gears, rpm, weight, etc.

I tow long distances often (2500+miles per trip). And head wind can have one tank at 9 mpg. Turn around and go with the wind and the next tank will be 15mpg. That is not an exaggeration!

Tow vehicle is a GMC 2500 Duramax diesel towing a 31ft trailer that is lightweight (6-7000lbs).

 

[meep]

Thanks everyone for your comments, especially the graphs are very useful (in theory anyway).

Basically my thoughts on this are: while it can hold 7th (first overdrive) in this scenario, it's not very happy about it. It will be quick to downshift, and it has no power in reserve. In 6th it's now in direct drive, better for the transmission, and it will usually hold 6th no problem until it encounter bigger slopes and hills.

So I've preferred 6th so far, but was just wondering if there was any significant mpg cost associated with this, and it looks like there probably isn't anything that one could reliably measure. My personal guess being less < 1mpg as I can't see any difference based on dash readouts (which isn't accurate of course, but still.)

Perhaps if the scenario was changed to a camry unloaded getting 30 mph, and then dropping 1 gear etc, there might be more of a difference.

But while towing, the other variables in this scenario like weight and aero profile of the trailer are so large and dominant that any possible "loss" in mpg by the increased RPMs is completely overshadowed by them.

If 6th is your 1:1 drive ratio thats the best place to be. There was a detailed study I read years ago in 4x4 magazine years ago that looked at gear stress and internal temperatures in heavy towing applications. The conclusion was that gears that were true overdrive, that spun the tail shaft faster than the input shaft, should be avoided during heavy loads.

i installed a gauge on one AT I had and could correlate the results, though I only had one measurement while they had multiple - under heavy loads the AT ran hotter when I tried OD. I only did that to experiment - I generally wouldnt tow in OD. In the ford 10 speed, “tow mode” seems to effectively lock out 9th and 10th, both of which i understand to be overdrive gears.

 

[Allaneddie]

So many variables!! Higher friction at higher rpm. possibly higher or lower bsfc #'s at higher or lower RPM.
In general you will use more fuel at higher rpm just due to friction alone. The only time that may not be the case is if the load and rpm puts you in a better efficiency area of the bsfc map

Or, at the lower rpm the ecm is being forced to constantly pull timing to keep out of detonation thus lowering efficiency, where at a higher rpm and less load the engine can take all the timing the ecm wants to throw at it.

Since it seems you are driving a hemi and the manual recommends you use 89 octane while towing it's possible you are pulling a lot of timing at lower rpm if you're running 87

I like that reasoning. Not sure if it right but the power equivalent idea makes sense.