MetroMPG.com mailbag: is lowest RPM really best for max MPG with big displacement?

[MetroMPG]

Got this question recently from a MetroMPG.com visitor:

I've been recently wondering about the whole rpm's and speed thing and I've tried it myself, I know that if I go 50 in my Jeep that's where it shifts, then I can slow down to about 47 without it downshifting.

I Have a 95' Jeep Grand Cherokee (ZJ) and It runs the 242 4.0L I6 And when going 47 I achieve around 1400 RPM. At 50 I run around 1475 RPM at 55 I do around 1530 and at 60 I do 1690. I've driven at all these speeds and I think that my best mpg is achieved at around 60mph.

I don't know why, but it seems that after experiencing driving with other vehicles that, vehicles with larger engines get better MPG at higher speeds.

I don't know why this is true, but it might have to do with the large engine displacement, and that a lot of fuel is being wasted at lower rpm's because maybe there is almost the same amount of fuel being put into the engine at those speeds.

This isn't the first time I've read a comment like this about larger displacement motors.

I can say definitively that in all the vehicles I've driven with fuel economy instrumentation, the best cruising efficiency is achieved in top gear at the the lowest RPM. (Level road, constant speed, no other traffic creating aerodynamic effects.)

That said, most of the vehicles I've driven have had small displacement 3 & 4-cyl motors. I've only tried a couple of V6's with instrumentation.

I'm not saying it's not possible that much bigger 6- and 8-cyl motors may behave differently. I just don't have experience with them.

The only way to know for sure is to get instrumentation and try it.

[RH77]

Theoretically, but I recall a discussion that an engine has an ideal "piston speed" of 1350 ft/min (total travel) for the best Economy. I know in my 1.8L, constant load at 1500 RPM is less efficient than the 1800-1900 RPM, which is closer to the 1350 speed mark...

More testing/research may be required

RH77

Originally Posted by MetroMPG

the best cruising efficiency is achieved in top gear at the the lowest RPM. (Level road, constant speed, no other traffic creating aerodynamic effects.)

[MetroMPG]

Piston speed? Have to say that's a new one to me.

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Had a followup question from the Jeep guy:

Oh, I read it, I was wanting to do it w/ instrumentation, but I Can't do that because my car is before 96. Do you know of any other guages that can monitor this without the plug in?

Fortunately, we can answer that one.

Pre-OBD2 vehicles like yours can use a custom electronic gauge called the SuperMID. Check this thread for more information: [CRXMPG FAQ] SuperMID and 1990 Toyota Truck

[Peakster]

I remember my dad's Chrysler Intrepid having a flat MPG curve to it too.


Originally posted April 24th, 2007:

I took the Intrepid out on the TC highway going west From Regina to Moose Jaw (very flat road) and this is the FE laydown:

Temperature 16*C, winds SE @ 20km/h
Each FE reading was done after 2 miles of driving at the stated speed

40 mph = 32.3 mpg
45 mph = 34.9 mpg
50 mph = 35.7 mpg
55 mph = 32.7 mpg
60 mph = 29.9 mpg
65 mph = 29.4 mpg
70 mph = 29.2 mpg

I find it interesting that a): the mileage gets worse below 50mph (maybe due to the torque converter not locking?) and b): the mileage seems to plateau with speeds over 60mph. Evidence of good aerodynamics? It's pretty sad that the V6, 3600 pound Intrepid gets just a few less mpg's less than the Geo @ 70 mph .

[Lazarus]

A jeep is not very aero. I would think that penalty alone should kill the FE with speed. Plus with out instrumentation who knows what really going on. I have also heard this before but not from anyone who was using instrumentation.

Interesting. Hopefully some one will chime in who drive a car with excessive power.

[DifferentPointofView]

Hey, "The Jeep Guy" here, I just wanted to say that, I don't seem to see any difference between 55 and 60mph, I actually see a better miles per tank when I drive 60 rather than 55. My ZJ has a 23 gallon fuel tank, so I'm guessing that it's pretty accurate at fill-up because more gallons are used than smaller vehicles. I've taken my factory roof racks to try and improve the mpg, but I don't feel or see a difference at fill up. But one thing I do notice while driving is it's much easier to accelerate on a half full tank than a completely full one. I've also noticed that I've gotten a slight but noticeable increase in fuel economy after replacing my OEM Air filter to a K&N Filter.

I'm Also thinking back to the whole truck thing where a truck actually gets better highway mpg with the tailgate up than down because of the dome effect created within the tailgate. Theoretically this would create drag, but in a wind tunnel it in fact creates the opposite.

With a large displacement engine, there's also usually a large car. I think that if your going at 60mph, the weight of the vehicle causes the momentum to continue on longer and, since momentum has built up at this higher speed it's easier on the engine cause the weight is pulling the car forward and helping out the engine. I've been in V8 Cadillac's with Instant fuel economy, and when I watch it, it seems that when as the car accelerates to a faster speed, the fuel economy goes up when they put on the gas. I'm guessing that from the large displacement, lets say 4.7L, at idle speed of like 15 mph there is still alot of extra fuel being injected into the engine and fuel is wasted, but with a little bit on the gas you go faster and there isn't much more fuel being injected cause it doesn't take much more RPM to get it from 0-60. I never have to go over 2000 RPM to go anywhere, and My Jeep shifts at 1500 if I accelerate lightly. I can easily get from 0 to sixty hitting the gas barely at all. If I wait long enough The Jeep can Idle at 20mph.

This could be why that Intrepid had a flat mpg curve at speeds over 60, it was a heavier car, and once you get a heavy car going, it goes, and the car doesn't have to do much more to go from 60 or from 70mph. I love coasting in my Jeep, cause it coasts for miles and my rpm's are near idle.

Also, with the SuperMid, Where do I buy one and can I get it in US Customary?

[DifferentPointofView]

ah, yea, but it did somehow lower my rpm's a bit after moving to a K&N, why is this?

What are some other things that can improve the MPG of my Jeep, Not including removing unnecessary weight.

[Lazarus]

Originally Posted by DifferentPointofView

ah, yea, but it did somehow lower my rpm's a bit after moving to a K&N, why is this?

What are some other things that can improve the MPG of my Jeep, Not including removing unnecessary weight.

Driver technique is the biggest saver of all. Take a look here for ideas. Also increasing tire pressure to max listed on the sidewall will help. Sounds like you do a lot of highway miles check out the aero tips also.

[DifferentPointofView]

I always drive economically, especially when no one is looking. I can't really drop any weight, I could dump the spare, but you never know with the Jeep (the side of the road seems to be more interesting than the road itself!) I've got the tires about 1.5psi overinflated and removed the roof racks. I was wondering about making a home-made intake from pvc and the cold air intake filter with some heat deflectors to try and make it breath a little better. How much difference in mpg is a clean Jeep vs a Muddy one?

[RH77]

This varies widely among vehicle manufacturers, but you may want to actually warming-up the incoming air.

I'm not sure about Jeeps or Chrysler brands, but Saturns and some Hondas respond to this.

Same for cold starts -- intake air temp is generally one of the feedback parameters for the FI system. Generally, warmer = more efficient.

As far as mud, it depends on how thick the coating is

RH77

Originally Posted by DifferentPointofView

I always drive economically, especially when no one is looking. I can't really drop any weight, I could dump the spare, but you never know with the Jeep (the side of the road seems to be more interesting than the road itself!) I've got the tires about 1.5psi overinflated and removed the roof racks. I was wondering about making a home-made intake from pvc and the cold air intake filter with some heat deflectors to try and make it breath a little better. How much difference in mpg is a clean Jeep vs a Muddy one?