Efficiency. Engine Guru's please chime in.

Alright folks, here is my question.

On a stock engine, wtih stock gearing and stock tires (Or the equivalent through proper gearing) what RPM range provides the best fuel economy?

I have read conflicting reports on this topic, ranging from wherever your engine makes the most torque to wherever your engine makes the most horsepower and various others in between. Now at first glance it would seem to me that the lower the engine RPM, the bette the mileage, because you are not burning as much fuel obviously. With a litlle thought however, it would seem to me that your engine would get the best mileage at wherever in the powerband it is most efficient. So, where is it?

No, I am not looking to get crazy good mileage. If I wanted that I would get a Geo Metro and live with half as many cylinders as I currently have. So no snarky remarks like: "Get a honda" Please :laugh3:.

I'm just more curious than anything and this seems like a damn fine place to ask.

When I'm just driving around town, I usually shift around 1750-1900 RPM (With a five speed on both a 91' and a 00' before you start screaming for data :laugh and that seems to be working nicely for me, leaving me at around 1500 RPM for steady cruising speed. That leaves me with enough throttle to stay in gear and move around or to drop a gear and haul if someone decides to be a wanker. Now I don't pay any attention to the little idiot light that tells me when to shift at all, if I did I would be cruising around at 1100 RPM.

Is this an appropriate range for shifting/cruising? Where do you guys prefer to leave it?

Thank you for your time. :jester:

now ya know this is all a matter of opinion, but ''my'' opinion is that the lower the engine speed (rpm's) the less fuel is used. but thats just my theory.
I mean theoreticaly, lower rpm's = less fuel used per minute. example 1500 rpm's using x amount of fuel will be less than 3000 rpm's using y amount of fuel in the same exact alloted amount of time. Does that make sense? I managed to confuse myself there. personaly Ive got a 96 4.0 with an auto, I cruise around all day in 4th gear turning 1500 rpm's works fine for me. just my .02

When I was in ground school, we were encouraged to learn the torque curve of the aircraft's engine. Why? Because "best cruise" nearly always was right around "peak torque." ("Peak horsepower" was far too high for sustained operation.)

When your engine is operating at peak torque, it is maximising airflow. You get just about the highest energy output per unit fuel burned.

With modern control systems, this is maximised - since fuel metering is regulated by the ECU, rather than just the carburettor. The more "extra torque" you have available at that crankshaft speed, the more efficiently you use fuel.

I've gotten best results (~19-21mpg) with cruise at 2500-2800rpm. As a result, I have long since quit using fifth gear - it made cruise too low to keep any fuel efficiency (and this is something I experimented with for about four months. I'm starting to wish I'd retained my notes on it.) "Peak torque" for the AMC242 is right around 2500-2800rpm, and it's a nice, flat "curve" - which gives you some wiggle room.

The 3.07 gearing used behind the manual gearbox is a mistake, plain and simple. I once crunched the numbers on it (I should probably do so again,) and found that ~3.73 gearing would work well with 28-31" tyres.

Peak Torque = Peak Volumetric Efficiency = Peak Fuel Efficiency. There it is.

Oh - and thanks for using the correct word....

"Fuel Efficiency" - getting the most unit energy per unit fuel consumed.
"Fuel Economy" - getting the most distance travelled per unit fuel consumed.

They are, in fact, different measurements.

Edit - I quit paying attention years ago, but I think I tend to upshift around 3500rpm. I long since pulled that little I-D-TEN-T light out, I got tired of it trying to get my attention. I'll hit redline (5500rpm) about once or twice a year just to make sure everything is still balanced, but I'll rarely even touch 4Krpm beyond that...

So how about fuel economy? My instinct is that this one will be a bit more complicated.

so 2200-2500prm is best for efficency of combustion, and what about economy then? I'd like to know the best cruising speed for mpg in my Jeep... light load. What should I do with my slushbox? Use '3' when I drive? This really is quite interesting.

We haven't heard from Eagle in a few moons, but as I recall he reported the best overall mileage from keeping it at around 2000-2500 and trying not to fall below 2000, which for the manual gearing means staying in fourth gear much of the time, because the ratios are so tall. Oddly, I think the AW4 is geared more for set-and-forget economy than the stick. I never gave it a thought on my 87 and averaged about 23 mpg over the years I had it, but I never had a tach on that one either. One problem with running the manual in fifth gear at lower than freeway speeds is that it requires a lot of pedal to accelerate, and that seems to cancel out much of the benefit of the low running speed the rest of the time.

The AW4 is much more readily complemented by the 3.55 gearing than the five-speed (BA-10/5 or AX-15. Or, I think, the NV3550 is by the 3.07.) Overdrive is quite useless for an "economy" or "efficiency" standpoint with the 3.07 gearing - I quit using fifth years ago. The top range isn't really useful until I get to 85-90mph - and the local constabulary takes quite a dim view of that rate for some odd reason. (I guess California is more broke than I thought...)

Fourth-gear cruise with a stick isn't a problem at all, and it will do that all day at 65-70mph without breathing hard. You'll also still have some "get out of the way" available that just isn't there when you're cruising under 2000rpm crankshaft speed.

I honestly think I'm not guilty of talking through my hat here - as I said, I did verify this experimentally (hardly scientific experiments, but good enough for the level I was working at) and I honestly am wishing now I'd retained that notebook, because I'd post the numbers I got over time. However, I don't think you'll have any difficulty duplicating the results yourself - 28" (stock,) 30", or 31" tyres should all be able to cruise happily in fourth gear at 65-70mph without difficulty, and getting increased fuel efficiency.

I use 3rd with my AW4 at all times and keep it between 2250 to 3000 and get decent mileage, but i run 33's and 3.55's in a place where there are alot of hills and it does not like to hold 4th anyway. Can't give you exact numbers 'cause my speedo and odometer don't work, but seems the fuel lasts longer in this range.

I am by no means an expert here, but keeping mine between 1500-2000rpm.. I still get 19mpg, which I consider decent and about average from what I have read. My truck rarely sees 3k, then again I dont drive the highway either... Mine is a 91 4.0, 5spd with 30x9.5x15s.

I get about 19mpg city and 22mpg highway, keeping the rpms almost always under 2000rpm, (3.07 stock gearing with the manual tranny) but that might be because I'm very gentle on the skinny pedal. Maybe I should try crusing around 2500-3000rpm just to see what happens.

ditto, 55 mph is about 1800 rpm, and about 20mpg.
No major hills around here.
this would sound more like an "economy" point to make.
Maybe the discussion is veering off in a technical way?
sort of side note.... does this mean i probably have the 2.89 gears in my Dana that I saw in the FSM?

A simple statement about rpm's does not take into account the load on the engine. Back in the big gas crisis in the 70's, it was found that monitoring the vacuum was a good indication of the best fuel consumption. The higher the vacuum at a given speed, the better mileage.

old_man said:

A simple statement about rpm's does not take into account the load on the engine. Back in the big gas crisis in the 70's, it was found that monitoring the vacuum was a good indication of the best fuel consumption. The higher the vacuum at a given speed, the better mileage.

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True - but without installing a vacuum gage (something I've not gotten around to yet,) it's difficult to make any statements that would be inclusive of vacuum.

Also, that would have to take into account a number of other factors not readily quanifiable - vehicle loading, aerodynamic profile, and general terrain being at the top of the list...

It's something I'm planning on doing one of these days...

5-90 said:

True - but without installing a vacuum gage (something I've not gotten around to yet,) it's difficult to make any statements that would be inclusive of vacuum.

Also, that would have to take into account a number of other factors not readily quanifiable - vehicle loading, aerodynamic profile, and general terrain being at the top of the list...

It's something I'm planning on doing one of these days...

Click to expand...

I keep planning on doing that and not doing it, but one thing you can do as a temporary measure if you want to monitor vacuum is to run the hose out the hood and tape the gauge to the side view mirror. I've done that for diagnostics and it works nicely.

But generally any time you have to drop the hammer, vacuum goes down, and it's difficult on any uneven terrain to keep a steady pedal in fifth gear, at least around here where it's hilly. I think you'll find a vacuum gauge agrees with your fourth gear habit. I still use fifth a lot too, but of course, half the time I'm going down hill!

Matthew Currie said:

I keep planning on doing that and not doing it, but one thing you can do as a temporary measure if you want to monitor vacuum is to run the hose out the hood and tape the gauge to the side view mirror. I've done that for diagnostics and it works nicely.

But generally any time you have to drop the hammer, vacuum goes down, and it's difficult on any uneven terrain to keep a steady pedal in fifth gear, at least around here where it's hilly. I think you'll find a vacuum gauge agrees with your fourth gear habit. I still use fifth a lot too, but of course, half the time I'm going down hill!

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On a long downhill run (like the backside of the Grapevine or Pacheco Pass) I like to use Mexican Overdrive - neutral and idle down the hill. We can get four- or five-mile downhill stretches, so it's possible.

Since I plan on redesigning the entire instrument cluster, I'll work in a vacuum gage somewhere (along with the six-cylinder pyrometer, and a few other goodies...) I tend to like information about my engine...

Where would I be able to find a BTSC map for the 89 242?? Seems that those tell peak engine efficiency in regards to RPM...

5-90 said:

...I tend to like information about my engine...

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x2:wave: I hate idiot lights,:wow:

For those with the 'low rpm=mpg' mentality, consider the load on the engine (more throttle=more air and fuel going through, and this increases with engine speed), since running at 1250rpm (which I can get my slushbox XJ to do once its in OD and I barely have my foot on throttle... it's kinda hard to do) sure on level ground it's easy to maintain and the engine isn't needing as much fuel as it would in 3rd with a bit more throttle to maintain the increased engine speed.

Going up a small hill (that lets me maintain a low rpm like the above) I have to put my foot down a lot more than I would if it were in a lower gear (more rpm). So, I think the ideal number is somewhere around 1500rpm, which is 45mph with stock tires and gears, OD in the slushbox.

ChevelleSSLS6 said:

........

For those with the 'low rpm=mpg' mentality, consider the load on the engine (more throttle=more air and fuel going through, and this increases with engine speed), since running at 1250rpm (which I can get my slushbox XJ to do once its in OD and I barely have my foot on throttle... it's kinda hard to do) sure on level ground it's easy to maintain and the engine isn't needing as much fuel as it would in 3rd with a bit more throttle to maintain the increased engine speed.

Going up a small hill (that lets me maintain a low rpm like the above) I have to put my foot down a lot more than I would if it were in a lower gear (more rpm). So, I think the ideal number is somewhere around 1500rpm, which is 45mph with stock tires and gears, OD in the slushbox.

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With my 4.0's slushbox, I pull about 1750 RPM at 55 MPH. That's in OD on the slushbox. 3rd gear will hit about 2700 RPM. It's enough for me to have a bit of scoot availble if I need it, without too much pedal.

This is an interesting thread for me to have re-discovered. Temperatures are starting to even out (on the high side of 40) here, and northern IN is rather flat. I think I'm going to have to try this out with the next couple of tanks of fuel, and see how it relates to fuel economy.

I'm right there with 5-90. I have a 96 with the ax15 and peak efficiency is around 2500-3200. I like keeping the revs above 2k and shifting gears in the efficiency range. I did swap out the 3.07s for some auto 3.55s($30) and love it w/ my 27" tire(Jeep definitely shouldn't have used 3.07s). I was trying to get some take out 3.73s, but as some of you know, they were a rare option and at the time I couldn't afford $200 for new 3.73s.