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I was running late for a funeral yesterday. I got stuck by a train that stopped. Was stuck in town for 30 min. I ended up making a 55-60 min hypermiling drive in about 35 min.

Anyways, as I was keeping my RPMs around the 2500 mark I thought to myself about fuel consumption at a given RPM. I don't think that it would be linear. But, does anyone have any info on fuel consumption at a given RPM. Does it vary too much depending on throttle position to get good data? Does it vary dependent on what gear you're in? If so does it also vary based on gear ratios?

I have found that under certain circumstances I have had better mileage in a lower gear than in a higher gear. For example, maybe 45mph, uphill grade, steady pace - 4th gear is probably more efficient than 5th gear. Due to sweet spot rpms in 4th and engine load/pumping losses in 5th.

http://www.metrompg.com/posts/rpm-mpg.htm

Well not the answer I was hoping for, but it did answer my questions. Thank you Pale.

Damion, I've never had better iMPG in a lower gear than a higher one, provided I wasn't lugging the engine in the higher gear (MT). As far as MPG at a given speed or RPM, it's the grade that affects it the most in my experience. Setting cruise will hold my RPM's pretty constant (MT remember...), and slight downgrades increase my iMPG substantially, while upgrades lower it substantially. Thus DWL techniques to smooth out those variables.

I am wondering more about the iFCD than the iMPG. Just wondering if there is any relation between RPM and fuel consumption. Of course given all other things equal. Such as a level road, smooth traffic conditions, etc.

I am wondering more about the iFCD than the iMPG. Just wondering if there is any relation between RPM and fuel consumption. Of course given all other things equal. Such as a level road, smooth traffic conditions, etc.
I think experimenting with this this might be a way to find the sweet spot in your particular engine's rpm range.
I've read various things here like "keep it below 2000 rpm" and "the engine is most efficient at the peak of the torque curve" and "Car X gets better MPG at 60 than 55."
I imagine this varies for each engine, car, and the speed range you are trying to find an optimum within.

Unfortunately I have no way to test this. No scangauge. Part of the reason I submitted the question. I do realize that it varies from car to car. But, I thought there might also be some similarities.

The peak of the torque curve is often at a higher RPM than many
of us would ever want to run. But that's okay, we don't need
to get there -- just running a *reasonably* high torque at a lower
RPM seems to be the trick for a healthy majority of engine/tranny
setups. Recall the BSFC diagram (http://techno-fandom.org/~hobbit/cars/bmep-Final_29.gif) with the lines of constant power
drawn through it -- you generally hit maximum *efficiency* before
you reach maximum *torque*, most likely because the mere fact you're
not burning a cylinder charge as often results in less waste heat
overall and still getting a reasonable amount of shaft grunt to
apply to the road. Less overall energy waste is better for getting
down the road on less fuel, even if it means slower acceleration
and hill-climbing. It's counterintuitive, since it feels like
less *time* burning fuel when using faster acceleration, but remember
that the only units in play are *miles* per *gallon*, and time is
really not a factor.
.
[*Note: the tc.gc.ca report that diagram came from has apparently
gone poof, but fortunately I snagged the graphics and still have
local copies.]
.
_H*

Fascinating diagram Hobbit!
Clarify one thing please;
Are the lines marked 1.70 through 0.42 the BSFC lines?

Thx!

Here's an article expanding on the BSFC concept: http://www.autospeed.com/cms/A_110216/article.html

Hobbit - you have a copy of that report? Can you please email it to me, or maybe upload it here for hosting, or something? It'd be a real loss for that go vanish.

email: palemelanesian at gmail dot com

Unfortunately I don't seem to have pulled the article itself,
just some of the diagrams. It was a webpage, not a PDF.
.
I have no idea why they took it down, maybe someone should try
to contact the site owners? The link is in my linkfarm; search
for "Final_Report".
.
_H*

I think tasdrouille (sp?) over at EM has contacted them. He has a huge linkfarm for these types of papers. I haven't found any archives in my searching, either.

Here is his list. Later in the thread, he's getting the beaurocratic run-around trying to find a copy of this report. http://ecomodder.com/forum/showthread.php/fuel-economy-related-papers-1814.html

I've noticed my car gets better fuel economy at 60-65 than at 45 or 50, according to the ScanGauge. It's a 3.8 liter (231 cubic inch) V6, though, with lots of torque starting at about 1500 RPM and remaining fairly flat up to somewhere around 5000 RPM's. The engine seems to fall on its face after that point. (could be due to a dirty fuel filter, as that is ancient)

So I have to surmise I'm getting into the peak torque band at around 60-65 without expending much fuel.

According to the scangauge, I get better mileage at 30mph than I do at 47.
At 47 I'm in 5th gear...I'm not sure what gear is 30mph...it's either 3rd or 4th.

DWL I'll get in the mid 50mpg's @ 30mph and high 40's-low 50's @ 47mph.

So that long stretch of 30mph road that I have to take to my house twice/day which I thought was killing my mileage is actually helping it!

I'll try to verify those numbers later tonight, but I think they are pretty close.
-Mike

Based on non-negotiable laws of physics, higher air speed will result in more drag, requiring more fuel per unit of distance to maintain that speed. I don't know the formula but there is a square involved, meaning drag increases faster than speed. I suppose engine efficiency might increase at certain RPM values resulting in better MPG at higher engine speed, and that may tend to offset the drag increase effect of more fuel required. In general though, higher speed = more drag = more fuel per distance (GPM) = lower MPG.

99, I'm thinking that setting your sg to show the trip MPG; driving about 10 miles at 60-ish; resetting and driving the same 10 miles in the same direction on the same road in similar traffic at 45-ish would give you a more accurate comparison than attempting to monitor the iMPG value...