Buger

Hi everyone, For a long time, I have wondered what rpms an engine should be run at to get the best mileage. After doing some searches, I found a couple of references that said that the best mileage would come when a given engine is run at the rpm where it's torque peak is at. While this would seem to make sense if you are going up a hill and need the torque, it would seem to be rather ridiculous that a car with it's torque peak at 7500 would be more fuel efficient in second gear rather than 5th or 6th while going 55 or 60 mph. The manual of my Mazda Tribute has what seem to be relatively low recommended shift points. I assume that the reason for that is because the next higher gear would normally be more fuel efficient given a flat road? Can anybody expound on this subject a little bit and educate me? Thanks in advance, Brian

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Grimace

Keep the revs low, but not so low the car feels like its bogging down or struggling. Good rule of thumb.

Donny Boy

This depends on a number of things. Displacement, torque curves, weight, aerodynamics, horsepower, rolling resistance to name a few. High mileage cars tend to be smaller in weight and engine displacement and also in the fun factor. The RX-8, however, should get decent mileage, considering the performance level of the car, estimated to be around 29 on the highway at a steady 60 mph.

wakeech

i think he just wanted to know how to better manage his throttle while driving, Don... but ya, you're right about all that stuff (ESPECIALLY the fun factor... ugh, i drive a freakin' Echo, which is the Tercel next gen...)

the most fuel efficient way of driving is tricky to figure out emprically, but you can take a good guess at it... often, it's in the highest (or a higher) gear, at fairly low rpms... "WHERE IN THE REV RANGE???" you're screaming at me in your mind?? well, it depends (do i sound like an econ major, or what?? )... but it's the point at which you can maintain a high speed whilst keeping the throttle closed as much as you can...
if you understand math, it's the point of intersection between the marginal increase in fuel usage (how fast the throttle is opened more to maintain the speed), and the marginal velocity increase (how quick you're speed is increasing)... so, like for a wild guess, 6th gear at like 2.2K rpm or something... you could really only get maximum fuel utility on a flat (or even better, for obvious reasons) or downward sloping road...

Buger

Thanks everyone,

After thinking about it a little more, it would make sense that it would be the highest gear at the lowest rpm. I was just confused that I found several references that said the peak torque rpms were the most efficient.

If anybody here has ever taken a looong drive, you find that your mind often wanders a bit. We went on a trip from Denver to San Diego earlier this year and I found myself thinking about what the most efficient speed/rpms were in 5th (top) gear.

At the same time, if you are going about 65 mph in your top gear and start facing a small hill and start slowly losing speed, you either have to press on the gas more or downshift. At what point would it be most efficent do downshift?

I just started thinking about it again recently because the renesis is an engine that begs to be geared more. It has peak torque and hp at high rpms and it has a very wide powerband. I would like the gearing to be closely spaced from 1st to 6th but the freeway gas mileage would look to be compromised a little bit if 6th gear were something like .9:1.

Brian

Grimace

I did an experiment where I drove my car two different ways on a 3 hour trip. The first time I did it, I accelerated lightly, shifted under 3000 RPM (usually around 2500 RPM), and while cruising kept the engine around 1500 RPM (except on the highway where my gearing doesn't let me cruise at less than 2700 RPM at highway speeds).
The second time around I dipped into the gas pedal more during acceleration, enough that I usually skipped 4th gear (1-2-3-5). Cruised around at 2500 RPM or more through towns. Same highway speeds. Got better mileage.
Same ambient conditions (it was the same day). Only conclusion is that by not lugging the engine, and keeping it closer to its most efficient RPM (not necessarily its torque peak) I was able to achieve better mileage.

BryanH

I read an article in (I believe) Car And Driver that supports Grimace's experiment. They basically said you'll get the best gas mileage if you go WOT to your cruising speed. Sounds crazy but that's what I recall from the article.

Salo

I'd thought I would revive an old thread....

Buger, I understand why you were confused about the efficiency of an engine. The sources you found were correct. An engine is most efficient when operating at peak torque (not hp). At this point, you are getting the most amount of useful "power" out of the engine for the amount of air/fuel entering the engine.

Standalone engines are typically operated at their most efficient rpm - which, if you notice isn't super low rpm. An example of this that most people have experience is a generator. You'll notice that they are operated at a steady rpm which provides the best conversion of chemical to mechanical to electrical power.

This, however, does not mean that your car will get the best mileage when running at peak torque rpm. A car is much more than just a simple engine. Accessories like pumps, fans, alternator all have a parasitic effect - decreasing usable engine output with increase in rpm at non-linear rates. What I'm trying to say is that a car engine does not operate at constant load which thoroughly muddies up the efficiency.

This biggest draw on engine efficiency is aerodynamic drag. For a typical sized car with an average frontal area, drag becomes the biggest loss of power around 30 mph or so.

The drag force can be expressed as:
F_d = C_d * A * rho_g * u^2

Where: C_d = coefficient of drag
A = area factor of soild body
rho_g = fluid density
u = velocity

The coefficient of drag is dependent on the shape of the object and speed of the object/fluid (laminar or turbulent). The Reynolds number is ratio of inertial forces to viscous forces.

Re = (d * u * rho_g)/mu_g

Where: d = characteristic length
mu_g = kinematic viscosity of fluid

What I'm trying to get across here is that the force of drag is extremely velocity dependent. So when operating a car you want to maximize your conservation of kinetic energy and minimize your fuel consumption. When you are driving you car and there is minimal load (i.e. driving downhill) your maximum fuel economy will be achieved by keeping the throttle open just enough to keep the engine running. At this point your aren't losing engine power to the accessories and aren't fighting the drag force.

Now to specifically answer your questions. A car engine should be operated at as low of rpm as possible to maximize efficiency. Ascertaining what rpms to shift at is a little more challenging since it depends on the car's velocity. The rule of thumb of keeping the rpms low should, however, suffice in providing the best mileage.

Hopefully this makes since. I've been working on a report all day and my eyes are getting a little glassy.

visitor

In normal city driving, my truck gets 20-22mpg. For highway, it's somewhere around 25, depending how fast I cruise. However, on one trip from the 5 in San Diego, to the 101 junction in LA (approximately 100 miles), I got ~35mpg. I swear I did the math twice, I started with a full tank in LA and filled it up full again just as I got on the 101. It was definitely 35mpg. How'd this happen? F'in traffic all the way from San Diego to the 101 junction. I don't think I ever reached above 3rd gear in my 5 speed. And I was always shifting the entire time. Every car's got a point where you can shift into the next gear without making the car shake too bad or stall the engine. As long as you hit the necessary RPM's to shift into the next gear safely, you'll probably be maximizing fuel efficiency. Of course where this point is on the tach will vary on the road your driving, ie downhill, uphill, or flat. Anyways, that was my observation when I did 35mpg.

And folks, please shift. I hate it when people just stay in 2nd when they can obviously be in 4th. It feels like the cars gonna blow up any minute. If your excuse not to shift is because you're too lazy....well, maybe your better off with an auto and psuedo paddle shifters. It maybe your car and you can do whatever you want, but you're just wasting natural resources for being lazy.

Buger

Hi Salo,

Thanks for taking the time to post but I did come to pretty much the same conclusion in my below post a couple of months ago:

http://www.rx8forum.com/showthread.p...6319#post16319

I'm sure it's a subject that many people have wondered about at one time or another.

Brian

Salo

Oops, I must have missed your other post. Sorry for restating your conclusions...

I took a lot of Transport Phenomena classes (Heat, Mass and Momentum transfer) - so it was fun remembering some of the equations. I got a little carried away... :o

Smoker

I'm not too much into math equations or technical details. The thing I go by in terms of "the most efficient speed" is just by feel. I think you if have driven enough cars over a long period of time, you can kind of feel what speed the car "wants" to be cruising on.

There is always seems to be a certain point at the top gear where it seems if you push just a little harder, the engine will kind of tells you that it will now have to work a little hard or if you back off a little too much, the engine will tell you that it can run a bit smoother at a higher RPM.

I go on business trips on a regular basis and on each trip I usually get a different rental car to drive and it seems that every car seems to have comfort speed where I can get the best mileage out of them.

FYI: what scared a bit was a 2002 Maxima SE i got one time, seems to me its comfort speed is all the way at 145km/h ....