How do I get the most mileage out of my manual-transmission car?
October 2, 2012 7:21 AM   Subscribe

Can you help me understand, once and for all, what I need to do to get the best mileage out of my manual-transmission car?

This seems like it ought to be a really simple question and I feel a bit stupid asking it, but I get a lot of conflicting advice which to me is indicative of some kind of popular misunderstanding on the subject. Certainly my own understanding is rather hazy. I feel like I can trust the good people of MetaFilter to set me straight for good and all, so here I am.

I have a 5-speed manual transmission car, a 2002 Honda Accord. I am trying to figure out how I should use the transmission if I want to maximize fuel economy. I'm not talking about trying to maintain a balance of economy and power, and for the purposes of this discussion I am separating acceleration and cruising, assuming that I will never have to unexpectedly accelerate and that I will always have time to change gears if necessary before making a maneuver. I realize that in the real world this is not the case, and am just trying to simplify the problem conceptually for myself.

My current understanding is that if I'm cruising I should pretty much just drive smoothly, as you would in an automatic, and that I should be in the highest gear that the car is able to sustain so as to minimize RPM. This makes sense to me. Lower RPM = fewer combustions = less fuel used, right?

However when I try to confirm this I also hear a lot about using the transmission to keep the car in the most efficient part of its power band, which would seem to mean that I should be aiming for some optimal RPM rather than just the lowest RPM that my car can easily sustain for a given speed. Is this the case? If so, why? And how do I know what that optimal RPM is for my specific car?

When accelerating, I tend to adopt a similar strategy. I try to accelerate smoothly and gently, again as one would in an automatic, and I shift up as soon as the car is going fast enough to let me do so. This is again intended to reduce engine RPM under the assumption that higher RPM = worse economy.

Is that correct, or is there again some optimal RPM I should be aiming for when accelerating if I want to minimize fuel consumption? Is this the same as the optimal RPM for cruising, or is it different? How do I know what it is?

My theory on the subject is rather hazy and if anyone feels like they have a really solid grasp of it and can explain it in a clear and simple manner then I would love to know about it. Mainly though I'd be happy if I could just be sure that I wasn't unnecessarily burning fuel while trying to drive economically.

Thanks for helping me settle this one for myself.
posted by Scientist to Travel & Transportation (27 answers total) 5 users marked this as a favorite
Is this a scientific exercise, or are you trying to get a measurabe improvement in gas mileage?

When driving a manual, you want to keep it in the highest gear that you can, given the traffic conditions.

The most economical gear is 5th.

The difference between driving like a gas saving loon, and driving like a person, might be 5 gallons annually. Certainly in a 2002 Honda.

Keeping your tires inflated to the correct PSI, staying up-to-date on your tune-ups and not jackrabbiting at starts will save as much gas and you won't be sitting there doing computations in your head.
posted by Ruthless Bunny at 7:29 AM on October 2, 2012 [4 favorites]

Also, if that's a car of model year 1997 or later and it doesn't have a mileage gauge, get an OBD II reader so that you have some better feedback for optimizing your mileage. I have a ScanGauge II in my truck (which happens to be the one vehicle that doesn't put distance information out on the OBD II port, which means I get GPM but not MPG), I have no particular reason to recommend that device other than that it works (aside from the larger issue with 1997 Ford Ranger/Mazda B-series pickups), but real-time "Oh, when I'm doing this my mileage goes to hell" will help you a lot.
posted by straw at 7:34 AM on October 2, 2012

Yes, I'm trying to maximize economy while maintaining some semblance of normal driving behavior. I'm not going for no-holds-barred hypermiling here, I am looking for an overall technique that I can use day-to-day without pissing off/endangering people around me.

Also yeah I know that I should keep my tires properly inflated, stay up on regular maintenance, and accelerate gently and smoothly. I do all of that already. I just want to know if I should also keep on driving in the highest gear that my car will let me, or if that is Wrong Wrong Wrong and there is some other strategy I am supposed to be using.
posted by Scientist at 7:34 AM on October 2, 2012

One common mistake I hear people making is shifting into neutral to coast. Not only does that cause you to be out of gear in case of a necessary evasive maneuver, but your car will shut off the fuel injectors while coasting in gear. As long as the rotational velocity of the wheels can keep the engine turning, the engine will need less/no fuel to turn. If you shift into neutral, you're consuming just as much gas as idling at a stop light.
posted by dobi at 7:35 AM on October 2, 2012 [1 favorite]

But if you throttle down in gear, the engine will slow you down, and if you forget, you'll bog down eventually. I put the clutch in to coast up to red lights & stop signs or down hills as much as possible, since I can coast a lot further than with it idling down in gear. I get between 28 & 30 mpg in town, in a Celica that's rated at 25-27 city.
posted by Devils Rancher at 7:49 AM on October 2, 2012

"Assuming that the engine is operating in its powerband (roughly 1200 RPM to 4000 RPM on two-carb corvairs), and throttle is between zero and 80% of travel, the slower the engine is turning, the better fuel economy will be. So upshift early--fuel economy is always best in top gear. This is especially true with a Corvair, because the power consumption of the cooling fan is a cubic function of RPM".
posted by hot soup at 7:50 AM on October 2, 2012

According the manual on my car 2000-2500 rpm is the ideal for reducing gas consumption. The bonus of a manual transmission is that you can more quickly change gears and remain in this sweet RPM window easier and faster than an automatic transmission.

Cars are geared differently, but for my 6 speed, if I'm city driving, stop light to stop light, I tend to stay in 3rd or 4th gear to keep it around 30 mph. Of course, interstate driving is what the top gear is designed for and because I've got 6 gears instead of 5, my 2500 RPM results in a slightly faster MPH than my previous manual transmission.

I was also told by my dad when I first started driving that using the clutch to hold your car rather than the break is extremely expensive. Because it wears out the clutch faster, it means that you have to replace the clutch and it's way cheaper to replace brake pads than clutches.
posted by teleri025 at 7:50 AM on October 2, 2012 [1 favorite]

There are lots of articles out there on Hypermiling (I've never knowingly read an of them!).

I don't know how usful this is, but I used to own a car that had all sorts of fuel-saving features -one of which was a MPG estimate. I found I could (at least according to the MPG guage) use least fuel by (as Scientist said) changing up to a high as quickly as pos (requiring a fairly open road), and then when up to a cruising speed (55mph was best, probably, as it's where many MPG stats measure), then subtly lifting off the gas. I used to think of my foot, rather than depressing the trottle, being used as an end-stop (ie. not applying pressure to the pedal, but let the pedal's return spring push against my foot). This *seemed* to reduce the MPG guage to almost nothing.
{Oh, as dobi said, about neutral. My old car could be set (City Mode) to switch off the engine at lights, rather than idle}

But, as usual YMMD! :)
posted by Dub at 7:51 AM on October 2, 2012

So far it looks like three votes for shifting up as soon as possible, and one (backed up by their car's owner's manual) for maintaining an optimal RPM. Are these really two competing theories or are they really the same thing and I'm just looking at them wrong? Thus far I remain confused.
posted by Scientist at 7:56 AM on October 2, 2012

I agree, the highest gear at which the car cruises smoothly will bring you the best mileage.
posted by HuronBob at 8:07 AM on October 2, 2012

The problem is that if you shift too soon (i.e., not hitting the optimal RPM-- I like to never drop under 2k), you'll be past the 80%+ of gas-pedal travel that hot soup mentions for even minor speed corrections. I know when I'm in the wrong gear if: (a) lifting off the gas jerks the speed down, or (b) I have to floor the gas to make minor speed increases.
posted by supercres at 8:08 AM on October 2, 2012

The "highest gear possible" theory is overly simplistic.

If you buy an OBD scanner ( mine cost about $20 and bluetooths to my android ) you'll often see that there is a "load" graph available. You want to keep the load as minimal as possible because that is where it's taking as little fuel as possible to keep the engine turning over.

Without a scanner, it's a case of using your common sense - if the engine feels like it's struggling to cope then you should be in a lower gear. There will be a sweet spot but it's going to vary on lots of things so it's not as simple as telling you "change at 2000rpm".

The coasting thing varies depending on the car (in my experience). My 12 year old Toyota Corollas (small petrol VVTi engines) get a better fuel economy out of gear. My 8 year old diesel pug gets massively better economy if you leave it in gear when you coast.

One of the things that I believe hypermillers tend to argue ( or did last time I checked ) was that you should accelerate like you have all the time in the world. In my experience, you get better economy by being brisk and getting to cruise speed. This is a judgement thing and will change a lot based on traffic, how far you can go before slowing down, your car, etc, etc...

The most important thing though, as far as I've seen (after keeping your car in good condition), is keeping your cruise speed to somewhere around the speed that they publish highway fuel economy stats for ( ie, if they base stats on 70mph, aim for 70mph ) because car manufacturers seem to tune cars for economy at around those speeds. My pug gets great economy at 70mph but really sucks at 80.

(on preview, waaaay too long)
posted by sodium lights the horizon at 8:12 AM on October 2, 2012 [1 favorite]

I have wondered this too, as a driver of a manual car, so I am interested to hear the advice here. But I wanted to add that there is also a give-and-take, pro-and-con with coasting your car to stoplights/using your brakes, versus gearing down to slow down for traffic or red lights. You would also have to consider more wear and tear on your brakes, even if your gas mileage improves.

In addition to brakes, you'd also want to consider the life of the stick shift itself. If you are always running in slightly too high of a gear, my impression is that this is bad for the car, lugs down the engine, and is hard on the manual.

As someone who doesn't know much about cars but contemplates these things as a standard driver, those are some other thoughts to consider in maximizing the life/expense of your car.
posted by shortyJBot at 8:15 AM on October 2, 2012

I am really uninterested in whether it is worthwhile to coast to a stoplight/down a hill vs. using the engine to assist braking. Brockles solved that one for me in this MetaTalk thread.

I do however think that it's coming together for me regarding what kind of shifting behavior I should use to maintain maximum fuel economy. I should be driving in the highest gear possible, provided that I am also in the power band for my engine, no? I think my problem here was a misunderstanding of the term "power band" which I took to mean "optimal RPM". It sounds like for all intents and purposes the power band is actually a pretty wide RPM range in which the engine is neither threatening to stall nor screaming as hard as it can.

If I am able to drive along without having to put a lot of pressure on the pedal just to keep the car going at the speed that I want, then I am in the power band. As long as a light touch on the pedal is sufficient, I'm good. So I should be driving in the highest gear that allows the car to cruise along with only a light touch on the pedal. Is that correct?
posted by Scientist at 8:23 AM on October 2, 2012

There are two kinds of, let's say program, governing your car's engine. 1) There's the electronic computer that is hooked up to sensors (air, temperature, o2 levels post-combustion), and there is the camshaft (number 11 in the linked diagram), which mechanically controls the valves on each cylinder.

The two of these - the electronic computer and the camshaft - determine the nature of the combustion in the cylinder. The computer is flexible in what it does, while the camshaft is fixed. It's just a rotating piece of metal. However, the results of the controls exerted by each of these systems changes at different RPMs.

The computer, being flexible, will mostly try to optimize the combustion for a balance of fuel economy, performance, and low emissions, with this balance largely determined by the manufacturer, and somewhat determined by the state of the engine (engine computers can adapt to engines that have problems, and some can adapt to a driver's long-term habits).

The camshaft, being the hunk of machined metal, is inflexible. As it rotates, lobes on the shaft push valves open: to let air into the cylinder, to let the burnt fuel and air (combustion products) out. When a lobe no longer pushes a valve open, a spring pushes the valves closed (to trap air and fuel under pressure). Inefficiencies at high RPM come in part because the valves have mass and springs can only close them so fast.

Now, here's where it gets interesting. The timing of the opening and closing of the valves is determined by the shape of the lobes on the shaft and by the speed of rotation on the shaft. The slower the shaft rotates, the slower the rate of opening and closing of the valves, and the longer they remain open and closed. So at low RPM, the valves are behaving in a predictable way.

At high rpm, the valves are opening and closing much faster, and the time where the cylinder has all valves closed is shorter. At high enough rpm, the valves can no longer shut quickly enough, or open long enough, for a perfect burn. Instead of clean air and fuel and a spark in a closed cylinder, for maximum bank, it may be a mix of clean and dirty air, with some valves still partly open, for a less efficient burn.

So ... you've got the computer and the camshaft, the engine speed, and we haven't even gotten to the manual transmission and aerodynamics yet ...

Broadly speaking, you will get your most efficient ride at an RPM in the middle of the car's "power band," which is, on a non-turbocharged and non-supercharged car like an Accord, determined by the shape of the lobes on the camshaft.

You will, at this RPM, get the best results rolling well under 65mph, to minimize aerodynamic drag.

You will, again at this RPM, get the best results at the highest gear comfortable for the engine (no lugging or knocking).

tl;dr engines are complex, but the middle of the power band is where the components are most likely balanced out for maximum fuel efficiency.
posted by zippy at 8:26 AM on October 2, 2012 [2 favorites]

Rolling around out of gear is, I'm pretty sure, illegal. When your car is out of gear, you are not fully in control of your vehicle: you can not accelerate.

With a manual transmission, you should rapidly develop a "feel" for when the car is "happy".
posted by at 8:27 AM on October 2, 2012

2nd-ing straw's suggestion of a ScanGuage II. I bought one for my automatic Subaru when the speedometer failed and now I use it pretty much for milking the maximum mileage out of it. I also use it in my manual pickup truck. In the pickup I get the most mileage out of it by running the RPMs as low as possible, like usually around 1200-1400.

In either vehicle I can milk way more than the cars' listed EPA MPG ratings if I'm careful.
posted by MonsieurBon at 8:27 AM on October 2, 2012

(edited above)
posted by zippy at 8:28 AM on October 2, 2012

In practice it's very simple.

Dont labour the engine (engine going very slow, throttle pressed very hard, nothing much happening). The engine will be out of its comfort (efficient) zone and will be trying to trying to compensate by using a lot of fuel.

Dont race the engine (engine going very fast, throttle pressed very hard). Although the engine will be quite comfortable here, it's moving a lot of mixture, so is wasting fuel.

Really; the throttle determines how much fuel the engine uses. Get the engine in a gear where you need to use the throttle least. That's the where the best economy is.
posted by BadMiker at 8:31 AM on October 2, 2012

Are these really two competing theories or are they really the same thing and I'm just looking at them wrong?

I think that in practice, they're the same thing. If you look at how that article addresses the power band, it's basically defined as any engine speed where the engine is making significant power (the range between the torque peak and the horsepower peak).

I think "optimal" is the word that's throwing you off. The optimal engine speed for mileage is, as you say, as low RPM as possible while still operating within the power band. There are other articles about optimal shift points that are describing optimal for maximum acceleration, which is at a much higher engine speed.
posted by hot soup at 8:32 AM on October 2, 2012

I've always owned cars with built in instant-fuel mileage readings. I play a 'game' where I try to keep the instant mileage as high as possible (without driving like an idiot of course).

The basics:
1. Accelerate slowly. Acceleration is the enemy of good gas mileage in general, but accelerating slowly is better than accelerating quickly.

2. As others have mentioned, use your RPM meter to find the earliest reasonable place to shift gears, where 'reasonable' means the lowest RPM you can shift at without bogging down the car. I actually find this is less important than #1 and #3 (especially), but it does help.

3. Probably the most important in my experience - use your brakes less. Every time you brake, you're wasting energy you've just burned gas to generate. Using your brakes less safely means leaving plenty of room between you and the car ahead of you so that if they brake moderately and then re-accelerate, you don't need to brake at all. In town, coast to stoplights rather than accelerating to it and then braking once you reach it.

4. Drive slower on the freeway. The 'best' speed depends on your exact car, but in general driving faster that 65 significantly worsens gas mileage because there is a power law involved having to do with drag (that is, doubling the car speed more than doubles the power input needed to maintain the speed).
posted by zug at 8:38 AM on October 2, 2012

Nevermind, I got confused. I think zippy's answer and BadMiker's throttle rule make the most sense. I misread this part of that article:

"An engine (no accessories), running at full throttle (we're ignoring power valves and other enrichment gadgets) has its lowest fuel consumption/unit power produced at the torque peak. The horsepower peak is a mathematical artifact of increasing RPM coupled with decreasing efficiency, and is not usually a good place to run for max. economy. Unfortunately, unless you're running a generator, or pump, or something like that, it's not a particularly relevant value, because automobile drivers *do* have engine accessories and power valves (late corvairs, anyway) and *don't* spend much time at full throttle at the torque peak."
posted by hot soup at 8:39 AM on October 2, 2012

An engine running with the accelerator mashed down, on my car, perhaps the Accord too, enters a special routine of the engine control computer that says

- oh, you're flooring it?
- ignore efficiency!

The computer stops trying to be all ideal and efficient because maybe you're about to get rear-ended by a truck, or maybe you want to burn out from the stop light. So it puts in more fuel than can be combusted to guarantee maximum safe power from the engine (safe here meaning avoiding a lean burn under load which could make the engine run too hot).
posted by zippy at 9:26 AM on October 2, 2012

(edited above)

[I know it's brand new, so no worries, but please note that this is not what the edit feature is for. Correct typos and such, but do not add chunks to existing comments or move stuff from one comment to the other. Commenting twice in a thread has always been fine and still is. FAQ.]

posted by cortex at 9:40 AM on October 2, 2012

Lower RPM = fewer combustions = less fuel used, right?

Possibly a bit more abstract than what you were looking for and someone like Brockles, who has MUCH more practical knowledge than I do, can feel free to amend/correct me here.

Oddly enough the mechanical engineer in me would rather you consider another aspect of this interaction.

Instead of thinking in terms of lower RPM meaning less fuel used, think about the torque (at the tires) required to maintain a given speed. That torque has to come from somewhere and, all other things aside, is a constant. Where does it come from? There is no magic energy fairy in a car. Energy has to be converted to work and transferred to the street or 'da car no go. So, fuel right? It comes from the fuel.

Fuel (potential chemical energy) --> combustion (chemical reaction) --> pistons/crankshaft (kinetic mechanical energy) --> transmission --> differential --> tires/road (frictional force) with heat being produced as a waste byproduct of combustion and drivetrain friction.

Let's imagine a car with an engine that has an infinitely wide optimum powerband, that car will get the best mileage (at a constant speed) in the highest gear possible. Why? Because the friction and inertial losses in the engine (as heat) will be at their lowest Lower engine RPM means less internal movement and frictional losses (along with wear and tear I would think as well). At the same vehicle speed the same amount of energy will be transferred to the tires but, in a lower gear/higher engine RPM, more fuel will be used. This difference in fuel is all being wasted as frictional heat/losses.

So, why not run in 5th gear all the time? Stoplights and the nature of IC otto cycle engines are why. These realworld engines have a powerband that's only so wide. You have to turn that low torque @ high rpms in the otto engine into higher torque at lower rpm at the tires to get moving from a stop. The price you pay is the need for a transmission of some sort and subsequently higher engine RPMs.

So, long story short: Highest gear possible as quickly as possible while not straying from the vehicles powerband or accelerating too quickly (both of which can cause the ECU to push excessive fuel to the cylinders because it thinks "You need Powar now! Right boss?").
posted by RolandOfEld at 12:40 PM on October 2, 2012

Thirding a ScanGauge II reccomendation. One of the best Christmas gifts MrsEld has ever gotten me. I improved my mileage very quickly.

I did have to manually program in either the instantaneous or trip MPG gauge calculation however, I forget which one. It was simple and you'll be able to figure it out no problem. I was just surprised that it came with one and not the other...
posted by RolandOfEld at 12:46 PM on October 2, 2012 [1 favorite]

This is all N=1 and your car may be different, but...

A while ago I bought a Bluetooth OBD scanner. Combined with my smart-phone and a fancy app, I was able to monitor instantaneous-mpg-over-time on a graph. Using this combo, I learned some things about my car (1999 Audi A4, 2.8 V6, Manual) and how it uses gas.

I had thought that on the highway coasting-in-gear would be best, but I learned that coasting in neutral gave significantly better mileage (one can, of course, debate the safety of this practice).

After a watching a while, I ended up simplifying everything I saw into: "Minimize the number of revolutions the engine makes". In practice, that means:
- coast down hills out in neutral
- use the highest appropriate gear (while avoiding bogging down too much)

Taking the extra hyper-miling step of "alternately surging up to speed, then coasting down in neutral" gave even better mileage. On the other hand, it's quite annoying to anyone else in the car - and probably to cars behind too. I don't do that much.
posted by sarah_pdx at 2:48 PM on October 2, 2012

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