Better gas economy - this "manual" transmission utterly confounds me

So about a year ago I bought a Mustang GT. Manual transmission because that’s just more fun :smiley:

It was my first manual but by now it’s second nature.

What I’m wondering about? How to use a manual to maximize gasoline efficiency.

Let’s start with a simple situation. You have 0.5 km to travel. The road is completely flat, there is no wind. You are starting from a complete stop.

Which is more fuel efficient? Generalize to a basic manual tranny car assuming you won’t stall the thing

  1. Work through all the gears - in my case, say 1st up to 2500 RPM, then 2nd to 2500 RPM, then 3rd up to maybe 2000, then 4th, then 5th.

  2. Go quickly into 5th gear - start in first up to maybe 2000 RPM, then maybe 2nd up to 2000, then go right to 5th.

0.5 km? Personally, I’d walk.

Heh, point taken.

I’m still wondering since I make the same ~20 km commute to work every day. 0.5 km is about how far apart the lights are spaced

Well, probably more like 1 km apart.

In that case, the standard (no pun intended) advice is to accelerate and decelerate smoothly. You don’t want to burn energy only to ‘waste’ it by having your brakes turn it into heat. For half a km you probably don’t want to get into 5th at all. Use the most efficient gear for your speed.

And keep in mind that neutral is an option. For a 3 block drive on a flat surface, you can coast the last block.

But which is more efficient? 3rd gear at 3000 rpm or 5th gear at 1500 rpm? That’s really what I’m wondering.

in this case it would be 4th gear. ideally you’d want to accelerate gently, and upshift around 2500-300 RPM. 1500 rpm is about the low end of using 5th gear and if you try to accelerate hard from low RPM you’ll be overstressing interior engine components. think of what it’s like to ride a 12 speed bicycle when it’s in high gear and you’re running at a low speed. The resistancce your legs are feeling is somewhat analogous to what your engine’s piston tie-rods are feeling. If it’s a flat surface with no traffic you should be able to coast after about the 1/2 way point to the stop sign. If the speed limit is below 45 mph then it’s pointless to put it in 5th gear. 4th gear should be adequate for city driving at max legal speed.

Previously owned a 1998 GT both in Italy and later in small town/city eastern Oklahoma. Car delivered 25-28 miles per gallon in semi-rural driving - assuming my foot wasn’t into it. Regular gas too! My best method was in general driving to skip as many gears as practical; 1-3-5 or 1-2-4-5, 1-2-5. The engine is very flexible, sufficient torque even at very low rpms. Usually shifted slightly over 2000 rpm in first, rest of gear changes at a leisurely 2000 rpm or less. Let’s you hear the radio and still a great exhaust rumble.

Gas money comes out of take-home-pay and fun money. YMMV.

I can’t give a factual answer to that question, but someone here can.

But from a practical perspective: My other car is an MGB roadster with a four-speed gearbox and overdrive in third and fourth. Of the freeway I tend not to use OD, even though the 50 mph road would allow it. This is because I get my torque higher up. Driving a sports car or a GT is not about getting good mileage (although I do get good mileage with my 1.8l engine); it’s about maneuvering. Keeping the revs in the proper range allows the driver to maneuver more efficiently because the throttle is much more responsive. (I’m of the school that proper handling involves the throttle – though I don’t like to use the brakes.)

Personally, I believe (without facts or cites to back me up) that keeping the revs in the proper range does save gas because you don’t have to mash on the pedal and wait for the revs to match the situation. That is, if I want to pass someone I’m pressing down on the pedal for less time if I’m already at the correct RPM. But what about all of the steady driving? Don’t you burn less fuel chugging up from low RPM for a pass, as opposed to steady driving at higher RPM? (And bear in mind that your engine is three times larger than mine, so techniques will be different.) Not sure. But the spirit of this type of car is to rev higher.

Ntcrawler and Johnny make good points, but the question is not really about the proper way to drive, or the most fun way to drive. As far as maximizing fuel economy, SmithSB nailed it. You’ll generally get the best mileage with lower RPMs and wider throttle openings. In a small economy car, you’d be noticeably lugging the engine, but this car has a very large displacement, torquey V8 engine and it is perfectly happy to skip gears. It’s not fun, but essentially the best way to maximize fuel economy.

AFAIK Ford never tried it with the Mustang, but Chevrolet made a 1-4 shift lockout on many 6 speed Corvettes and F-Bodies in the interest of fuel economy. Putting along in traffic you don’t really need 2 and 3, but of course if you really hustle it, you can row through all the gears. Depending on how the Mustang is geared, 1-3-5 may be more comfortable.

For the Mustang, 1500 in 5th is going to be more efficient than 3000 in 3rd. Your engine friction is reduced (and roughly proportional to engine rpm), and I can tell you from personal experience (from my 98) that a Mustang GT has more than enough torque to smoothly pull away at 1500 rpm in most situations.

My grrl Fierra’s new Corvette has the 1-4 skip-shift. She finds it much smoother and easier to go 1-3-4-6 in everyday driving. Second is only used in hard acceleration, and 5th is very rarely used, if at all.

Lower RPM is always better. The one quirk of modern engines is that coasting may not be the best idea - if one is simply engine braking at high gear, Modern ECUs will (of course YMMV depending on how modern the engine is) cut off the fuel completely. This is better than coasting in neutral, because in neutral, the engine is still idling (and burning fuel).

It seems terribly counterintuitive -

Say I shift into 5th and the engine is running low, 1000-1500 rpm.

Is full throttle open (low acceleration) really efficient? I guess my perception is that full throttle open even if the car isn’t accelerating quickly is that a lot of gas is being combusted.

In my experience with manual transmissions, your case would involve shifting up exactly as described in the car’s manual. I say this because whenever I’ve read one of those things, they invariably recommend shifting up earlier than I normally would. It really seems they are going for conservative, efficient driving, rather than responsiveness and performance.

It just boils down to the old basic of shifting early to save gas and late to get some zoom zoom happening. I don’t think you need to worry about exactly what gear you’re in as an on-paper exercise like it’s being discussed here, and no need to consider lugging the engine unduly - just drive it like you’re going to church.

The problem is that you are thinking of the throttle in the context of directly controlling the engine’s supply of gasoline, when it actually is controlling the amount of airflow into the engine. (what follows is a very dumbed down explanation of basic car technology, there are very many variations on this topic, particularly drive by wire throttle in which there is no cable connection btw the throttle and gas pedal at all.)

In modern automobiles, gasoline is injected into the cylinders at a rate which is controlled by a computer in response to signals from various sensors, only one of which is the throttle position sensor. When you floor it, you essentially open up the airway to the engine and tell the computer you want more “go”, but the computer actually determines how much gas to inject via the fuel injectors based on input from all the sensors. This ensures that the gasoline is combusted much more efficiently rather than the scenario that you are thinking of where the throttle is directly controlling the supply via a “gas hose”.

This gets a bit complicated, but when I mentioned lower RPMs and wider throttle openings in my earlier post, I was essentially describing a “leaner” running condition in which more air was available to the engine, and the fuel flow to the engine was computer limited. Engine computers calculate fuel delivery based on input from the sensors via fuel “maps”, essentially the optimum amount of gas to inject for a given set of temperature, throttle, airflow in, oxygen % in exhaust, etc. inputs . The computer determines fuel flow electronically, so when fuel is being limited, and the car is accelerating slowly from a low RPM at a decent throttle opening, the engine is pretty much running at it’s maximum efficiency.

You have a GT and you’re asking about fuel economy? Probably should have sprung for the V6.

But, to answer the question, less RPMs = less fuel used. There are two ways to test this. The first would be to get up to 5th every time for a tank, and see how many miles you get on a tank. Then, keep it in 3rd on the next tank. The other would be to take an average of your RPMs and figure out which one keeps them lower. Keep in mind that “lugging” your engine will put more wear on the crankshaft than keeping the RPMs a bit higher.

But, like what I think Johnny L.A. was insinuating, you have a GT, man, have fun with it! (I drive an '06 Charger myself, so I’m kinda partial the pedal).


When I’m trying to beat someone off the line, I’m not worried about fuel efficiency.

When I’m making the same dull commute every day, I am.

…also I test drove the V6 and it felt exactly like my old Camry, so no deal :smiley:

To echo Stan Doubt, everything I’ve read says that low RPMs give you better efficiency, until they’re so low that the engine runs unevenly.

However, I have also read that if you are going to wind up at a certain speed, say 55 mph (though not on your 1/4 mile commute), it is better to accelerate fairly hard to get there than it is to accelerate gently. The point is that gasoline engines aren’t very efficient when they are aren’t working very hard. So, while working hard uses more fuel, it uses less fuel per mechanical energy produced. It takes the same mechanical energy to bring the car to a certain kinetic energy (which is just half the mass of the car times the square of its speed), so, better to deliver that mechanical energy at a point on the engine’s efficiency curve where the efficiency is higher, say, at 3/4 maximum horsepower output.

This last bit sounds reasonable to me, but it is a detailed quantitative issue - does anybody know more specifically?

Personally, I doubt this. Outside of a test facility, it can be very hard to measure small changes in fuel economy, because so many variables come into play (taking the exact route, having the same warm-up time, tire pressure, wind, ambient temperature, traffic flow…)

And why will the crankshaft have more “wear?” I’m calling cite on that. The main problem I know of with “lugging” is that it increases the chance of detonation.