[shadowx360]

Hi, this is more of a curiosity question than trying to MPG-whore. I have an automatic E90 N52 and I always drive on M/T mode. I was wondering what is the best shifting/throttle point for accelerating from a red light? I drive a lot of high speed city roads and was just wondering about the theoretical best point. A quick Google for our cards only returns results of 2/3 throttle point (with no source) and only for those with a true M/T, not Steptronic.

Obviously, too light of a throttle will get honks and lack of efficiency from driving at low speeds but too heavy of a throttle got pretty bad results last time C&D ran tests (though with only a ~10% different between a very light foot and a very heavy foot).

[smallseafishie]

Let the car shift on its own while going easy on the throttle...

[NemesisX]

Originally Posted by smallseafishie

Let the car shift on its own while going easy on the throttle...

Yeah pretty much this. Unofficially I've found that the optimal cruising speed to maximize MPG is somewhere around 55-60 MPH. It's slightly downhill from there.

I get something like 28-30 MPG cruising gingerly at 55 MPH, 26 MPG cruising at 65, and 22-24 cruising at 75-80 MPH. Add in any appreciable city driving and my usual average is around an even 20 MPG. This is all on (approximately) flat ground.

[Dozhdbog]

When there's no one around to annoy with this technique, slowly speed up to cruising, then coast down for a bit, then slowly speed back up, etc.

[VTECaddict]

Originally Posted by Dozhdbog

When there's no one around to annoy with this technique, slowly speed up to cruising, then coast down for a bit, then slowly speed back up, etc.

This wastes more fuel unless you do some extreme measures. Accelerating takes more fuel than maintaining speed.

[avocet]

I shift at redline for best MPH... Oh wait you said mpg...

Never mind.

[Dozhdbog]

Originally Posted by VTECaddict

This wastes more fuel unless you do some extreme measures. Accelerating takes more fuel than maintaining speed.

This is considered a valid technique, but it depends on the car:

Burn and coast[edit source | editbeta]
Burn and coast is also known as pulse and glide. This method consists of rapid acceleration to a given speed (the "burn" or "pulse"), followed by a period of coasting down to a lower speed, at which point the burn-coast sequence is repeated.[25] Coasting is most efficient when the engine is not running, although some gains can be realized with the engine on (to maintain power to brakes, steering and ancillaries) and the vehicle in neutral, or even with the vehicle remaining in gear[citation needed].
Some hybrid vehicles are well-suited to performing the burn and coast. In a series-parallel hybrid (see Hybrid vehicle drivetrain), the internal combustion engine and charging system can be shut off for the glide by simply manipulating the accelerator. However based on simulation, more gains in economy are obtained in non-hybrid vehicles.[25]
Causes of pulse-and-glide energy saving[edit source | editbeta]
Much of the time, automobile engines operate at only a fraction of their maximal efficiency, resulting in lower fuel economy (or what is the same thing, higher specific fuel consumption (SFC)).[26] Charts that show the SFC for every feasible combination of torque (or Brake Mean Effective Pressure) and RPM are called Brake specific fuel consumption maps. Using such a map, one can find the efficiency of the engine at various rpms, torques, etc.
During the pulse (acceleration) phase of pulse and glide, the efficiency is near maximal due to the high torque and much of this energy is stored as kinetic energy of the moving vehicle. This efficiently-obtained kinetic energy is then used in the glide phase to overcome rolling resistance and aerodynamic drag. In other words, going between periods of very efficient acceleration and gliding gives an overall efficiency that is usually significantly higher than just cruising at a constant speed. Computer calculations have predicted that in rare cases (at low speeds where the torque required for cruising at steady speed is low) it's possible to double (or even triple) fuel economy.[25]
These two- or three-fold improvements in fuel economy are possible only at city driving speeds of say 25 or 35 miles/hour. This is because cruising (steady speed) at such low speeds is very inefficient since the torque needed is so low that the efficiency read on a BSFC map is very poor. Pulse and glide significantly improves this. Unfortunately, city driving often involves many stops at signals and stop signs which were absent in the computer simulation which showed such multiple fold improvements. In other words, in the real world one is unlikely to see fuel efficiency double or triple. Such a failure is due to signals, stop signs, and considerations for other traffic; all of these factors interfering with the pulse and glide technique. But improvements in fuel economy of 20% or so are still feasible.[25][27]

http://en.wikipedia.org/wiki/Energy-..._.28braking.29

[John 070]

I just turn the motor off whenever possible, it's called hypermiling. When in doubt, turn it off. I spent most of 2009 staring and comparing. Without hypermiling, my mpg was 21.7. With hypermiling, it was 21.9.

[asus389]

Get lighter/narrower rims/tires. I get like 35mpg on the highway when running on my 16" non-RFT Michelin snows. It drops to 30 with my wider bridgestone summer tires/and 17" 161 wheels.

As far as shifting goes, getting to the higher gears faster is the game. You can use the MPG meter to help you figure out when to shift. I dunno about with a "shiftable" auto is as far as I know you can't skip gears. If you can, you could go like 1-3-6 with as you would with an MT.

[VTECaddict]

Originally Posted by Dozhdbog

This is considered a valid technique, but it depends on the car:

I know it's a valid technique, but in order for it to work you have to do more than just simply speed up and slow down. That in and of itself will not save much fuel and will probably burn more. Like you said, it depends on the car and a modern BMW is not such a car.

For it to work best you need to be "gliding" in neutral with the engine off. This is not only cumbersome but will likely cause more damage and long term wear on the car and starter. Not to mention the heavy computer control of everything in a modern BMW, push button start, automatic transmission, and generally high repair costs for a German car. You're going to end up spending more money in repairs than the minuscule amount of fuel you'll save.

Also note that for MT, the manual recommends against 'bump starting' the car (using the clutch to restart the engine when the car is moving).

[spitfirecwp]

I just purchased a 2007 335i auto with 86000 mi on it on my 4 hr drive back I got 32 mpg wooing nothing other then putting the cruise control at 80 I was completely amazed. I'm sunrised to see that some of you get way less. I know the n55 is rated a little lower mpg then the n54 but I didn't realize it was that much

[Dozhdbog]

Originally Posted by VTECaddict

I know it's a valid technique, but in order for it to work you have to do more than just simply speed up and slow down. That in and of itself will not save much fuel and will probably burn more. Like you said, it depends on the car and a modern BMW is not such a car.

For it to work best you need to be "gliding" in neutral with the engine off. This is not only cumbersome but will likely cause more damage and long term wear on the car and starter. Not to mention the heavy computer control of everything in a modern BMW, push button start, automatic transmission, and generally high repair costs for a German car. You're going to end up spending more money in repairs than the minuscule amount of fuel you'll save.

Also note that for MT, the manual recommends against 'bump starting' the car (using the clutch to restart the engine when the car is moving).

Actually no. BMW's use NO gas when coasting. No need to turn off the car and glide. This technique works perfectly well with BMW's, actually. Works even better if you time your coasts up an incline and the throttle on the downhill.

[VTECaddict]

Originally Posted by Dozhdbog

Actually no. BMW's use NO gas when coasting. No need to turn off the car and glide. This technique works perfectly well with BMW's, actually. Works even better if you time your coasts up an incline and the throttle on the downhill.

All fuel injected cars use no gas when costing in gear. It's called deceleration fuel cut off (DFCO). The problem is when you coast in gear, your kinetic energy is now being used to turn the transmission, engine, and all the accessories like AC and alternator. With that comes all of the friction and air compression that goes on inside the engine, which slows the car down much faster than if it was just freewheeling in neutral. This is also known as engine braking.

So your stored kinetic energy is actually being wasted to turn the engine and slow the car down. This defeats the purpose of "pulse and glide" because you're not only squandering your stored kinetic energy during your "glide", but since the car is slowing down faster you will need to "pulse" more (eg use more gas) to bring the car back up to speed.

So like I said, this trying to pulse and glide in gear completely defeats the purpose and any fuel savings will be negligible at best.

[Eddien123]

My auto shifts at 1900-2300 (when I'm going easy) and the auto knows best you can't get better mpg by autoshifting.

[Dozhdbog]

Originally Posted by VTECaddict

All fuel injected cars use no gas when costing in gear. It's called deceleration fuel cut off (DFCO). The problem is when you coast in gear, your kinetic energy is now being used to turn the transmission, engine, and all the accessories like AC and alternator. With that comes all of the friction and air compression that goes on inside the engine, which slows the car down much faster than if it was just freewheeling in neutral. This is also known as engine braking.

So your stored kinetic energy is actually being wasted to turn the engine and slow the car down. This defeats the purpose of "pulse and glide" because you're not only squandering your stored kinetic energy during your "glide", but since the car is slowing down faster you will need to "pulse" more (eg use more gas) to bring the car back up to speed.

So like I said, this trying to pulse and glide in gear completely defeats the purpose and any fuel savings will be negligible at best.

I guess I think "negligible" isn't so negligable then. All I know is I've seen a 3-4mpg bump vs my typical driving.