[richlux]

I don't fully understand what the gear ratios mean, but I had a question about how the gears affect fuel efficiency. Aside from the obvious that the revs lower as you shift to a higher gear, how does driving in one gear versus another affect gas mileage. Will driving in 6th at 2k rpms give you better fuel economy than driving in 5th at 2.5k rpms? Personally, I like to keep the car closer to 3k all the time because I like having the power available without having to downshift. Am I losing fuel efficiency because of this?

Thanks,
Rich

[NFS]

Originally Posted by richlux

I don't fully understand what the gear ratios mean, but I had a question about how the gears affect fuel efficiency. Aside from the obvious that the revs lower as you shift to a higher gear, how does driving in one gear versus another affect gas mileage. Will driving in 6th at 2k rpms give you better fuel economy than driving in 5th at 2.5k rpms? Personally, I like to keep the car closer to 3k all the time because I like having the power available without having to downshift. Am I losing fuel efficiency because of this?

Thanks,
Rich

For economy you should always drive in the highest gear possible. The only time this slightly varies is - if you are going up a steep hill you might have to be on a lot of throttle in 6th to maintain your speed in which case you are better off dropping a gear.

[SoYank]

Originally Posted by richlux

I don't fully understand what the gear ratios mean, but I had a question about how the gears affect fuel efficiency. Aside from the obvious that the revs lower as you shift to a higher gear, how does driving in one gear versus another affect gas mileage. Will driving in 6th at 2k rpms give you better fuel economy than driving in 5th at 2.5k rpms? Personally, I like to keep the car closer to 3k all the time because I like having the power available without having to downshift. Am I losing fuel efficiency because of this?

Thanks,
Rich

The gear ratio is the ratio of crankshaft revolutions to the revolutions of the drive wheels or how many times the engine rotates in order to rotate the drive wheels once. The lower this is, the better fuel economy you realize.

On the US 330i, 1st gear is 4.35:1, 2nd is 2.50:1, 3rd is 1.67:1, 4th is 1.23:1, 5th is 1.00:1, and 6th is 0.85:1.

Take 5th gear for example. For every 1.00 revolutions of the crankshaft, the rear wheels will make 1 revolution. If the wheel is 1 meter in diameter (for explanation purposes only) and the engine is running at 2500 rpm, then you move down the road 2500 meters in one minute. In 6th gear at 2000 rpm you move 2,353 meters. However, in one minute the engine has fired 15,000 times in 5th gear vs. 12,000 times in 6th. The engine has fired 25% more to go a mere 6% further.

To paraphrase needforspeed, the higher the gear and lower the rpms the better the fuel economy. Grant you, you won't get 50% economy driving strictly 6th gear as you would in 5th because there are other factors that actually cause a greater load on the engine. For example, an engine running at 2000 rpm moving a heavy load will use more fuel to maintain that 2000 rpm than an engine moving a lighter load at the same rate or even moving a given load up a hill at 2000 rpm compared to the same load and rpm on a level road.

Don't try driving long distances in reverse and expect good gas mileage. Its ratio is 3.93:1. You'll be mighty disappointed. Not to mention tired.

[bavarian19]

watch your MPG gague as you drive... youll figure out how your driving affects the economy...

sometimes, the higher gear is not the best economy. If your cruising on a flat road it will help you. If your going uphill and you are hard on the throttle in 6th gear, you can use more fuel than if you were in 5th because of the strain you are putting on the engine...

[petesamprs]

Follow up question: someone here once mentioned that it's better (solely from a gas mileage perspective) to coast down a hill in a gear vs. in neutral. The argument was that, despite the higher RPM when in gear, the turning wheels were keeping the engine at that RPM, versus neutral where the engine was doing all the work keeping itself at idle.

Is there truth to this? I tend to think that higher RPM results in higher fuel consumption.

[shimshimhada]

Originally Posted by petesamprs

Follow up question: someone here once mentioned that it's better (solely from a gas mileage perspective) to coast down a hill in a gear vs. in neutral. The argument was that, despite the higher RPM when in gear, the turning wheels were keeping the engine at that RPM, versus neutral where the engine was doing all the work keeping itself at idle.

Is there truth to this? I tend to think that higher RPM results in higher fuel consumption.

Yes. Even if you're RPM is at 4000 in 2nd let's say, you're completely off the throttle and the only thing moving the car is momentum. If you stick it in neutral, the only moving the car is still momentum, but the engine is still firing to keep the engine idle. Why would anyone want to put the car out of gear downhill anyway?

[petesamprs]

That is very interesting, and implies that RPMs are not directly correlated to fuel efficiency, but rather the source of the RPM.

I guess it makes sense from a common sense perspective. Going down a hill at a constant speed in 2nd @ 4k is basically using gravity to move the car. Going up a hill at a constant speed in 2nd @ 4k is using all fuel to move the car.

In the downhill/no throttle case, then, is there no gas being fed into the engine?

[shimshimhada]

Originally Posted by petesamprs

That is very interesting, and implies that RPMs are not directly correlated to fuel efficiency, but rather the source of the RPM.

I guess it makes sense from a common sense perspective. Going down a hill at a constant speed in 2nd @ 4k is basically using gravity to move the car. Going up a hill at a constant speed in 2nd @ 4k is using all fuel to move the car.

In the downhill/no throttle case, then, is there no gas being fed into the engine?

On rolling roads, I like to get my speed up when I'm going downhill by applying a little throttle, so by the time I don't lose speed by the time I reach the next crest. I'm pretty sure there's no gas being fed, just look at the MPG meter drop to infinity! Okay, neutral probably does that too, so the differences would be really minimal, it's better practice to leave it in gear anyway.

[NFS]

Originally Posted by petesamprs

Follow up question: someone here once mentioned that it's better (solely from a gas mileage perspective) to coast down a hill in a gear vs. in neutral. The argument was that, despite the higher RPM when in gear, the turning wheels were keeping the engine at that RPM, versus neutral where the engine was doing all the work keeping itself at idle.

Is there truth to this? I tend to think that higher RPM results in higher fuel consumption.

Coasting in neutral is not ideal - it's not particularly safe because the car is not really under control and I believe it's illegal in many places.

[froman]

The August issue of Car and Driver has a column by Larry Webster on this very subject, even using the C and D Long-Term Test 330i six-speed as the car. He experiments with various fuel-saving measures in the BMW, namely:
1. Always cruise in top gear
2. Drive slower (duh)
3. Turn off the A/C (yeah right, in Houston?)
4. Keep your tires properly inflated
5. Accelerate moderately

[richlux]

Originally Posted by froman

5. Accelerate moderately

I will for the next 600 miles (still in the break in period), but then all bets are off :-)

Thanks guys,
Rich

[akhbhaat]

Originally Posted by petesamprs

Follow up question: someone here once mentioned that it's better (solely from a gas mileage perspective) to coast down a hill in a gear vs. in neutral. The argument was that, despite the higher RPM when in gear, the turning wheels were keeping the engine at that RPM, versus neutral where the engine was doing all the work keeping itself at idle.

Is there truth to this? I tend to think that higher RPM results in higher fuel consumption.

The car should never be in neutral while moving, unless you're on the verge of stopping.

[SC 330i]

why does the MPG gauge displays higher fuel comsumption right before every stop? is this due to engine breaking? my foot is off the throttle yet the mpg is at 12 and then goes up when the car is at a complete stop. i have a step

[David328M-Sport]

Originally Posted by SC 330i

why does the MPG gauge displays higher fuel comsumption right before every stop? is this due to engine breaking? my foot is off the throttle yet the mpg is at 12 and then goes up when the car is at a complete stop. i have a step

from what i understand theoretically at least the car is consuming infinity mpg at standstill. using fuel but literally not going anywhere. so the economy guage should, as it did in my old e46, display infinity mpg.

now from lots of feedback bmw decided to change the display to zero in the e90, so people would not panic as much. with todays fuel prices, it was a good chce. however, the reality is that the car is still consuming infinity mpg till you push off again.

[ANovak]

Originally Posted by David325Australia

from what i understand theoretically at least the car is consuming infinity mpg at standstill. using fuel but literally not going anywhere. so the economy guage should, as it did in my old e46, display infinity mpg.

now from lots of feedback bmw decided to change the display to zero in the e90, so people would not panic as much. with todays fuel prices, it was a good chce. however, the reality is that the car is still consuming infinity mpg till you push off again.

At a complete stop while the engine is idling you get 0 MPG not Infinity. The E46 had a solenoid that disengaged the MPG gauge below 15 MPH.