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Old 05-04-07, 02:50 PM   #1
phantomcow2
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Traffic Jam advice. And Q on gas mileage

So I was caught in my first traffic jam with my accord today . Good practice getting into 1st from a full stop!
This was the traffic where you would go for 20mph for 150ft maybe, and then come to a stop. I stayed in 1st gear until I had to stop again. But, I didn't like running the motor at nearly 4k rpm. I tried shifting up into 2nd gear, but it seems kind of silly because I would have to basically stop in just a few seconds.
What is the best method? Is it best to keep the car in 1st gear and pretty much rev the engine? Should I just deal with the extra shifting and go into 2nd?

The other question is about gas mileage. I understand running your engine at high rpm will typically mean lower gas mileage. Being in 3rd gear @ 65mph probably won't give you the same gas mileage as being in 5th.
What about if I was cruising along at ~50mph? In 4th gear, this would be at roughly 2500rpm, maybe a hair more. My dad encourages me to get into 5th gear as soon as possible, usually around 40 if the road is flat. At 50mph in 5th gear, my engine is just under 2k rpm. So it does run at a lower rpm obviously. But at the same time, it has to generate more torque to supply the demand (assume a flat road). Does this extra demand on torque end up requiring more gas than running at a higher rpm, but lower demand on torque?
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Old 05-04-07, 03:00 PM   #2
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First question. I'll probably get argument on this but here's what I do if I'm only going to go a couple hundred feet before I have to stop again. Take off, as soon as you have enough speed to coast the rest of the way to the next stop let off the gas, push in the clutch and coast.

Second question. Think of your engine as an air pump. Exactly what it is. The faster it turns the more times each cylinder fills with air/fuel. The slower it turns the fewer times each cylinder fills with air/fuel. The less cylinder fills of air/fuel, the less fuel used.
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Old 05-04-07, 03:03 PM   #3
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Originally Posted by SoonerBent
First question. I'll probably get argument on this but here's what I do if I'm only going to go a couple hundred feet before I have to stop again. Take off, as soon as you have enough speed to coast the rest of the way to the next stop let off the gas, push in the clutch and coast.
I do the same, more or less. Though I try not to ride the clutch and instead take it completely out of gear.
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Old 05-04-07, 03:05 PM   #4
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I do the same, more or less. Though I try not to ride the clutch and instead take it completely out of gear.
what i do too.
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Old 05-04-07, 03:07 PM   #5
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Originally Posted by x136
Try to keep a smooth, steady speed.
i do this too. if you were in front of me and looked in your rearview mirror i'd be waaay back there.
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Old 05-04-07, 04:22 PM   #6
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As for the traffic jam situation, get out of the habit of rushing up to the car ahead of you and jamming on the brakes. This is what causes a lot of traffic jams in the first place. Try to keep a smooth, steady speed. Slow enough that the car in front of you is starting up again before you get there. You may find a large space opening in front of you, but that's okay. If someone pulls into that space, just hang back some more.

Exactly. This is especially important when driving a large truck like I used to drive, or anything thats slow off the line. Notice 18 wheelers in the right lane during traffic 'jams' will, usually, always be crawling along while the cars in the middle and left lanes are constantly starting and stopping. If everyone just chilled, they'd probably save so much more wear and tear on their cars as well as saving gas, and preserving their patience too.

I've been able to anticipate stops too on certain stretchs of road that curve ahead of me. I'll see 500 feet down and the cars will be doing this:



And if I'm careful (and nobody cuts in front of me), I can adjust my speed so I'll still be cruising by the time the person in front of starts again. And you can do it for a lot intersections controlled by traffic lights.

Too bad this simple concept is lost on people. Speeding up to stop 100 feet ahead of you is ridiculous. Especially over and over again...




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Not only will you still arrive at your destination in about the same amount of time, you'll actually smooth out the traffic behind you as well (I find it takes a lot of the stress out of driving in traffic, too).
It does smooth the traffic immediatly behind you, but I'd have assume, that since your driving so slow compared to the speed limit, cars that first encounter the slowed the down traffic (unless the back up is for an entire stretch of road) are probably going to press hard on their brakes, slowing themselves down to a rate less than that of yours, but then increasing it to catch up, and slowing down again, repeat ad nauseam for the cars behind them and another 'wave' appears.

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Old 05-04-07, 04:29 PM   #7
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What I'll do in traffic is just get myself into second gear, sometimes from a dead stop. My car will get going in 2nd without any gas, just careful clutch work. Then, I give myself plenty room and just let the car roll along in second without any gas. By the time I catch up to the guy in front of me, the traffic has begun moving and I woin't have to stop or clutch even. Of course, this all depends on the dinks around me leaving my buffer zone alone.
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Old 05-04-07, 07:44 PM   #8
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...avoid the Spaulding at 5ish...
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Old 05-04-07, 09:25 PM   #9
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Quote:
Originally Posted by SoonerBent
Second question. Think of your engine as an air pump. Exactly what it is. The faster it turns the more times each cylinder fills with air/fuel. The slower it turns the fewer times each cylinder fills with air/fuel. The less cylinder fills of air/fuel, the less fuel used.
Your engine is a big air pump, but it's not quite that simple. It's also about the fuel demand signal for the power required. For example, I've got a diesel pickup truck. I could roll down the road at 55-60 with the engine just loping along and get about 21 or 22 MPG (yeah, I know, but I've done it; amazing for a vehicle this big!). However, at that same 55-60, turning the same RPMs, towing 7500#, I'll only get about 13-14 MPG.

That said, yes, smoothly accelerating to the point you can let the engine idle while you coast along will save some gas. And, if you can work it so you don't have to come to a complete stop, you'll do better also (don't have to overcome the object at rest and all).

Unfortunately, traffic jams are going to cost you fuel/MPG because the bottom line is you're burning gas and not moving!
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Old 05-04-07, 09:33 PM   #10
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Originally Posted by SoonerBent
Second question. Think of your engine as an air pump. Exactly what it is. The faster it turns the more times each cylinder fills with air/fuel. The slower it turns the fewer times each cylinder fills with air/fuel. The less cylinder fills of air/fuel, the less fuel used.
Best pseudo-interpetation of the internal combustion language EVER!!
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Old 05-05-07, 12:34 AM   #11
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Best pseudo-interpetation of the internal combustion language EVER!!
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Old 05-05-07, 06:24 AM   #12
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I just ride past all the stopped cars using the sidewalk when this happens

The main arterial road I used to have to ride on for my daily commute would come to nearly a standstill as people tried to push and shove to get onto the highway. I finally got sick of being stopped, and filtering in a traffic jam isn't always a good idea as people are always trying to change lanes. Sidewalk for the win!

If you've gotta be stuck in a car, though, I actually try to even out the stop and go. I'll move up really slowly, and as I'm approaching the car in front of me, traffic will often move up another 150 feet or so... and so on. This allows me, and those behind me, to move along at 7-10 MPH constantly (me idling in first or second gear), while the dweebs in front of us are going 25, 0, 25, 0, 25, 0, 25, 0.
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