Shifting and Fuel Economy... - Fuel Economy, Hypermiling, EcoModding News and Forum

davyboy · 03-19-2011, 06:40 PM

Okay I know it's been ages since I've been on here last, but now that I've got le small efficient car again, and there is no longer 5 feet of snow around, its time to make er more efficient...

Now a question, since my car is a 5speed manual, I'll post some numbers, at 2000rpm in each gear and the speed I'm traveling...

1st - 20kmph
2nd - 32kmph
3rd - 43kmph
4th - 62kmph
5th - 84kmph

These are rough numbers, but fairly accurate, now in first gear, I push the gas harder than a feather touch and with the gravel and everything on the road it spins the tires, so my question is as follows...

Is it more or less fuel efficient to start in second gear (skip first) and when is the best rpm range to shift (if I shift and the rpm are near or below 1000 it bogs and I can't imagine it is good for fuel economy etc)...

gone-ot · 03-19-2011, 07:49 PM

...but, *if* you're really interested in fuel economy, you'll *start* out in the highest gear that you can use (2nd or 3rd or even 4th, if possible) so as to get "up to" your intended speed and into the highest gear possible as quickly as possible, because two things "eat" fuel: acceleration (time & velocity) and speed (velocity).

...of course, exactly *which* gear you actually start in will depend upon traffic, vehicle load, engine warm-up, etc.

PaleMelanesian · 03-21-2011, 12:37 PM

I often start in 2nd gear.

I shift when I reach 2000/2100 rpm, or when the next gear will be >1400 rpm when I land there.

Really, though, I do 2nd and 3rd so fast, get into 4th by 20 mph (32kph). The shift into 5th at ~35 mph (55 kph).

1400-2100 rpm is the best range on my car, and likely many others. The lower end of that is best for cruising, the higher end for accelerating.

Arragonis · 03-21-2011, 07:20 PM

I have to disagree with the crowd on this one, do not start in 2nd/3rd.

Why ?

You have to slip the clutch more to build up speed before you can fully engage (i.e. lift off) it. A slipping clutch is a wearing clutch and a worn clutch is spendy, nasty, and prone to all sorts of other issues when "Mr Mechanic" starts to work on it. I have £2K (about $3K) invested in my clutch so I know

Try the following.

Start in 1st, gentle throttle - maybe even only idle to get moving. Rev gently to 2.5K to avoid wheelspin - getting this right feels nice.

This means you can fully lift off the clutch peddle asap.

Then press down fully and shift to 3rd, let the engine go down to 1K and lift off the clutch peddle again fully. There will be less jerk because the speed difference (and also clutch wear) is minimised.

Then Gently accelerate. Then repeat and change to 4th/5th depending on your chosen cruise speed.

With this approach you get out of the high-rev scenario quickly and also your clutch is full in / out as much as possible, so less slip and less wear.

The key here is clutch out at as low RPM as your engine can take given the circumstances.

j12piprius · 03-21-2011, 08:22 PM

Quote:

Originally Posted by Arragonis

Start in 1st, gentle throttle - maybe even only idle to get moving. Rev gently to 2.5K to avoid wheelspin - getting this right feels nice.

This means you can fully lift off the clutch peddle asap.

Then press down fully and shift to 3rd, let the engine go down to 1K and lift off the clutch peddle again fully. There will be less jerk because the speed difference (and also clutch wear) is minimised.

With this approach you get out of the high-rev scenario quickly and also your clutch is full in / out as much as possible, so less slip and less wear.

So you go from 1st at 2500 rmp -> to 3rd at 1000 rpm?

Why do you let the clutch out all the way, and then have to wait?

I've found shifting from 1st to 2nd is very smooth by letting the clutch only part way out, because I can shift sooner, more quickly and at much lower rpm.

Letting the clutch all the way out from 1st to 2nd would result in jerking, as you say, because that takes more time and the rpm's get too high.

JMac · 03-21-2011, 09:05 PM

I'm not convinced that starting a car in anything but first gear (unless you're on a downslope and can get some movement before using the clutch) is a good idea. Here's why:

Starting the car in 2nd or 3rd will make the clutch wear out exponentially faster.

Let's say, for the sake of argument that 2nd gear will spin the clutch pad twice as fast as 1st gear. One might think, "So my clutch might wear out twice as fast, no big deal, I can still make it last 75k miles..." However, when you factor in heat and amount of time needed to slip the clutch, we're talking about an exponential factor. A hot clutch wears its material out much faster. Also, if you need to slip it for twice as long, at twice the speed; we're looking at clutch wear of (2*2)x= y where x equals some sort of heat factor and y equals the amount of clutch material used relative to using first. So, we're using four times as much clutch material before multiplying it by however much the added heat affects it.

I'm sure my equation is not quite right, and there are some other factors. I'm a musician, not a physicist (I'm sure I'm ignoring weight and a few other variables).

So, how much is a clutch worth? I will admit one miiiight be able to squeeze a bit more FE, but I'm not convinced of that, especially since all that slippage is wasted energy, and that energy is being converted to heat on the clutch which increases wear.

That's just my $0.02.

JMac

Arragonis · 03-22-2011, 07:06 AM

Quote:

Originally Posted by johnlvs2run

So you go from 1st at 2500 rmp -> to 3rd at 1000 rpm?

Why do you let the clutch out all the way, and then have to wait?

I've found shifting from 1st to 2nd is very smooth by letting the clutch only part way out, because I can shift sooner, more quickly and at much lower rpm.

Letting the clutch all the way out from 1st to 2nd would result in jerking, as you say, because that takes more time and the rpm's get too high.

I'm not meaning like count 1 to 10, I mean a quick pause - a second or so. Its hard to explain these techniques, we need a video

The delay is to allow the engine (Petrol in this case so light flywheel) to settle down to 1K. When you lift the clutch peddle out the speeds usually mesh quite well and you're ready to go.

Maybe that was in the wrong order.

Start in 1st, go to 2.5
Clutch in, throttle off
wait for revs to drop
Into 3rd
Clutch out.
Away.

Jobs a gud'un.

Kodak · 03-22-2011, 08:58 AM

I can totally see how the lower RPM shifting would be a bit smoother.

However, I wonder if this method would prolong the amount of time it takes to get into optimum FE range.

I've been accelerating a little harder lately in an attempt to get up to high gear more quickly when possible. No ScanGauge (my results are by the tank), so I can't attest to the effectiveness of this brisk acceleration. However, I got the idea from a thread I read on here.

Skipping second gear would get you to third gear more quickly, but the poor acceleration at 1K would mean a good deal of time before getting into fourth. Maybe there would be a net gain by using second gear, but reducing the amount of time spent in third.

Perhaps this is a scenario in which tank-to-tank data is not specific enough to declare a clear winner.

PaleMelanesian · 03-22-2011, 10:18 AM

It depends on your gear ratios. My first gear is really short. I know some cars with a taller first than my second gear.

185,000 miles and no slipping on my clutch.

slowmover · 03-24-2011, 11:30 PM

I find that with my pickup truck -- over 7,000-lbs -- that First gear starts are a very easy way to get all that mass underway with minimal engine work. The time in gear is short, granted, but it beats slipping the clutch to start in Second. (Which itself is relative as Second is used by most with this spec truck). As the Gross Combind Weight Rating is 20,000-lbs, the use of First is a bit overkill when solo/empty, but all shifts proceed in the same manner afterwards which is like finding a rhythm and sticking with it.

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