Economy question. Revs vs Load

[Adamal]

So I've always driven around at ~60kph in 4th gear for as long as I can remember.
Up until recently, I had a work car, a 1.3L Sirion. Now I no longer have one, I had to get my own car, and typical me, got an N/A SW20.
Having gone from a low torque car to a high(er) torque car, I find myself sitting quite comfortably in 5th gear when going ~60kph.
It got me to thinking, at first I though "Gee, this is probably going to be economical!" but then remembered being back in 5th form physics, where the teacher told me energy in = energy out.
So it got me thinking even more, would it not even matter if I were in 5th or 4th at the same speed?
Would the load in the higher gear counter the revs of the lower, so that fuel usage would be the same?
I know theres a few smarties here on TS who would probably be able to provide an answer with a good explination!

[Pelo]

Generally the BSFC (brake specific fuel consumption = amount of fuel consumed per unit of power made) is lowest at the engines torque peak.
High revs & Low load means high vacuum and high pumping losses.
Low revs & high load means high airflow and fuel flow.

In real life the easiest way to measure this would be with a fuel flow gauge probably. Old vacuum gauges always had the green economy section between 20 - 10 in.Hg.
As long as your motors not lugging or buzzing with revs I guess its running pretty efficiently.
4th sounds about right for that speed in a 2 litre, I tend to leave 5th gear till 70+ kph

Interesting autospeed article here: http://www.autospeed.com.au/cms/A_110216/article.html

[KinLoud]

Lower revs and bigger throttle opening will TEND to give lower fuel consumption.
- less internal power loss at lower revs for the same power output
- less pumping loss with bigger throttle opening

Complex subject.
Lots of the BSFC (brake specific fuel consumption) charts are related to wide open throttle at that rpm - so not as relevant for part throttle cruising.

Ken
Ham
021 408 863

[QikStarlie]

put a link in it and watch the injector duty cycle

[85AW20v]

I know that in my auto Lancer on the daily run to Turangi from Taupo, I drop it out of overdrive as I have less throttle on for the same speed on certain parts of the road that are slightly uphill. If I keep the throttle position the same and drop out of overdrive, I get to about 120km/hr. So in this case, it's more revs = less load = better economy.

[barryogen]

completely anecdotal, but in the last few weeks I had the RunX I tried driving in 4th at 50km/h instead of 5th, and the fuel economy seemed to be better.

my mileage/tank tended to be fairly consistent, and that was the only change I made(that I can think of).

[neon_spork]

Yes, energy in ~ energy out (less some for inefficiencies etc) but the engine will run most efficiently at a certain RPM. What ever gear is closest to that RPM at your desired car speed will give you the better fuel efficiency

[KinLoud]

Look at this page - interesting stuff about fuel consumption and BSFC of turbo vs NA!!

http://www.tc.gc.ca/programs/environmen ... Report.htm

Ken
Ham
021 408 863

[ChaosAD]

put a link in it and watch the injector duty cycle

Bet me to it.

Or get an obdII logger if the sw20 has it

[Rob]

Its all good thinking about fuel consumption........Tried thinking about all the extra load you putting on the mechanical parts by driving it in 5th at such a slow speed???? All that labouring can't be goor for it, if you want to drive around in 5th get a six speed box

[Stealer Of Souls]

completely anecdotal, but in the last few weeks I had the RunX I tried driving in 4th at 50km/h instead of 5th, and the fuel economy seemed to be better.

my mileage/tank tended to be fairly consistent, and that was the only change I made(that I can think of).

Yet more anecdotal info. But i've had similar experiences with a couple of lower powered cars... Economy seemed slightly better or about the same in 4th as 5th when trundling around town.