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Registered:
August 2002
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Fuel usage at same RPM levels in different gears
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Mon, 16 June 2003 11:21
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This is something ive thought about for a while as i cruise in a lower gear than i really should so i have power on tap when i want it etc.
Anyways what i was thinking is sitting on 3000rpm in 3rd, would i be using as much fuel as 3000rpm in 1st gear?
Also, will this vary with throttle position, ie. WOT in 1st at 3000rpm and no throttle at 3000rpm in 3rd?
Im guessing the maps will be enriched in WOT territory and once backed off the fuel will drop off, but im still turning the engine over at 3000rpm.
Ideas and comments welcome.
Will search if you tell me what to search for 
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Location:
Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Mon, 16 June 2003 11:59
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1st gear - 3000 rpm
3rd gear - 3000 rpm
same fuel usage ?
This is your question yeah ?
simple answer i think 3d gear would use more as because of wind resistance, the engine is under more load hence it needs to try harder to maintain speed
in regards to backing off throttle.....im pretty sure newer cars shut off their injectors alltogether when this happens
im not 100% on my response though
Thanx
Mani
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Location:
Brisbane
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Mon, 16 June 2003 12:13
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Yeah, it's pointless comparing fuel economy at different speeds because factors like wind resistance and rolling friction have more effect than the engine's efficiency.
As a rough guide, your engine is at its most efficient when it's developing peak torque. Even that's not clear-cut though, because some engines develop their peak torque at relatively high rpm, and the engine's internal losses increase with rpm!
I think it's safe to say that spinning the engine at a lower speed at any given road speed is the most fuel efficient, provided the engine isn't labouring.
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Registered:
August 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 10:34
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Ok , assume dyno conditions where there is no wind interferance
1st - 3000rpm
4th - 3000rpm
both under equal load.
Do they both recieve identical amounts of fuel?
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Location:
Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 11:00
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Yeah id also like to know this
I thought about it once but never really got into it for some reason
for some reason i still think itll take more fuel but i dunno y i say that
Mani
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 11:42
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4th would be using more.
although the load at the tires would be the same, as a result of the gearbox ratios the load on the engine would be a lot greater for 4th gear than for 1st gear, so 4th would use more fuel.
if the load was adjusted for the gear ratios, in theory 1st and 4th would use identical amounts of fuel (apart from the frictional and centrifugal forces generated by the wheels and internal components between gearbox and dyno)
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 12:08
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we all missed the fact at 3000rpm in every gear, the engine turns at the same speed so gearing is used for greater speed! load increases with high gearing and weight{downforce,people etc} of course.
mick
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Location:
Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 12:09
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Yeah i was gonna refer to gearing but thought that speed would cancel it out
guess not
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 12:23
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just another slant on it..
an engine will use (assuming injected and equal AFR's) an amount of fuel that is directly proportional to the air being consumed.
since torque is directly proportional to air consumed (or volumetric efficiency), disregarding internal losses for different rpm, the amount of fuel for a given torque output will be equal.
given that for cruising at a given speed will have the same air resistance, the torque at the wheels will need to be the same, so fuel consumed is equal.
from this you add in losses...so in a higher gear (like 3rd instead of 4th) you have more engine thrashing around, so more losses....
but it also depends on your torque curve...
if you are below your peak, and the car slows a little because of a rise in the road, you have to apply more throttle, cos oyu have less torque at lower rpm with same throttle
if you are higher than your torque peak, then if the car slows, the torque increases, so car speeds up etc, all with your foot in same place....
this is of course assuming that the WOT curve applies to part throttle...
also in a higher gear, if you want to speed up, you have more torque advantage, so takes less fuel than in a lower gear.
all depend son a lot of things, but for a given road speed on a dyno, you will have more fuel used at higher rpm. same goes for on the road with wind resistance... but the difference may not be very much at all (and is basically the mechanical losses between the engine speeds). if you have EFI, borrow an oscilliscope and measure the pulse widths of the injectors on a dyno at same speed with different engine rpm... longer pulsewidth = more fuel...
Cya, Stewart
PS, if ya had a Megasquirt you could just look at the datalog to see what the pulsewidth was in milliseconds, then multiply by the number of injection events, based on rpm and you'd know exactly.. 
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Location:
North Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 13:21
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Something also you guys arn't considering.
Load takes into account wind, the weight of the car and everything. Your throttle will match the load on the car, ie you'll put your foot down the more load is put on the car be it more wind or a hill coming up.
Load is the major factor in fuel consumption.
Half throttle in 3rd at 3000rpm will chew alot more fuel then light throttle 3000rpm in 4th.
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Location:
Sydney
Registered:
July 2002
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Re: Fuel usage at same RPM levels in different gears
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Tue, 17 June 2003 23:29
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<my opinion>The 3rd gear to 1st gear comparison. 3rd gear will use more fuel.
When I had my turbo on, it wouldn't boost in neutral, and wouldn't have much boost 1st. That was because there was no load, and not as much airflow as when it was in 3rd gear. When there is more load, the engine will put more fuel (and air) in. Regardless of throttle position.
</my opinion>
Thinking about it again now, its all BS as it can't be relative to anything.
Driving in a higher gear when possible will use less fuel.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Fuel usage at same RPM levels in different gears
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Wed, 18 June 2003 04:05
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| SUPRAGTE wrote on Tue, 17 June 2003 23:21 |
Something also you guys arn't considering.
Load takes into account wind, the weight of the car and everything. Your throttle will match the load on the car, ie you'll put your foot down the more load is put on the car be it more wind or a hill coming up..
Load is the major factor in fuel consumption.
Half throttle in 3rd at 3000rpm will chew alot more fuel then light throttle 3000rpm in 4th.
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yeah, but if you are at a constant speed on the road, and change between 3rd and 4th, the load on the car from external factors is the same... half throttle in thr\ird will be accelerating much more than light throttle in 4th..
i know in my car that if i'm at 4500rpm, it uses less fuel than at 3500rpm, even tho 4500 is higher car speed, because the efficiency is much greater at 4500rpm...
i did this on the M5 going from the airport to camden and back, holding at those revs, or close to..... which is around 120-140km's... sure enough, higher revs used less fuel 
i think with bigger motors with lower torque peaks it would be a different story tho, and each engine is different anyhoo..
Cya, Stewat
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I supported Toymods
Location:
Berowra-Sydney
Registered:
July 2002
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Re: Fuel usage at same RPM levels in different gears
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Wed, 18 June 2003 07:59
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modern cars ar designed that when driving at 110km/h in your highest gear the engine will be running at its best fuel consumption 4 power.
Toyota will have 8 or so engines on a dyno running at this minute working all of that bull shit out, soif you have a modern car then look at what revs your doing at 110km/h in your overdrive gear and thats is the revs at which your engine runs best for fuel consumption.
got a guy at work who's job was in that Toyota Dyno room doing all of that crap
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Location:
Southern Sydney
Registered:
August 2002
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Re: Fuel usage at same RPM levels in different gears
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Wed, 18 June 2003 08:40
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if you have a modern car then look at what revs your doing at 110km/h in your overdrive gear and thats is the revs at which your engine runs best for fuel consumption.
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Surely this isn't correct for all cars?
They would have to be lowering the best fuel consumption to a lower speed on cars like echo's and corrola's as the majority of them are used in urban areas where the maximum speed reached is about 90km/h.
Mabye a Camry or other family sized car would be optimised for 110 but its seems a bit silly for a city car to be.
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I supported Toymods
Location:
Perth WA
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Thu, 19 June 2003 17:04
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Gearing depends on how they want to cook the books when they release "city" and "highway" fuel consumption figures. These are performed according to set protocols, and are important marketing tools as car buyers can compare them from car to car. In cars that have less of a mileage emphasis (ie Porsches or Bentleys etc) they probably tune their gearing for performance (i.e. second gear tops out at just over 100kph so you dont need to shift up when measuring performance times).
Also a gearbox may show a bias depending on driver preferences - American gearboxes behind V8's tend to have a very tall final gear as they like them just over idle rev on the freeway while cruising.
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Location:
Toowoomba
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Thu, 19 June 2003 23:25
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Lets look at this from a slightly different perspective. Say you are travelling 100km on a straight, flat road with no wind, first at 3000rpm in 1st gear (25km/h, 240mins) and secondly at 3000rpm in 3rd gear (60km/h, 100mins). This will give you about 720,000 revolutions for the whole trip in 1st or 300,000 in 3rd (not really acurate, will differ between cars, good enough for an example).
Now, we know how many revolutions for the trip (about) so now work out how much fuel pre revolution. In 1st you will use less pedal, there fore more vacume, less and and less fuel, so we will say 2ms injector timing in 1st would give you about 15L for the trip, now at about 3.5ms for 3rd gear would give you around 14L.
But if you take into account resistances like road, mechanical, wind, weight of car, variations in road condition and incline, how much stopping/starting/overtaking/blah blah blha. You'll see there is no real difference.
As you can see I have used alot of guestimation, estimate, aproximation, and talkcrapimation. So take what i've said with a grain of salt, there is some sense in my calculations but I don't have enough accurate figures to work it out properly. If some one could give me exact speeds at 3000rpm in 1st and 3rd, injector flow rates for that duty cycle at that speed, it would be much more accuate but still would not simulate real world conditions. You could however at take 1sec intervals the injector duty cycles and take an average of that for a trip but it's unlikely you could stay at 3000rpm in either gear for much of a time.  I'd say hope this helps but I don't think it would, so if you actually read all of this you have as much spare time as me .
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Location:
Sydney
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Fri, 20 June 2003 09:48
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nah he doesnt want to measure it from driving a distance
he wants to measure with time
Thanx
Mani
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Location:
Cabramatta, NSW
Registered:
May 2002
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Re: Fuel usage at same RPM levels in different gears
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Sat, 21 June 2003 01:59
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| GIN51E wrote on Wed, 18 June 2003 17:59 |
modern cars ar designed that when driving at 110km/h in your highest gear the engine will be running at its best fuel consumption 4 power.
Toyota will have 8 or so engines on a dyno running at this minute working all of that bull shit out, soif you have a modern car then look at what revs your doing at 110km/h in your overdrive gear and thats is the revs at which your engine runs best for fuel consumption.
got a guy at work who's job was in that Toyota Dyno room doing all of that crap
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Except that at 110, there's a lot more wind resistance than at 70. It might be a good guide, but I wouldn't say that those revs are right in all real world situations.
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