which would consume more fuel? - Fuelly Forums

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Old 10-01-2018, 12:59 AM   #1
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which would consume more fuel?

Hey all, first post here.

Been doing some research about fuel efficient driving lately and got conflicting results.

first of all, my car is a 1995 nissan sentra 1.6 M/T. Interesting to note though, it doesnt have an o2 sensor, no catalytic converter, egr, and evaporative emissions control.

now to the question(s), is it more efficient to shift at lower vs higher rpms? (2000 vs 3000) after watching the youtube video of engineering explained about shifting for the best fuel economy, where he said its more efficient if the engine is loaded and at a low rpm. and its more efficient to accelerate on a higher gear (lower gear ratio) than a lower gear (higher gear ratio). but some say the exact opposite... its more efficient to accelerate at a lower gear as you are stepping on the throttle less and the injector whatever time is also less...

if i step on the throttle more at a low rpm to accelerate (engineering explained method), the engine will be loaded more. wouldnt the ecu retard the timing and inject more fuel as the load is higher? would less pumping losses negate that effect?

what if i downshift so my rpms are higher but im stepping on the throttle less (some other guy in the internet method)? the ecu would advance the timing, and (as they say) the injector time is less. therefore better fuel economy. would higher pumping losses negate that effect?

the user manual of my car says to shift at 2500 rpm. lets say im at second gear and at 1800 rpm. not accelerating, just keeping up with traffic in front. should i hold it at second till it reaches at 2500 or shift to third? if i shift to third, my rpm will be at 1200 and i suddenly need to accelerate. would it be more efficient to accelerate from 1200 rpm, or downshift back to second?

sorry if its all confusing or jumbled up, i didnt know how i would write my questions or where to start.

any input appreciated, thanks.
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Old 10-01-2018, 01:31 AM   #2
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Welcome. I have been through all this in the past year as I try to improve my mpg.
There is a thing called Brake Specific Fuel Consumption where the relative consumption is measured against load and rpm. There is a specific band where the engine is most efficient, round about the max torque. For my engine it is around 2,500 rpm.
It is best to accelerate, with a fairly wide throttle, until you reach that speed, then change up. I often change gear early if there is little load on the engine.
I have a ScanGauge E fitted to my OBD port and it shows instantaneous mpg. I find the lower revs with an open throttle gives better numbers than higher revs with a lighter throttle. Worth looking into such a device, or the software that connects to your mobile phone. More than pays for itself in 12 - 18 fuel savings.
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Old 10-01-2018, 01:56 AM   #3
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Thanks for your response.

Fuel prices here have been rising rampantly lately. Although at first i was not really concerned, but after taking my other car ( 2011 hyundai elantra) to a 100km trip for the first time; i was shocked to see how much more fuel efficient it was compared to my sentra. it was a real eye opener for me

I never really calculated my fuel consumption before... but my sentra would consume half a tank of gas on a 100 km trip, whereas the hyundai would consume the same half a tank. only difference is i drive the sentra gently (or normally i suppose) while i drive the hyundai like an f1 driver would.

Unfortunately, my sentra doesnt have a proper OBD port. though my hyundai has an mpg meter built in.
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Old 10-01-2018, 06:11 AM   #4
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For per-OBD cars, a MPGuino is an option for a scangauge.
https://ecomodder.com/wiki/MPGuino
It is much more involved installing than a simple plug, so may not be suitable for some people.
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Old 10-01-2018, 08:56 AM   #5
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Some interesting stuff here.
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Old 10-01-2018, 08:29 PM   #6
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Thanks for your responses, I believe there is a cheap mpg meter here that might work in my pre-obdii car; as for its accuracy, i couldnt say for sure.

Im not really hypermilling per se, im just trying to get my nissan to be atleast almost as fuel efficient as my hyundai; as i think that the fuel economy of the 2 shouldnt be as massive as it currently is, since they both have 1.6l engines. only big difference they have (aside from the hyundai being 20 years newer than the nissan) is the hyundai has an automatic transmission.

The biggest reason, i think, why the fuel economy of my nissan is massively lower than the hyundai's is my driving style. i tend to keep the engine of the nissan fairly unloaded, i would downshift on the smallest incline and shift fairly late. while the elantra would hold the gear and sometimes even upshift on hills, keeping the engine loaded. though the hyundai would never shift below 2000 rpm no matter how light the pedal input, but i digress.
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Old 10-01-2018, 10:04 PM   #7
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Biggest thing that influences fuel consumption is the use of the brakes. If you think ahead, leave sufficient gaps from the traffic in front, and avoid having to brake by lifting off early, you can save a load of fuel. Every time you brake you are creating heat, which you used petrol to create. Petrol that was not used to get from A to B.
The Hyundai used Deceleration Fuel Cut Off, whereby, if the engine revs are above a threshold, and your foot is completely off the throttle, you will use NO PETROL AT ALL.
The Sentra may not have that system, or if it does, may not be as efficient as the newer model.
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Old 10-02-2018, 06:05 AM   #8
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Yes, manufacturers have gotten better at squeezing the efficiency out of a car in 20 years. Then the wear of 20 years may have reduced the Nissan's.
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Old 10-03-2018, 03:17 AM   #9
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yeah, im aware that braking is basically wasting energy. i normally tend to conserve my brakes by engine braking.

@trollbait, yes perhaps. been looking at my hyundai's built in mpg meter lately, and im surprised that it could achieve a fuel economy as good as 6L/100km if you drive economically, although most of my driving is downhill... I've yet to calculate the nissans.
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